SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
Dec. 31, 2020
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Basis of Presentation Southwest Airlines Co. (the "Company") operates Southwest Airlines, a major domestic airline. The Consolidated Financial Statements include the accounts of the Company and its wholly owned subsidiaries, which include AirTran Holdings, LLC, the successor to AirTran Holdings, Inc. ("AirTran Holdings"), the former parent company of AirTran Airways, Inc., and Triple Crown Assurance Co., an insurance captive. The accompanying Consolidated Financial Statements include the results of operations and cash flows for all periods presented and all significant inter-entity balances and transactions have been eliminated. The preparation of financial statements in conformity with generally accepted accounting principles in the United States ("GAAP") requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ from these estimates. Effective as of January 1, 2019, the Company adopted Accounting Standards Update ("ASU") No. 2016-02, Leases, codified in Accounting Standards Codification ("ASC") 842 (the "New Lease Standard"). All amounts and disclosures set forth in this Form 10-K, reflect the adoption of this ASU, while all periods prior to 2019 remain in accordance with prior accounting requirements. See Note 3 for further information. Cash and Cash Equivalents Cash in excess of that necessary for operating requirements is invested in short-term, highly liquid, income-producing investments. Investments with original maturities of three months or less when purchased are classified as cash and cash equivalents, which primarily consist of certificates of deposit, money market funds, and investment grade commercial paper issued by major corporations and financial institutions. Cash and cash equivalents are stated at cost, which approximates fair value. As of December 31, 2020 and 2019, $34 million and $25 million, respectively, in cash collateral deposits were held by the Company from its fuel hedge counterparties, and no cash collateral deposits were held by or provided by the Company to its interest rate hedge counterparties for both periods. Cash collateral amounts provided or held associated with fuel and interest rate derivative instruments are not restricted in any way and earn interest income at an agreed upon rate that approximates the rates earned on short-term securities issued by the U.S. Government. Depending on the fair value of the Company’s fuel and interest rate derivative instruments, the amounts of collateral deposits held or provided at any point in time can fluctuate significantly. See Note 11 for further information on these collateral deposits and fuel derivative instruments. Short-term and Noncurrent Investments Short-term investments consist of investments with original maturities of greater than three months but less than twelve months when purchased. These are primarily short-term securities issued by the U.S. Government and certificates of deposit issued by domestic banks. All of these investments are classified as available-for-sale securities and are stated at fair value, which approximates cost. For all short-term investments, at each reset period or upon reinvestment, the Company accounts for the transaction as Proceeds from sales of short-term investments for the security relinquished, and Purchases of short-investments for the security purchased, in the accompanying Consolidated Statement of Cash Flows. Unrealized gains and losses, net of tax, if any, are recognized in Accumulated other comprehensive income (loss) ("AOCI") in the accompanying Consolidated Balance Sheet. Realized net gains and losses on specific investments, if any, are reflected in Interest income in the accompanying Consolidated Statement of Income (Loss). Both unrealized and realized gains and/or losses associated with investments were immaterial for all years presented. Noncurrent investments consist of investments with maturities of greater than twelve months. Noncurrent investments are included as a component of Other assets in the Consolidated Balance Sheet. Accounts and Other Receivables Accounts and other receivables are initially recorded at cost and are evaluated for collectability in every period. They primarily consist of the amounts due from the Company's business partners and other suppliers, amounts due from business partners in the Company’s loyalty program, and tax receivables from overpayment or net operating losses that are allowed to be carried back to prior periods to claim refunds against prior taxes paid. See Note 16 for further information. The allowance for doubtful accounts was immaterial at December 31, 2020 and 2019. In addition, the provision for doubtful accounts and write-offs for 2020, 2019, and 2018 were each immaterial. Inventories Inventories primarily consist of aircraft fuel, flight equipment expendable parts, materials, and supplies. All of these items are carried at average cost, less an allowance for obsolescence. These items are generally charged to expense when issued for use. The reserve for obsolescence was immaterial at December 31, 2020, and 2019. In addition, the Company’s provision for obsolescence and write-offs for 2020, 2019, and 2018 were each immaterial. Property and Equipment Property and equipment is stated at cost. Capital expenditures include payments made for aircraft, other flight equipment, purchase deposits related to future aircraft deliveries, airport and other facility construction projects, and ground and other property and equipment. Depreciation is provided by the straight-line method to estimated residual values over periods of approximately 25 years for flight equipment, and 5 to 30 years for ground property and equipment. Residual values estimated for aircraft are approximately 15 percent, and generally range from 0 to 10 percent for ground property and equipment. Assets constructed for others consists of airport improvement projects in which the Company is considered to have control of the asset during the construction period. Once construction is effectively completed, the sale-leaseback model would apply when control passes from the lessee to the lessor. See Note 5 for further information. The Company evaluates its long-lived assets used in operations for impairment when events and circumstances indicate that the undiscounted cash flows to be generated by that asset are less than the carrying amounts of the asset and may not be recoverable. Factors that would indicate potential impairment include, but are not limited to, significant decreases in the market value of the long-lived asset(s), a significant change in the long-lived asset’s physical condition, and operating or cash flow losses associated with the use of the long-lived asset. If an asset is deemed to be impaired, an impairment loss is recorded for the excess of the asset book value in relation to its estimated fair value. As a result of the events and impacts surrounding the COVID-19 pandemic, including the Company's net loss incurred during the year ended December 31, 2020, and the significant number of aircraft that have been placed in storage, the Company assessed whether any impairment of its amortizable assets existed. No aircraft impairment charges were deemed necessary for the majority of 2020. For the purpose of impairment assessment, the Company evaluates its all Boeing fleet as one asset group. However, during fourth quarter 2020, the Company made the decision to permanently ground, and accelerate the retirement of, 20 of its 737-700 aircraft as of December 31, 2020. This action resulted in the Company recording $32 million in impairment charges associated with these 20 aircraft. Leases The Company determines if an arrangement is a lease at inception. Operating leases are included in Operating lease right-of-use assets, Current operating lease liabilities, and Noncurrent operating lease liabilities in the Consolidated Balance Sheet. Finance leases are included in Property and equipment, Current maturities of long-term debt, and Long-term debt less current maturities in the Consolidated Balance Sheet. Right-of-use assets represent the Company's right to use an underlying asset for the lease term, and lease liabilities represent the Company's obligation to make lease payments arising from the lease. The lease liability is measured as the present value of the unpaid lease payments, and the right-of-use asset value is derived from the calculation of the lease liability. Lease payments include fixed and in-substance fixed payments, variable payments based on an index or rate, reasonably certain purchase options, termination penalties, fees paid by the lessee to the owners of a special-purpose entity for restructuring the transaction, and probable amounts the lessee will owe under a residual value guarantee. Lease payments do not include (i) variable lease payments other than those that depend on an index or rate, (ii) any guarantee by the lessee of the lessor’s debt, or (iii) any amount allocated to non-lease components, if such election is made upon adoption, per the provisions of the New Lease Standard. The Company uses its estimated incremental borrowing rate, which is derived from information available at the lease commencement date, in determining the present value of lease payments, since the Company does not know the actual implicit rates in its leases. The Company gives consideration to its recent debt issuances as well as publicly available data for instruments with similar characteristics when calculating its incremental borrowing rate. Lease expense for operating lease payments is recognized on a straight-line basis over the lease term. The Company combines lease and nonlease components for all asset groups. The Company's lease term includes any option to extend the lease when it is reasonably certain to be exercised based on considering all relevant economic factors. Aircraft and Engine Maintenance The cost of scheduled inspections and repairs and routine maintenance costs for all aircraft and engines are charged to Maintenance materials and repairs expense within the accompanying Consolidated Statement of Income (Loss) as incurred. The Company has maintenance agreements related to certain of its aircraft engines with external service providers, including agreements that effectively transfer the risk of performance of such work to the service provider. Under the agreements where the risk of performance is deemed transferred to the counterparty, the appropriate expense is recorded commensurate with the period in which the corresponding level of service is provided. Generally, expense is recorded on a straight-line basis over the term of the agreement based on the Company's best estimate of expected future aircraft utilization. For its engine maintenance contracts that do not transfer risk to the service provider, the Company records expense on a time and materials basis when an engine repair event takes place. Modifications that significantly enhance the operating performance or extend the useful lives of aircraft or engines are capitalized and amortized over the remaining life of the asset. Goodwill and Intangible Assets The Company applies a fair value based impairment test to the carrying value of goodwill and indefinite-lived intangible assets annually on October 1st, or more frequently if certain events or circumstances indicate that an impairment loss may have been incurred. The Company assesses the value of goodwill and indefinite-lived intangible assets under either a qualitative or quantitative approach. Under a qualitative approach, the Company considers various market factors, including applicable key assumptions also used in the quantitative assessment listed below. These factors are analyzed to determine if events and circumstances could reasonably have affected the fair value of goodwill and indefinite-lived intangible assets. If the Company determines that it is more likely than not that an indefinite-lived intangible asset or reporting unit goodwill is impaired, the quantitative approach is used to assess the asset or reporting unit fair value and the amount of the impairment. Under a quantitative approach, the fair value of the Company's indefinite-lived intangible asset or reporting unit is calculated based on key market participant assumptions. If the indefinite-lived intangible assets' carrying value exceeds the fair value calculated using the quantitative approach, an impairment charge is recorded for the difference in fair value and carrying value. If the reporting unit carrying value exceeds the reporting unit fair value calculated using the quantitative approach, an impairment charge is recorded for the difference between fair value and carrying value, limited to the amount of goodwill in the reporting unit. When performing a quantitative impairment assessment of goodwill and indefinite-lived intangible assets, fair value is estimated based on (i) recent market transactions, where available, (ii) projected discounted cash flows (an income approach), or (iii) a combination of limited market transactions and the lease savings method (which reflects potential annual after-tax lease savings arising from owning the certain indefinite-lived intangibles rather than leasing them from another airline at market rates). The Company applied the quantitative approach during its annual 2020 impairment tests. Key assumptions and/or estimates made in the Company's 2020 impairment tests included the following: (i) a projection of revenues, expenses, and cash flows; (ii) terminal period revenue growth and cash flows; (iii) an estimated weighted average cost of capital; (iv) an assumed discount rate depending on the asset; (v) a tax rate; and (vi) market purchase prices and lease rates for comparable assets. The Company believes these assumptions are consistent with those a hypothetical market participant would use given circumstances that were present at the time the estimates were made. However, actual results and amounts may be significantly different from the Company's estimates. As a result of the annual impairment tests performed as of October 1, 2020, no impairment was determined to exist for Goodwill or indefinite-lived intangible assets, as the fair values of the reporting unit and indefinite-lived intangible assets exceeded their respective carrying values. The Company’s intangible assets primarily consist of acquired rights to certain airport owned takeoff and landing slots (a "slot" is the right of an air carrier, pursuant to regulations of the Federal Aviation Administration ("FAA"), to operate a takeoff or landing at a specific time at certain airports) at certain domestic slot-controlled airports. Indefinite lived slots of $295 million are included as a component of Other assets in the Company's Consolidated Balance Sheet, as of December 31, 2020 and 2019. Revenue Recognition Tickets sold are initially deferred as Air traffic liability. Passenger revenue is recognized and Air traffic liability is reduced when transportation is provided. Air traffic liability primarily represents tickets sold for future travel dates, funds that are past flight date and remain unused, but are expected to be used in the future, and the Company’s liability for loyalty benefits that are expected to be redeemed in the future. The majority of the Company’s tickets sold are nonrefundable. Southwest has a No Show policy that applies to fares that are not canceled or changed by a Customer at least ten minutes prior to a flight's scheduled departure. Nonrefundable tickets that are sold but not flown on the travel date, and are canceled in accordance with the No Show policy, can be applied to future travel. Refundable tickets that are sold but not flown on the travel date can also be applied to future travel. A small percentage of tickets (or partial tickets) expire unused. The Company estimates the amount of tickets that expire unused and recognizes such amounts in Passenger revenue once the scheduled flight date has lapsed in proportion to the pattern of flights taken by the Customers. Based on the Company's revenue recognition policy, revenue is recorded at the flight date for a Customer who does not change his/her itinerary and loses his/her funds as the Company has then fulfilled its performance obligation. Amounts collected from passengers for ancillary services are also recognized when the service is provided, which is typically the flight date. Initial spoilage estimates for both tickets and funds available for future use are routinely adjusted and ultimately finalized once the tickets expire, which is typically twelve months after the original purchase date. However, during 2020, the Company extended the expiration dates for a significant amount of tickets and funds beyond its normal twelve months. Spoilage estimates are based on the Company's Customers' historical travel behavior as well as assumptions about the Customers' future travel behavior. Assumptions used to generate spoilage estimates can be impacted by several factors including, but not limited to: fare increases, fare sales, changes to the Company's ticketing policies, changes to the Company’s refund, exchange, and unused funds policies, seat availability, and economic factors. Given the unprecedented amount of 2020 Customer flight cancellations and the amount of travel funds provided, the Company expects additional variability in the amount of spoilage revenue recorded in future periods, as the estimates of the portion of sold tickets that will expire unused may differ from historical experience. See Note 6 for further information. Approximately $184 million, approximately $615 million, and approximately $566 million of the Company's Operating revenues in 2020, 2019, and 2018, respectively, were attributable to foreign operations. The remainder of the Company's Operating revenues, approximately $8.9 billion, approximately $21.8 billion, and approximately $21.4 billion in 2020, 2019, and 2018, respectively, were attributable to domestic operations. Loyalty Program The Company records a liability for the relative fair value of providing free travel under its loyalty program for all points earned from flight activity or sold to companies participating in the Company’s Rapid Rewards loyalty program as business partners that are expected to be redeemed for future travel. The loyalty liability represents performance obligations that will be satisfied when a Rapid Rewards loyalty member redeems points for travel or other goods and services. Points earned from flight activity are valued at their relative standalone selling price by applying fair value based on historical redemption patterns. Points earned from business partner activity, which primarily consist of points sold, along with related marketing services, to companies participating in the Rapid Rewards loyalty program, are valued using a relative fair value methodology based on the contractual rate which partners pay to Southwest to award Rapid Rewards points to the business partner’s customers. For points that are expected to remain unused, the Company recognizes spoilage in proportion to the pattern of points used by the Customer, which approximates the average period over which the population of Rapid Reward Members redeem their points. The Company records passenger revenue related to air transportation when the transportation is delivered. The marketing elements are recognized as Other - net revenue when earned. The Company’s liability for loyalty benefits includes a portion that is expected to be redeemed during the following twelve months (classified as a component of Air traffic liability), and a portion that is not expected to be redeemed during the following twelve months (classified as Air traffic liability - noncurrent). The Company continually updates this analysis and adjusts the split between current and non-current liabilities as appropriate. See Note 6 for further information. Advertising Advertising costs are charged to expense as incurred. Advertising and promotions expense for the years ended December 31, 2020, 2019, and 2018 was $156 million, $212 million, and $215 million, respectively, and is included as a component of Other operating expense in the accompanying Consolidated Statement of Income (Loss). Share-based Employee Compensation The Company has share-based compensation plans covering certain Employees, including a plan that also covers the Company’s Board of Directors. The Company accounts for share-based compensation based on its grant date fair value. See Note 10 for further information. Financial Derivative Instruments The Company accounts for financial derivative instruments at fair value and applies hedge accounting rules where appropriate. The Company utilizes various derivative instruments, including jet fuel, crude oil, unleaded gasoline, and heating oil-based derivatives, to attempt to reduce the risk of its exposure to jet fuel price increases. These instruments are accounted for as cash flow hedges upon proper qualification. The Company also has had interest rate swap agreements to convert certain floating-rate debt to a fixed-rate and had interest rate swap agreements to convert portions of its fixed-rate debt to floating rates. The Company has forward-stating interest rate swap agreements, the primary objective of which is to hedge forecasted debt issuances. These interest rate hedges are appropriately designated as cash flow hedges. Since the majority of the Company’s financial derivative instruments are not traded on a market exchange, the Company estimates their fair values. Depending on the type of instrument, the values are determined by the use of present value methods or option value models with assumptions about commodity prices based on those observed in underlying markets. All cash flows associated with purchasing and selling derivatives are classified as operating cash flows in the Consolidated Statement of Cash Flows, within Changes in certain assets and liabilities. The Company classifies its cash collateral provided to or held from counterparties in a "net" presentation on the Consolidated Balance Sheet against the fair value of the derivative positions with those counterparties. See Note 11 for further information. Software Capitalization The Company capitalizes certain internal and external costs related to the acquisition and development of internal use software during the application development stages of projects. The Company amortizes these costs using the straight-line method over the estimated useful life of the software, which is typically five to fifteen years. Costs incurred during the preliminary project or the post-implementation/operation stages of the project are expensed as incurred. Capitalized computer software, included as a component of Ground property and equipment in the accompanying Consolidated Balance Sheet, net of accumulated depreciation, was $697 million and $630 million at December 31, 2020, and 2019, respectively. Computer software depreciation expense was $203 million, $177 million, and $155 million for the years ended December 31, 2020, 2019, and 2018, respectively, and is included as a component of Depreciation and amortization expense in the accompanying Consolidated Statement of Income (Loss). The Company evaluates internal use software for impairment on a quarterly basis; if it is determined the value of an asset was not recoverable or it qualifies for impairment, a charge will be recorded to write down the software to the lower of its carrying value or fair value. The Company had no significant impairments during 2020, 2019, or 2018. Insurance Reserves The Company uses a combination of insurance and self-insurance mechanisms, including a wholly-owned captive insurance entity and participation in a reinsurance treaty, to provide for the potential liabilities associated with certain risks, including workers’ compensation, healthcare benefits, general liability, and aviation liability. Liabilities associated with the risks that are retained by the Company are not discounted and are estimated, in part, by considering historical claims experience, demographics, exposure and severity factors and other actuarial assumptions. Income Taxes The Company accounts for deferred income taxes utilizing an asset and liability method, whereby deferred tax assets and liabilities are recognized based on the tax effect of temporary differences between the financial statements and the tax basis of assets and liabilities, as measured by current enacted tax rates. The Company also evaluates the need for a valuation allowance to reduce deferred tax assets to estimated recoverable amounts. The Company’s policy for recording interest and penalties associated with uncertain tax positions is to record such items as a component of Income (loss) before income taxes. Penalties are recorded in Other (gains) losses, net, and interest paid or received is recorded in Interest expense or Interest income, respectively, in the accompanying Consolidated Statement of Income (Loss). There were no material amounts recorded for penalties and interest related to uncertain tax positions for all years presented. See Note 15 for further information. Concentration Risk Approximately 83 percent of the Company’s full-time equivalent Employees are unionized and are covered by collective-bargaining agreements. A percentage of the Company's unionized Employees, including its Pilots, Flight Attendants, Customer Service Agents, Dispatchers, Aircraft Appearance Technicians, and Meteorologists, which had contracts that became amendable on or before December 31, 2020, are in discussions on labor agreements. Those unionized Employee groups in discussions represent approximately 55 percent of the Company’s full-time equivalent Employees as of December 31, 2020. The Company attempts to minimize its concentration risk with regards to its cash, cash equivalents, and its investment portfolio. This is accomplished by diversifying and limiting amounts among different counterparties, the type of investment, and the amount invested in any individual security or money market fund. To manage risk associated with financial derivative instruments held, the Company selects and will periodically review counterparties based on credit ratings, limits its exposure to a single counterparty, and monitors the market position of the program and its relative market position with each counterparty. The Company also has agreements with counterparties containing early termination rights and/or bilateral collateral provisions whereby security is required if market risk exposure exceeds a specified threshold amount or credit ratings fall below certain levels. Collateral deposits provided to or held from counterparties serve to decrease, but not totally eliminate, the credit risk associated with the Company’s hedging program. See Note 11 for further information. As of December 31, 2020, the Company operated an all-Boeing fleet, all of which are variations of the Boeing 737. The Boeing 737 MAX aircraft ("MAX") are crucial to the Company’s growth plans and fleet modernization initiatives. On March 13, 2019, the FAA issued an emergency order for all U.S. airlines to ground the MAX aircraft, including the 34 MAX aircraft in the Company's fleet. On November 18, 2020, the FAA rescinded its order to ground the MAX fleet. The Company is currently working to meet the FAA's requirements by modifying certain operating procedures, implementing enhanced Pilot training requirements, installing FAA-approved flight control software updates, and completing other required maintenance tasks specific to the MAX aircraft. The Company currently estimates that the MAX will return to service on March 11, 2021 after all active Pilots have received updated, MAX-related training. The MAX fleet grounding has adversely affected the Company's operating results, and could have a material adverse effect on the Company's operating results in future periods. Boeing no longer manufactures versions of the 737 other than the 737 MAX family of aircraft. If the 737 MAX aircraft were to again become unavailable for the Company’s flight operations, the Company’s growth would be restricted unless and until it could procure and operate other types of aircraft from Boeing or another manufacturer, seller, or lessor, and the Company’s operations would be materially adversely affected. In particular, if the Company’s growth were to be dependent upon the introduction of a new aircraft make and model to the Company’s fleet, the Company would need to, among other things, (i) develop and implement new maintenance, operating, and training programs, (ii) secure extensive regulatory approvals, and (iii) implement new technologies. The requirements associated with operating a new aircraft make and model could take an extended period of time to fulfill and would likely impose substantial costs on the Company. A shift away from a single fleet type could also add complexity to the Company’s operations, present operational and compliance risks, and materially increase the Company's costs. Any of these events would have a material, adverse effect on the Company's business, operating results, and financial condition. The Company could also be materially adversely affected if the pricing or operational attributes of its aircraft were to become less competitive. See Note 17 for further information. The Company is also dependent on sole or limited suppliers for aircraft engines and certain other aircraft parts and services and would, therefore, also be materially adversely impacted in the event of the unavailability of, inadequate support for, or a mechanical or regulatory issue associated with, engines and other parts. The Company has historically entered into agreements with some of its co-brand, payment, and loyalty partners that contain exclusivity aspects which place certain confidential restrictions on the Company from entering into certain arrangements with other payment and loyalty partners. Some of these agreements automatically renew on an annual basis, unless either party objects to such extension. None of these agreements are more than 10 years in length. The Company believes the financial benefits generated by the exclusivity aspects of these arrangements outweigh the risks involved with such agreements.

Southwest Airlines Co. (the "Company") operates Southwest Airlines, a major domestic airline. The Consolidated Financial Statements include the accounts of the Company and its wholly owned subsidiaries, which include AirTran Holdings, LLC, the successor to AirTran Holdings, Inc. ("AirTran Holdings"), the former parent company of AirTran Airways, Inc., and Triple Crown Assurance Co., an insurance captive. The accompanying Consolidated Financial Statements include the results of operations and cash flows for all periods presented and all significant inter-entity balances and transactions have been eliminated. The preparation of financial statements in conformity with generally accepted accounting principles in the United States ("GAAP") requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ from these estimates.

Cash in excess of that necessary for operating requirements is invested in short-term, highly liquid, income-producing investments. Investments with original maturities of three months or less when purchased are classified as cash and cash equivalents, which primarily consist of certificates of deposit, money market funds, and investment grade commercial paper issued by major corporations and financial institutions. Cash and cash equivalents are stated at cost, which approximates fair value.

As of December 31, 2020 and 2019, $34 million and $25 million, respectively, in cash collateral deposits were held by the Company from its fuel hedge counterparties, and no cash collateral deposits were held by or provided by the Company to its interest rate hedge counterparties for both periods. Cash collateral amounts provided or held associated with fuel and interest rate derivative instruments are not restricted in any way and earn interest income at an agreed upon rate that approximates the rates earned on short-term securities issued by the U.S. Government. Depending on the fair value of the Company’s fuel and interest rate derivative instruments, the amounts of collateral deposits held or provided at any point in time can fluctuate significantly. See Note 11 for further information on these collateral deposits and fuel derivative instruments.

Short-term investments consist of investments with original maturities of greater than three months but less than twelve months when purchased. These are primarily short-term securities issued by the U.S. Government and certificates of deposit issued by domestic banks. All of these investments are classified as available-for-sale securities and are stated at fair value, which approximates cost. For all short-term investments, at each reset period or upon reinvestment, the Company accounts for the transaction as Proceeds from sales of short-term investments for the security relinquished, and Purchases of short-investments for the security purchased, in the accompanying Consolidated Statement of Cash Flows. Unrealized gains and losses, net of tax, if any, are recognized in Accumulated other comprehensive income (loss) ("AOCI") in the accompanying Consolidated Balance Sheet. Realized net gains and losses on specific investments, if any, are reflected in Interest income in the accompanying Consolidated Statement of Income (Loss). Both unrealized and realized gains and/or losses associated with investments were immaterial for all years presented.

Accounts and other receivables are initially recorded at cost and are evaluated for collectability in every period. They primarily consist of the amounts due from the Company's business partners and other suppliers, amounts due from business partners in the Company’s loyalty program, and tax receivables from overpayment or net operating losses that are allowed to be carried back to prior periods to claim refunds against prior taxes paid. See Note 16 for further information. The allowance for doubtful accounts was immaterial at December 31, 2020 and 2019. In addition, the provision for doubtful accounts and write-offs for 2020, 2019, and 2018 were each immaterial.

Inventories primarily consist of aircraft fuel, flight equipment expendable parts, materials, and supplies. All of these items are carried at average cost, less an allowance for obsolescence. These items are generally charged to expense when issued for use. The reserve for obsolescence was immaterial at December 31, 2020, and 2019. In addition, the Company’s provision for obsolescence and write-offs for 2020, 2019, and 2018 were each immaterial.

Property and equipment is stated at cost. Capital expenditures include payments made for aircraft, other flight equipment, purchase deposits related to future aircraft deliveries, airport and other facility construction projects, and ground and other property and equipment. Depreciation is provided by the straight-line method to estimated residual values over periods of approximately 25 years for flight equipment, and 5 to 30 years for ground property and equipment. Residual values estimated for aircraft are approximately 15 percent, and generally range from 0 to 10 percent for ground property and equipment. Assets constructed for others consists of airport improvement projects in which the Company is considered to have control of the asset during the construction period. Once construction is effectively completed, the sale-leaseback model would apply when control passes from the lessee to the lessor. See Note 5 for further information.

The Company evaluates its long-lived assets used in operations for impairment when events and circumstances indicate that the undiscounted cash flows to be generated by that asset are less than the carrying amounts of the asset and may not be recoverable. Factors that would indicate potential impairment include, but are not limited to, significant decreases in the market value of the long-lived asset(s), a significant change in the long-lived asset’s physical condition, and operating or cash flow losses associated with the use of the long-lived asset. If an asset is deemed to be impaired, an impairment loss is recorded for the excess of the asset book value in relation to its estimated fair value. As a result of the events and impacts surrounding the COVID-19 pandemic, including the Company's net loss incurred during the year ended December 31, 2020, and the significant number of aircraft that have been placed in storage, the Company assessed whether any impairment of its amortizable assets existed. No aircraft impairment charges were deemed necessary for the majority of 2020. For the purpose of impairment assessment, the Company evaluates its all Boeing fleet as one asset group. However, during fourth quarter 2020, the Company made the decision to permanently ground, and accelerate the retirement of, 20 of its 737-700 aircraft as of December 31, 2020. This action resulted in the Company recording $32 million in impairment charges associated with these 20 aircraft.

The Company determines if an arrangement is a lease at inception. Operating leases are included in Operating lease right-of-use assets, Current operating lease liabilities, and Noncurrent operating lease liabilities in the Consolidated Balance Sheet. Finance leases are included in Property and equipment, Current maturities of long-term debt, and Long-term debt less current maturities in the Consolidated Balance Sheet.

Right-of-use assets represent the Company's right to use an underlying asset for the lease term, and lease liabilities represent the Company's obligation to make lease payments arising from the lease. The lease liability is measured as the present value of the unpaid lease payments, and the right-of-use asset value is derived from the calculation of the lease liability. Lease payments include fixed and in-substance fixed payments, variable payments based on an index or rate, reasonably certain purchase options, termination penalties, fees paid by the lessee to the owners of a special-purpose entity for restructuring the transaction, and probable amounts the lessee will owe under a residual value guarantee. Lease payments do not include (i) variable lease payments other than those that depend on an index or rate, (ii) any guarantee by the lessee of the lessor’s debt, or (iii) any amount allocated to non-lease components, if such election is made upon adoption, per the provisions of the New Lease Standard. The Company uses its estimated incremental borrowing rate, which is derived from information available at the lease commencement date, in determining the present value of lease payments, since the Company does not know the actual implicit rates in its leases. The Company gives consideration to its recent debt issuances as well as publicly available data for instruments with similar characteristics when calculating its incremental borrowing rate. Lease expense for operating lease payments is recognized on a straight-line basis over the lease term. The Company combines lease and nonlease components for all asset groups. The Company's lease term includes any option to extend the lease when it is reasonably certain to be exercised based on considering all relevant economic factors.

The Company has maintenance agreements related to certain of its aircraft engines with external service providers, including agreements that effectively transfer the risk of performance of such work to the service provider. Under the agreements where the risk of performance is deemed transferred to the counterparty, the appropriate expense is recorded commensurate with the period in which the corresponding level of service is provided. Generally, expense is recorded on a straight-line basis over the term of the agreement based on the Company's best estimate of expected future aircraft utilization. For its engine maintenance contracts that do not transfer risk to the service provider, the Company records expense on a time and materials basis when an engine repair event takes place.

The Company applies a fair value based impairment test to the carrying value of goodwill and indefinite-lived intangible assets annually on October 1st, or more frequently if certain events or circumstances indicate that an impairment loss may have been incurred. The Company assesses the value of goodwill and indefinite-lived intangible assets under either a qualitative or quantitative approach. Under a qualitative approach, the Company considers various market factors, including applicable key assumptions also used in the quantitative assessment listed below. These factors are analyzed to determine if events and circumstances could reasonably have affected the fair value of goodwill and indefinite-lived intangible assets. If the Company determines that it is more likely than not that an indefinite-lived intangible asset or reporting unit goodwill is impaired, the quantitative approach is used to assess the asset or reporting unit fair value and the amount of the impairment. Under a quantitative approach, the fair value of the Company's indefinite-lived intangible asset or reporting unit is calculated based on key market participant assumptions. If the indefinite-lived intangible assets' carrying value exceeds the fair value calculated using the quantitative approach, an impairment charge is recorded for the difference in fair value and carrying value. If the reporting unit carrying value exceeds the reporting unit fair value calculated using the quantitative approach, an impairment charge is recorded for the difference between fair value and carrying value, limited to the amount of goodwill in the reporting unit.

The Company applied the quantitative approach during its annual 2020 impairment tests. Key assumptions and/or estimates made in the Company's 2020 impairment tests included the following: (i) a projection of revenues, expenses, and cash flows; (ii) terminal period revenue growth and cash flows; (iii) an estimated weighted average cost of capital; (iv) an assumed discount rate depending on the asset; (v) a tax rate; and (vi) market purchase prices and lease rates for comparable assets. The Company believes these assumptions are consistent with those a hypothetical market participant would use given circumstances that were present at the time the estimates were made. However, actual results and amounts may be significantly different from the Company's estimates. As a result of the annual impairment tests performed as of October 1, 2020, no impairment was determined to exist for Goodwill or indefinite-lived intangible assets, as the fair values of the reporting unit and indefinite-lived intangible assets exceeded their respective carrying values.

The Company’s intangible assets primarily consist of acquired rights to certain airport owned takeoff and landing slots (a "slot" is the right of an air carrier, pursuant to regulations of the Federal Aviation Administration ("FAA"), to operate a takeoff or landing at a specific time at certain airports) at certain domestic slot-controlled airports. Indefinite lived slots of $295 million are included as a component of Other assets in the Company's Consolidated Balance Sheet, as of December 31, 2020 and 2019.

Tickets sold are initially deferred as Air traffic liability. Passenger revenue is recognized and Air traffic liability is reduced when transportation is provided. Air traffic liability primarily represents tickets sold for future travel dates, funds that are past flight date and remain unused, but are expected to be used in the future, and the Company’s liability for loyalty benefits that are expected to be redeemed in the future. The majority of the Company’s tickets sold are nonrefundable. Southwest has a No Show policy that applies to fares that are not canceled or changed by a Customer at least ten minutes prior to a flight's scheduled departure. Nonrefundable tickets that are sold but not flown on the travel date, and are canceled in accordance with the No Show policy, can be applied to future travel. Refundable tickets that are sold but not flown on the travel date can also be applied to future travel. A small percentage of tickets (or partial tickets) expire unused. The Company estimates the amount of tickets that expire unused and recognizes such amounts in Passenger revenue once the scheduled flight date has lapsed in proportion to the pattern of flights taken by the Customers. Based on the Company's revenue recognition policy, revenue is recorded at the flight date for a Customer who does not change his/her itinerary and loses his/her funds as the Company has then fulfilled its performance obligation. Amounts collected from passengers for ancillary services are also recognized when the service is provided, which is typically the flight date.

Initial spoilage estimates for both tickets and funds available for future use are routinely adjusted and ultimately finalized once the tickets expire, which is typically twelve months after the original purchase date. However, during 2020, the Company extended the expiration dates for a significant amount of tickets and funds beyond its normal twelve months. Spoilage estimates are based on the Company's Customers' historical travel behavior as well as assumptions about the Customers' future travel behavior. Assumptions used to generate spoilage estimates can be impacted by several factors including, but not limited to: fare increases, fare sales, changes to the Company's ticketing policies, changes to the Company’s refund, exchange, and unused funds policies, seat availability, and economic factors. Given the unprecedented amount of 2020 Customer flight cancellations and the amount of travel funds provided, the Company expects additional variability in the amount of spoilage revenue recorded in future periods, as the estimates of the portion of sold tickets that will expire unused may differ from historical experience. See Note 6 for further information.

The Company records a liability for the relative fair value of providing free travel under its loyalty program for all points earned from flight activity or sold to companies participating in the Company’s Rapid Rewards loyalty program as business partners that are expected to be redeemed for future travel. The loyalty liability represents performance obligations that will be satisfied when a Rapid Rewards loyalty member redeems points for travel or other goods and services. Points earned from flight activity are valued at their relative standalone selling price by applying fair value based on historical redemption patterns. Points earned from business partner activity, which primarily consist of points sold, along with related marketing services, to companies participating in the Rapid Rewards loyalty program, are valued using a relative fair value methodology based on the contractual rate which partners pay to Southwest to award Rapid Rewards points to the business partner’s customers. For points that are expected to remain unused, the Company recognizes spoilage in proportion to the pattern of points used by the Customer, which approximates the average period over which the population of Rapid Reward Members redeem their points. The Company records passenger revenue related to air transportation when the transportation is delivered. The marketing elements are recognized as Other - net revenue when earned. The Company’s liability for loyalty benefits includes a portion that is expected to be redeemed during the following twelve months (classified as a component of Air traffic liability), and a portion that is not expected to be redeemed during the following twelve months (classified as Air traffic liability - noncurrent). The Company continually updates this analysis and adjusts the split between current and non-current liabilities as appropriate. See Note 6 for further information.

The Company accounts for financial derivative instruments at fair value and applies hedge accounting rules where appropriate. The Company utilizes various derivative instruments, including jet fuel, crude oil, unleaded gasoline, and heating oil-based derivatives, to attempt to reduce the risk of its exposure to jet fuel price increases. These instruments are accounted for as cash flow hedges upon proper qualification. The Company also has had interest rate swap agreements to convert certain floating-rate debt to a fixed-rate and had interest rate swap agreements to convert portions of its fixed-rate debt to floating rates. The Company has forward-stating interest rate swap agreements, the primary objective of which is to hedge forecasted debt issuances. These interest rate hedges are appropriately designated as cash flow hedges.

The Company capitalizes certain internal and external costs related to the acquisition and development of internal use software during the application development stages of projects. The Company amortizes these costs using the straight-line method over the estimated useful life of the software, which is typically five to fifteen years. Costs incurred during the preliminary project or the post-implementation/operation stages of the project are expensed as incurred. Capitalized computer software, included as a component of Ground property and equipment in the accompanying Consolidated Balance Sheet, net of accumulated depreciation, was $697 million and $630 million at December 31, 2020, and 2019, respectively. Computer software depreciation expense was $203 million, $177 million, and $155 million for the years ended December 31, 2020, 2019, and 2018, respectively, and is included as a component of Depreciation and amortization expense in the accompanying Consolidated Statement of Income (Loss). The Company evaluates internal use software for impairment on a quarterly basis; if it is determined the value of an asset was not recoverable or it qualifies for impairment, a charge will be recorded to write down the software to the lower of its carrying value or fair value. The Company had no significant impairments during 2020, 2019, or 2018.

The Company uses a combination of insurance and self-insurance mechanisms, including a wholly-owned captive insurance entity and participation in a reinsurance treaty, to provide for the potential liabilities associated with certain risks, including workers’ compensation, healthcare benefits, general liability, and aviation liability. Liabilities associated with the risks that are retained by the Company are not discounted and are estimated, in part, by considering historical claims experience, demographics, exposure and severity factors and other actuarial assumptions.

The Company accounts for deferred income taxes utilizing an asset and liability method, whereby deferred tax assets and liabilities are recognized based on the tax effect of temporary differences between the financial statements and the tax basis of assets and liabilities, as measured by current enacted tax rates. The Company also evaluates the need for a valuation allowance to reduce deferred tax assets to estimated recoverable amounts.

The Company’s policy for recording interest and penalties associated with uncertain tax positions is to record such items as a component of Income (loss) before income taxes. Penalties are recorded in Other (gains) losses, net, and interest paid or received is recorded in Interest expense or Interest income, respectively, in the accompanying Consolidated Statement of Income (Loss). There were no material amounts recorded for penalties and interest related to uncertain tax positions for all years presented. See Note 15 for further information.

Approximately 83 percent of the Company’s full-time equivalent Employees are unionized and are covered by collective-bargaining agreements. A percentage of the Company's unionized Employees, including its Pilots, Flight Attendants, Customer Service Agents, Dispatchers, Aircraft Appearance Technicians, and Meteorologists, which had contracts that became amendable on or before December 31, 2020, are in discussions on labor agreements. Those unionized Employee groups in discussions represent approximately 55 percent of the Company’s full-time equivalent Employees as of December 31, 2020.

To manage risk associated with financial derivative instruments held, the Company selects and will periodically review counterparties based on credit ratings, limits its exposure to a single counterparty, and monitors the market position of the program and its relative market position with each counterparty. The Company also has agreements with counterparties containing early termination rights and/or bilateral collateral provisions whereby security is required if market risk exposure exceeds a specified threshold amount or credit ratings fall below certain levels. Collateral deposits provided to or held from counterparties serve to decrease, but not totally eliminate, the credit risk associated with the Company’s hedging program. See Note 11 for further information.

As of December 31, 2020, the Company operated an all-Boeing fleet, all of which are variations of the Boeing 737. The Boeing 737 MAX aircraft ("MAX") are crucial to the Company’s growth plans and fleet modernization initiatives. On March 13, 2019, the FAA issued an emergency order for all U.S. airlines to ground the MAX aircraft, including the 34 MAX aircraft in the Company's fleet. On November 18, 2020, the FAA rescinded its order to ground the MAX fleet. The Company is currently working to meet the FAA's requirements by modifying certain operating procedures, implementing enhanced Pilot training requirements, installing FAA-approved flight control software updates, and completing other required maintenance tasks specific to the MAX aircraft. The Company currently estimates that the MAX will return to service on March 11, 2021 after all active Pilots have received updated, MAX-related training.

Boeing no longer manufactures versions of the 737 other than the 737 MAX family of aircraft. If the 737 MAX aircraft were to again become unavailable for the Company’s flight operations, the Company’s growth would be restricted unless and until it could procure and operate other types of aircraft from Boeing or another manufacturer, seller, or lessor, and the Company’s operations would be materially adversely affected. In particular, if the Company’s growth were to be dependent upon the introduction of a new aircraft make and model to the Company’s fleet, the Company would need to, among other things, (i) develop and implement new maintenance, operating, and training programs, (ii) secure extensive regulatory approvals, and (iii) implement new technologies. The requirements associated with operating a new aircraft make and model could take an extended period of time to fulfill and would likely impose substantial costs on the Company. A shift away from a single fleet type could also add complexity to the Company’s operations, present operational and compliance risks, and materially increase the Company's costs. Any of these events would have a material, adverse effect on the Company's business, operating results, and financial condition. The Company could also be materially adversely affected if the pricing or operational attributes of its aircraft were to become less competitive. See Note 17 for further information.

The Company has historically entered into agreements with some of its co-brand, payment, and loyalty partners that contain exclusivity aspects which place certain confidential restrictions on the Company from entering into certain arrangements with other payment and loyalty partners. Some of these agreements automatically renew on an annual basis, unless either party objects to such extension. None of these agreements are more than 10 years in length. The Company believes the financial benefits generated by the exclusivity aspects of these arrangements outweigh the risks involved with such agreements.