Nature Of Operations And Summary Of Significant Accounting Policies (Policy)	12 Months Ended
Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Company Structure And Principles Of Consolidation	Company Structure and Principles of Consolidation. NiSource, a Delaware corporation headquartered in Merrillville, Indiana, is an energy holding company whose subsidiaries are fully regulated natural gas and electric utility companies serving approximately 3.9 million customers in seven states. NiSource generates substantially all of its operating income through these rate-regulated businesses. The consolidated financial statements include the accounts of NiSource and its majority-owned subsidiaries after the elimination of all intercompany accounts and transactions.
Use Of Estimates	Use of Estimates. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Cash, Cash Equivalents, And Restricted Cash	Cash, Cash Equivalents and Restricted Cash. NiSource considers all highly liquid investments with original maturities of three months or less to be cash equivalents. NiSource reports amounts deposited in brokerage accounts for margin requirements as restricted cash. In addition, NiSource has amounts deposited in trust to satisfy requirements for the provision of various property, liability, workers compensation, and long-term disability insurance, which is classified as restricted cash and disclosed as cash and cash equivalents on the Statements of Consolidated Cash Flows.
Accounts Receivable And Unbilled Revenue	Accounts Receivable and Unbilled Revenue. Accounts receivable on the Consolidated Balance Sheets includes both billed and unbilled amounts. Unbilled amounts of accounts receivable relate to a portion of a customer’s consumption of gas or electricity from the date of the last cycle billing date through the last day of the month (balance sheet date). Factors taken into consideration when estimating unbilled revenue include historical usage, customer rates and weather. Accounts receivable fluctuates from year to year depending in large part on weather impacts and price volatility. NiSource's accounts receivable on the Consolidated Balance Sheets include unbilled revenue, less reserves, in the amounts of $359.4 million and $329.7 million as of December 31, 2017 and 2016, respectively. The reserve for uncollectible receivables is NiSource’s best estimate of the amount of probable credit losses in the existing accounts receivable. NiSource determined the reserve based on historical experience and in consideration of current market conditions. Account balances are charged against the allowance when it is anticipated the receivable will not be recovered.
Investments In Debt And Equity Securities	Investments in Debt Securities. NiSource’s investments in debt securities are carried at fair value and are designated as available-for-sale. These investments are included within “Other investments” on the Consolidated Balance Sheets. Unrealized gains and losses, net of deferred income taxes, are reflected as accumulated other comprehensive income. These investments are monitored for other than temporary declines in market value. Realized gains and losses and permanent impairments are reflected in the Statements of Consolidated Income. No material impairment charges were recorded for the years ended December 31, 2017, 2016 or 2015. Refer to Note 16, "Fair Value," for additional information.
Basis Of Accounting For Rate-Regulated Subsidiaries	Basis of Accounting for Rate-Regulated Subsidiaries. Rate-regulated subsidiaries account for and report assets and liabilities consistent with the economic effect of the way in which regulators establish rates, if the rates established are designed to recover the costs of providing the regulated service and it is probable that such rates can be charged and collected. Certain expenses and credits subject to utility regulation or rate determination normally reflected in income are deferred on the Consolidated Balance Sheets and are later recognized in income as the related amounts are included in customer rates and recovered from or refunded to customers. In the event that regulation significantly changes the opportunity for NiSource to recover its costs in the future, all or a portion of NiSource’s regulated operations may no longer meet the criteria for regulatory accounting. In such an event, a write-down of all or a portion of NiSource’s existing regulatory assets and liabilities could result. If transition cost recovery was approved by the appropriate regulatory bodies that would meet the requirements under GAAP for continued accounting as regulatory assets and liabilities during such recovery period, the regulatory assets and liabilities would be reported at the recoverable amounts. If unable to continue to apply the provisions of regulatory accounting, NiSource would be required to apply the provisions of ASC 980-20, Discontinuation of Rate-Regulated Accounting. In management’s opinion, NiSource’s regulated subsidiaries will be subject to regulatory accounting for the foreseeable future. Refer to Note 8, "Regulatory Matters," for additional information.
Utility Plant And Other Property And Related Depreciation And Maintenance	Plant and Other Property and Related Depreciation and Maintenance. Property, plant and equipment (principally utility plant) is stated at cost. The rate-regulated subsidiaries record depreciation using composite rates on a straight-line basis over the remaining service lives of the electric, gas and common properties as approved by the appropriate regulators. Non-utility property is generally depreciated on a straight-line basis over the life of the associated asset. Refer to Note 5, "Property, Plant and Equipment," for additional information related to depreciation expense at Units 7 and 8 at Bailly Generating Station. For rate-regulated companies, AFUDC is capitalized on all classes of property except organization costs, land, autos, office equipment, tools and other general property purchases. The allowance is applied to construction costs for that period of time between the date of the expenditure and the date on which such project is placed in service. The pre-tax rate for AFUDC was 4.0% in 2017, 4.5% in 2016 and 4.7% in 2015. Generally, NiSource’s subsidiaries follow the practice of charging maintenance and repairs, including the cost of removal of minor items of property, to expense as incurred. When NiSource’s subsidiaries retire regulated property, plant and equipment, original cost plus the cost of retirement, less salvage value, is charged to accumulated depreciation. However, when it becomes probable a regulated asset will be retired substantially in advance of its original expected useful life or is abandoned, the cost of the asset and the corresponding accumulated depreciation is recognized as a separate asset. If the asset is still in operation, the net amount is classified as "Other property, at cost, less accumulated depreciation" on the Consolidated Balance Sheets. If the asset is no longer operating, the net amount is classified in "Regulatory assets" on the Consolidated Balance Sheets. If NiSource is able to recover a full return of and on investment, the carrying value of the asset is based on historical cost. If NiSource is not able to recover a full return on investment, a loss on impairment is recognized to the extent the net book value of the asset exceeds the present value of future revenues discounted at the incremental borrowing rate. When NiSource’s subsidiaries sell entire regulated operating units, or retire or sell nonregulated properties, the original cost and accumulated depreciation and amortization balances are removed from "Property, Plant and Equipment" on the Consolidated Balance Sheets. Any gain or loss is recorded in earnings, unless otherwise required by the applicable regulatory body. Refer to Note 5, "Property, Plant and Equipment," for further information. External and internal costs associated with computer software developed for internal use are capitalized. Capitalization of such costs commences upon the completion of the preliminary stage of each project. Once the installed software is ready for its intended use, such capitalized costs are amortized on a straight-line basis generally over a period of five years, except for certain significant enterprise-wide technology investments which are amortized over a ten-year period.
Goodwill And Other Intangible Assets	Goodwill and Other Intangible Assets. Substantially all of NiSource's goodwill relates to the excess of cost over the fair value of the net assets acquired in the Columbia acquisition on November 1, 2000. NiSource tests its goodwill for impairment annually as of May 1, or more frequently if events and circumstances indicate that goodwill might be impaired. Fair value of NiSource's reporting units is determined using a combination of income and market approaches. NiSource has other intangible assets consisting primarily of franchise rights apart from goodwill that were identified as part of the purchase price allocations associated with the acquisition of Columbia of Massachusetts which is being amortized on a straight-line basis over forty years from the date of acquisition. See Note 6, "Goodwill and Other Intangible Assets," for additional information
Revenue Recognition	Revenue Recognition. Revenue is recorded as products and services are delivered. Utility revenues are billed to customers monthly on a cycle basis. Revenues are recorded on the accrual basis and also include estimates for electricity and gas delivered but not billed. The accruals for unbilled revenues are reversed in the subsequent accounting period when meters are actually read and customers are billed. On occasion, NiSource's regulated subsidiaries are permitted to implement new rates that have not been formally approved by their state regulatory commissions, which are subject to refund. As permitted by accounting principles generally accepted in the United States, each regulated subsidiary recognizes this revenue and establishes a reserve for amounts that could be refunded based on its experience for the jurisdiction in which the rates were implemented. In connection with such revenues, estimated rate refund liabilities are recorded which reflect management’s current judgment of the ultimate outcomes of the proceedings. No provisions are made when, in the opinion of management, the facts and circumstances preclude a reasonable estimate of the outcome.
Accounts Receivable Transfer Program	Accounts Receivable Transfer Program. Certain of NiSource’s subsidiaries have agreements with third parties to sell certain accounts receivable without recourse. These transfers of accounts receivable are accounted for as secured borrowings. The entire gross receivables balance remains on the December 31, 2017 and 2016 Consolidated Balance Sheets and short-term debt is recorded in the amount of proceeds received from the purchasers involved in the transactions. Refer to Note 17, "Transfers of Financial Assets," for further information.
Fuel Adjustment Clause	Gas Cost and Fuel Adjustment Clause. NiSource’s regulated subsidiaries defer most differences between gas and fuel purchase costs and the recovery of such costs in revenues, and adjust future billings for such deferrals on a basis consistent with applicable state-approved tariff provisions. These deferred balances are recorded as "Regulatory assets" or "Regulatory liabilities," as appropriate, on the Consolidated Balance Sheets. Refer to Note 8, "Regulatory Matters," for additional information.
Gas Inventory	Inventory. Both the LIFO inventory methodology and the weighted average cost methodology are used to value natural gas in storage, as approved by regulators for all of NiSource’s regulated subsidiaries. Inventory valued using LIFO was $45.5 million and $46.1 million at December 31, 2017 and 2016, respectively. Based on the average cost of gas using the LIFO method, the estimated replacement cost of gas in storage was less than the stated LIFO cost by $17.4 million and $9.4 million at December 31, 2017 and 2016, respectively. Gas inventory valued using the weighted average cost methodology was $239.6 million at December 31, 2017 and $233.8 million at December 31, 2016. Electric production fuel is valued using the weighted average cost inventory methodology, as approved by NIPSCO's regulator. Materials and supplies are valued using the weighted average cost inventory methodology.
Accounting For Exchange And Balancing Arrangements Of Natural Gas	Accounting for Exchange and Balancing Arrangements of Natural Gas. NiSource’s Gas Distribution Operations segment enters into balancing and exchange arrangements of natural gas as part of its operations and off-system sales programs. NiSource records a receivable or payable for any of its respective cumulative gas imbalances, as well as for any gas inventory borrowed or lent under a Gas Distribution Operations exchange agreement. Exchange gas is valued based on individual regulatory jurisdiction requirements (for example, historical spot rate, spot at the beginning of the month). These receivables and payables are recorded as “Exchange gas receivable” or “Exchange gas payable” on NiSource’s Consolidated Balance Sheets, as appropriate.
Accounting For Risk Management Activities	Accounting for Risk Management Activities. NiSource accounts for its derivatives and hedging activities in accordance with ASC 815. NiSource recognizes all derivatives as either assets or liabilities on the Consolidated Balance Sheets at fair value, unless such contracts are exempted as a normal purchase normal sale under the provisions of the standard. The accounting for changes in the fair value of a derivative depends on the intended use of the derivative and resulting designation. NiSource has elected not to net fair value amounts for any of its derivative instruments or the fair value amounts recognized for its right to receive cash collateral or obligation to pay cash collateral arising from those derivative instruments recognized at fair value, which are executed with the same counterparty under a master netting arrangement. See Note 9, "Risk Management Activities," for additional information.
Income Taxes And Investment Tax Credits	Income Taxes and Investment Tax Credits. NiSource records income taxes to recognize full interperiod tax allocations. Under the asset and liability method, deferred income taxes are provided for the tax consequences of temporary differences by applying enacted statutory tax rates applicable to future years to differences between the financial statement carrying amount and the tax basis of existing assets and liabilities. Previously recorded investment tax credits of the regulated subsidiaries were deferred on the balance sheet and are being amortized to book income over the regulatory life of the related properties to conform to regulatory policy. To the extent certain deferred income taxes of the regulated companies are recoverable or payable through future rates, regulatory assets and liabilities have been established. Regulatory assets for income taxes are primarily attributable to property-related tax timing differences for which deferred taxes had not been provided in the past, when regulators did not recognize such taxes as costs in the ratemaking process. Regulatory liabilities for income taxes are primarily attributable to the regulated companies’ obligation to refund to ratepayers deferred income taxes provided at rates higher than the current Federal income tax rate. Such property-related amounts are credited to ratepayers using either the average rate assumption method or the reverse South Georgia method. Non property-related amounts are credited to ratepayers consistent with state utility commission direction. Pursuant to the U.S. Internal Revenue Code and relevant state taxing authorities, NiSource and its subsidiaries file consolidated income tax returns for Federal and certain state jurisdictions. NiSource and its subsidiaries are parties to an agreement (the “Intercompany Income Tax Allocation Agreement”) that provides for the allocation of consolidated tax liabilities. The Intercompany Income Tax Allocation Agreement generally provides that each party is allocated an amount of tax similar to that which would be owed had the party been separately subject to tax.
Environmental Expenditures	Environmental Expenditures. NiSource accrues for costs associated with environmental remediation obligations when the incurrence of such costs is probable and the amounts can be reasonably estimated, regardless of when the expenditures are actually made. The undiscounted estimated future expenditures are based on currently enacted laws and regulations, existing technology and estimated site-specific costs where assumptions may be made about the nature and extent of site contamination, the extent of cleanup efforts, costs of alternative cleanup methods and other variables. The liability is adjusted as further information is discovered or circumstances change. The reserves for estimated environmental expenditures are recorded on the Consolidated Balance Sheets in “Legal and environmental” for short-term portions of these liabilities and “Other noncurrent liabilities” for the respective long-term portions of these liabilities. Rate-regulated subsidiaries applying regulatory accounting establish regulatory assets on the Consolidated Balance Sheets to the extent that future recovery of environmental remediation costs is probable through the regulatory process. Refer to Note 18, "Other Commitments and Contingencies," for further information.
Excise Taxes	Excise Taxes. NiSource accounts for excise taxes that are customer liabilities by separately stating on its invoices the tax to its customers and recording amounts invoiced as liabilities payable to the applicable taxing jurisdiction. Such balances are presented within "Other accruals" on the Consolidated Balance Sheets. These types of taxes collected from customers, comprised largely of sales taxes, are presented on a net basis affecting neither revenues nor cost of sales. NiSource accounts for excise taxes for which it is liable by recording a liability for the expected tax with a corresponding charge to “Other taxes” expense on the Statements of Consolidated Income.
Accrued Insurance Liabilities	Accrued Insurance Liabilities. NiSource accrues for insurance costs related to workers compensation, automobile, property, general and employment practices liabilities based on the most probable value of each claim. Claim values are determined by professional, licensed loss adjusters who consider the facts of the claim, anticipated indemnification and legal expenses, and respective state rules. Claims are reviewed by NiSource at least quarterly and an adjustment is made to the accrual based on the most current information. NiSource’s actual exposure to liability is minimal due to coverage from its wholly-owned captive insurer who then transfers risk to third party insurance providers for the majority of costs paid to claimants above NiSource's deductible.

Nature Of Operations And Summary Of Significant Accounting Policies (Policy)

12 Months Ended

Dec. 31, 2017

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Company Structure And Principles Of Consolidation

Company Structure and Principles of Consolidation. NiSource, a Delaware corporation headquartered in Merrillville, Indiana, is an energy holding company whose subsidiaries are fully regulated natural gas and electric utility companies serving approximately 3.9 million customers in seven states. NiSource generates substantially all of its operating income through these rate-regulated businesses. The consolidated financial statements include the accounts of NiSource and its majority-owned subsidiaries after the elimination of all intercompany accounts and transactions.

Use Of Estimates

Use of Estimates. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.

Cash, Cash Equivalents, And Restricted Cash

Cash, Cash Equivalents and Restricted Cash. NiSource considers all highly liquid investments with original maturities of three months or less to be cash equivalents. NiSource reports amounts deposited in brokerage accounts for margin requirements as restricted cash. In addition, NiSource has amounts deposited in trust to satisfy requirements for the provision of various property, liability, workers compensation, and long-term disability insurance, which is classified as restricted cash and disclosed as cash and cash equivalents on the Statements of Consolidated Cash Flows.

Accounts Receivable And Unbilled Revenue

Accounts Receivable and Unbilled Revenue. Accounts receivable on the Consolidated Balance Sheets includes both billed and unbilled amounts. Unbilled amounts of accounts receivable relate to a portion of a customer’s consumption of gas or electricity from the date of the last cycle billing date through the last day of the month (balance sheet date). Factors taken into consideration when estimating unbilled revenue include historical usage, customer rates and weather. Accounts receivable fluctuates from year to year depending in large part on weather impacts and price volatility. NiSource's accounts receivable on the Consolidated Balance Sheets include unbilled revenue, less reserves, in the amounts of $359.4 million and $329.7 million as of December 31, 2017 and 2016, respectively. The reserve for uncollectible receivables is NiSource’s best estimate of the amount of probable credit losses in the existing accounts receivable. NiSource determined the reserve based on historical experience and in consideration of current market conditions. Account balances are charged against the allowance when it is anticipated the receivable will not be recovered.

Investments In Debt And Equity Securities

Investments in Debt Securities. NiSource’s investments in debt securities are carried at fair value and are designated as available-for-sale. These investments are included within “Other investments” on the Consolidated Balance Sheets. Unrealized gains and losses, net of deferred income taxes, are reflected as accumulated other comprehensive income. These investments are monitored for other than temporary declines in market value. Realized gains and losses and permanent impairments are reflected in the Statements of Consolidated Income. No material impairment charges were recorded for the years ended December 31, 2017, 2016 or 2015. Refer to Note 16, "Fair Value," for additional information.

Basis Of Accounting For Rate-Regulated Subsidiaries

Basis of Accounting for Rate-Regulated Subsidiaries. Rate-regulated subsidiaries account for and report assets and liabilities consistent with the economic effect of the way in which regulators establish rates, if the rates established are designed to recover the costs of providing the regulated service and it is probable that such rates can be charged and collected. Certain expenses and credits subject to utility regulation or rate determination normally reflected in income are deferred on the Consolidated Balance Sheets and are later recognized in income as the related amounts are included in customer rates and recovered from or refunded to customers.

In the event that regulation significantly changes the opportunity for NiSource to recover its costs in the future, all or a portion of NiSource’s regulated operations may no longer meet the criteria for regulatory accounting. In such an event, a write-down of all or a portion of NiSource’s existing regulatory assets and liabilities could result. If transition cost recovery was approved by the appropriate regulatory bodies that would meet the requirements under GAAP for continued accounting as regulatory assets and liabilities during such recovery period, the regulatory assets and liabilities would be reported at the recoverable amounts. If unable to continue to apply the provisions of regulatory accounting, NiSource would be required to apply the provisions of ASC 980-20, Discontinuation of Rate-Regulated Accounting. In management’s opinion, NiSource’s regulated subsidiaries will be subject to regulatory accounting for the foreseeable future. Refer to Note 8, "Regulatory Matters," for additional information.

Utility Plant And Other Property And Related Depreciation And Maintenance

Plant and Other Property and Related Depreciation and Maintenance. Property, plant and equipment (principally utility plant) is stated at cost. The rate-regulated subsidiaries record depreciation using composite rates on a straight-line basis over the remaining service lives of the electric, gas and common properties as approved by the appropriate regulators.

Non-utility property is generally depreciated on a straight-line basis over the life of the associated asset. Refer to Note 5, "Property, Plant and Equipment," for additional information related to depreciation expense at Units 7 and 8 at Bailly Generating Station.

For rate-regulated companies, AFUDC is capitalized on all classes of property except organization costs, land, autos, office equipment, tools and other general property purchases. The allowance is applied to construction costs for that period of time between the date of the expenditure and the date on which such project is placed in service. The pre-tax rate for AFUDC was 4.0% in 2017, 4.5% in 2016 and 4.7% in 2015.

Generally, NiSource’s subsidiaries follow the practice of charging maintenance and repairs, including the cost of removal of minor items of property, to expense as incurred. When NiSource’s subsidiaries retire regulated property, plant and equipment, original cost plus the cost of retirement, less salvage value, is charged to accumulated depreciation. However, when it becomes probable a regulated asset will be retired substantially in advance of its original expected useful life or is abandoned, the cost of the asset and the corresponding accumulated depreciation is recognized as a separate asset. If the asset is still in operation, the net amount is classified as "Other property, at cost, less accumulated depreciation" on the Consolidated Balance Sheets. If the asset is no longer operating, the net amount is classified in "Regulatory assets" on the Consolidated Balance Sheets. If NiSource is able to recover a full return of and on investment, the carrying value of the asset is based on historical cost. If NiSource is not able to recover a full return on investment, a loss on impairment is recognized to the extent the net book value of the asset exceeds the present value of future revenues discounted at the incremental borrowing rate.

When NiSource’s subsidiaries sell entire regulated operating units, or retire or sell nonregulated properties, the original cost and accumulated depreciation and amortization balances are removed from "Property, Plant and Equipment" on the Consolidated Balance Sheets. Any gain or loss is recorded in earnings, unless otherwise required by the applicable regulatory body. Refer to Note 5, "Property, Plant and Equipment," for further information.

External and internal costs associated with computer software developed for internal use are capitalized. Capitalization of such costs commences upon the completion of the preliminary stage of each project. Once the installed software is ready for its intended use, such capitalized costs are amortized on a straight-line basis generally over a period of five years, except for certain significant enterprise-wide technology investments which are amortized over a ten-year period.

Goodwill And Other Intangible Assets

Goodwill and Other Intangible Assets. Substantially all of NiSource's goodwill relates to the excess of cost over the fair value of the net assets acquired in the Columbia acquisition on November 1, 2000. NiSource tests its goodwill for impairment annually as of May 1, or more frequently if events and circumstances indicate that goodwill might be impaired. Fair value of NiSource's reporting units is determined using a combination of income and market approaches.

NiSource has other intangible assets consisting primarily of franchise rights apart from goodwill that were identified as part of the purchase price allocations associated with the acquisition of Columbia of Massachusetts which is being amortized on a straight-line basis over forty years from the date of acquisition. See Note 6, "Goodwill and Other Intangible Assets," for additional information

Revenue Recognition

Revenue Recognition. Revenue is recorded as products and services are delivered. Utility revenues are billed to customers monthly on a cycle basis. Revenues are recorded on the accrual basis and also include estimates for electricity and gas delivered but not billed. The accruals for unbilled revenues are reversed in the subsequent accounting period when meters are actually read and customers are billed.

On occasion, NiSource's regulated subsidiaries are permitted to implement new rates that have not been formally approved by their state regulatory commissions, which are subject to refund. As permitted by accounting principles generally accepted in the United States, each regulated subsidiary recognizes this revenue and establishes a reserve for amounts that could be refunded based on its experience for the jurisdiction in which the rates were implemented. In connection with such revenues, estimated rate refund liabilities are recorded which reflect management’s current judgment of the ultimate outcomes of the proceedings. No provisions are made when, in the opinion of management, the facts and circumstances preclude a reasonable estimate of the outcome.

Accounts Receivable Transfer Program

Accounts Receivable Transfer Program. Certain of NiSource’s subsidiaries have agreements with third parties to sell certain accounts receivable without recourse. These transfers of accounts receivable are accounted for as secured borrowings. The entire gross receivables balance remains on the December 31, 2017 and 2016 Consolidated Balance Sheets and short-term debt is recorded in the amount of proceeds received from the purchasers involved in the transactions. Refer to Note 17, "Transfers of Financial Assets," for further information.

Fuel Adjustment Clause

Gas Cost and Fuel Adjustment Clause. NiSource’s regulated subsidiaries defer most differences between gas and fuel purchase costs and the recovery of such costs in revenues, and adjust future billings for such deferrals on a basis consistent with applicable state-approved tariff provisions. These deferred balances are recorded as "Regulatory assets" or "Regulatory liabilities," as appropriate, on the Consolidated Balance Sheets. Refer to Note 8, "Regulatory Matters," for additional information.

Gas Inventory

Inventory. Both the LIFO inventory methodology and the weighted average cost methodology are used to value natural gas in storage, as approved by regulators for all of NiSource’s regulated subsidiaries. Inventory valued using LIFO was $45.5 million and $46.1 million at December 31, 2017 and 2016, respectively. Based on the average cost of gas using the LIFO method, the estimated replacement cost of gas in storage was less than the stated LIFO cost by $17.4 million and $9.4 million at December 31, 2017 and 2016, respectively. Gas inventory valued using the weighted average cost methodology was $239.6 million at December 31, 2017 and $233.8 million at December 31, 2016.

Electric production fuel is valued using the weighted average cost inventory methodology, as approved by NIPSCO's regulator.

Materials and supplies are valued using the weighted average cost inventory methodology.

Accounting For Exchange And Balancing Arrangements Of Natural Gas

Accounting for Exchange and Balancing Arrangements of Natural Gas. NiSource’s Gas Distribution Operations segment enters into balancing and exchange arrangements of natural gas as part of its operations and off-system sales programs. NiSource records a receivable or payable for any of its respective cumulative gas imbalances, as well as for any gas inventory borrowed or lent under a Gas Distribution Operations exchange agreement. Exchange gas is valued based on individual regulatory jurisdiction requirements (for example, historical spot rate, spot at the beginning of the month). These receivables and payables are recorded as “Exchange gas receivable” or “Exchange gas payable” on NiSource’s Consolidated Balance Sheets, as appropriate.

Accounting For Risk Management Activities

Accounting for Risk Management Activities. NiSource accounts for its derivatives and hedging activities in accordance with ASC 815. NiSource recognizes all derivatives as either assets or liabilities on the Consolidated Balance Sheets at fair value, unless such contracts are exempted as a normal purchase normal sale under the provisions of the standard. The accounting for changes in the fair value of a derivative depends on the intended use of the derivative and resulting designation.

NiSource has elected not to net fair value amounts for any of its derivative instruments or the fair value amounts recognized for its right to receive cash collateral or obligation to pay cash collateral arising from those derivative instruments recognized at fair value, which are executed with the same counterparty under a master netting arrangement. See Note 9, "Risk Management Activities," for additional information.

Income Taxes And Investment Tax Credits

Income Taxes and Investment Tax Credits. NiSource records income taxes to recognize full interperiod tax allocations. Under the asset and liability method, deferred income taxes are provided for the tax consequences of temporary differences by applying enacted statutory tax rates applicable to future years to differences between the financial statement carrying amount and the tax basis of existing assets and liabilities. Previously recorded investment tax credits of the regulated subsidiaries were deferred on the balance sheet and are being amortized to book income over the regulatory life of the related properties to conform to regulatory policy.

To the extent certain deferred income taxes of the regulated companies are recoverable or payable through future rates, regulatory assets and liabilities have been established. Regulatory assets for income taxes are primarily attributable to property-related tax timing differences for which deferred taxes had not been provided in the past, when regulators did not recognize such taxes as costs in the ratemaking process. Regulatory liabilities for income taxes are primarily attributable to the regulated companies’ obligation to refund to ratepayers deferred income taxes provided at rates higher than the current Federal income tax rate. Such property-related amounts are credited to ratepayers using either the average rate assumption method or the reverse South Georgia method. Non property-related amounts are credited to ratepayers consistent with state utility commission direction.

Pursuant to the U.S. Internal Revenue Code and relevant state taxing authorities, NiSource and its subsidiaries file consolidated income tax returns for Federal and certain state jurisdictions. NiSource and its subsidiaries are parties to an agreement (the “Intercompany Income Tax Allocation Agreement”) that provides for the allocation of consolidated tax liabilities. The Intercompany Income Tax Allocation Agreement generally provides that each party is allocated an amount of tax similar to that which would be owed had the party been separately subject to tax.

Environmental Expenditures

Environmental Expenditures. NiSource accrues for costs associated with environmental remediation obligations when the incurrence of such costs is probable and the amounts can be reasonably estimated, regardless of when the expenditures are actually made. The undiscounted estimated future expenditures are based on currently enacted laws and regulations, existing technology and estimated site-specific costs where assumptions may be made about the nature and extent of site contamination, the extent of cleanup efforts, costs of alternative cleanup methods and other variables. The liability is adjusted as further information is discovered or circumstances change. The reserves for estimated environmental expenditures are recorded on the Consolidated Balance Sheets in “Legal and environmental” for short-term portions of these liabilities and “Other noncurrent liabilities” for the respective long-term portions of these liabilities. Rate-regulated subsidiaries applying regulatory accounting establish regulatory assets on the Consolidated Balance Sheets to the extent that future recovery of environmental remediation costs is probable through the regulatory process. Refer to Note 18, "Other Commitments and Contingencies," for further information.

Excise Taxes

Excise Taxes. NiSource accounts for excise taxes that are customer liabilities by separately stating on its invoices the tax to its customers and recording amounts invoiced as liabilities payable to the applicable taxing jurisdiction. Such balances are presented within "Other accruals" on the Consolidated Balance Sheets. These types of taxes collected from customers, comprised largely of sales taxes, are presented on a net basis affecting neither revenues nor cost of sales. NiSource accounts for excise taxes for which it is liable by recording a liability for the expected tax with a corresponding charge to “Other taxes” expense on the Statements of Consolidated Income.

Accrued Insurance Liabilities

Accrued Insurance Liabilities. NiSource accrues for insurance costs related to workers compensation, automobile, property, general and employment practices liabilities based on the most probable value of each claim. Claim values are determined by professional, licensed loss adjusters who consider the facts of the claim, anticipated indemnification and legal expenses, and respective state rules. Claims are reviewed by NiSource at least quarterly and an adjustment is made to the accrual based on the most current information. NiSource’s actual exposure to liability is minimal due to coverage from its wholly-owned captive insurer who then transfers risk to third party insurance providers for the majority of costs paid to claimants above NiSource's deductible.