Background and Basis of Presentation (Policies)	9 Months Ended
Sep. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying condensed consolidated balance sheet as of September 30, 2019, the condensed consolidated statements of operations, the condensed consolidated statements of comprehensive loss, and the condensed consolidated statements of stockholder's equity for the three and nine months ended September 30, 2019 and 2018, the condensed consolidated statements of cash flows for the nine months ended September 30, 2019 and 2018 and the related footnote disclosures are unaudited. In the opinion of management, the accompanying unaudited condensed consolidated financial statements contain all adjustments, including normal recurring adjustments, necessary to present fairly our financial position as of September 30, 2019, our results of operations, results of comprehensive loss, and stockholder's equity for the three and nine months ended September 30, 2019 and 2018 and cash flows for the nine months ended September 30, 2019 and 2018. Our results of operations, stockholder's equity, and cash flows for the nine months ended September 30, 2019 are not necessarily indicative of the results to be expected for the full year. We operate in a single segment. Our fiscal year ends on December 31 and in this report we refer to the year ended December 31, 2018 as 2018. The condensed consolidated financial statements and related financial information should be read in conjunction with the audited consolidated financial statements and the related notes thereto that are included in our Annual Report on Form 10-K for the year ended December 31, 2018 (the Annual Report on Form 10-K) filed with the U.S. Securities and Exchange Commission (SEC). We have changed the captions on our condensed consolidated statements of cash flows from “purchases of marketable securities” to “purchases of investments” and from “maturities of marketable securities” to “maturities of investments.” This change does not impact any current or previously reported results. Except for our policies on leases and convertible senior notes, there have been no material changes to our significant accounting policies as compared to the significant accounting policies described in our Annual Report on Form 10-K.
Leases	Leases We determine if an arrangement is a lease at inception. Operating leases are included in operating lease right of use (ROU) assets, operating lease liabilities within current liabilities, and operating lease liabilities within long-term liabilities on our condensed consolidated balance sheet. Operating lease ROU assets and operating lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term at commencement date. Our leases do not provide an implicit rate and therefore we use our incremental borrowing rate based on the information available at commencement date in determining the present value of future minimum lease payments. Our incremental borrowing rate is estimated based on the estimated rate incurred to borrow, on a collateralized basis over a similar term as our leases, an amount equal to the lease payments in a similar economic environment. Our lease terms may include options to extend or terminate the lease when it is reasonably certain that we will exercise such options. We do not record leases on our condensed consolidated balance sheet with a term of one year or less. We do not separate lease and non-lease components but rather account for each separate component as a single lease component for all underlying classes of assets. Some of our leases include payments that are dependent on an index, such as the Consumer Price Index (CPI), and our minimum lease payments include payments based on the index at inception with any future changes in such indices recognized as an expense in the period of change. Where leases contain escalation clauses, rent abatements, or concessions, such as rent holidays and landlord or tenant incentives or allowances, we apply them in the determination of straight-line operating lease cost over the lease term.
Convertible Senior Notes, net	Convertible Senior Notes, net In March 2019, we issued $700 million in aggregate principal amount of 0.125% convertible senior notes due in 2025 (2025 notes) and in April 2019, the initial purchasers fully exercised their option to purchase $100 million of additional 2025 notes for aggregate total gross proceeds of $800 million. In April 2018, we issued $345 million in aggregate principal amount of 0.25% convertible senior notes due in 2023 (2023 notes). Collectively, the 2025 notes and the 2023 notes are referred to as the “notes.” In accounting for their issuance, we separated the notes into liability and equity components. The carrying amount of the liability component was calculated by measuring the fair value of similar liabilities that do not have an associated convertible feature. The carrying amount of the equity component representing the conversion option was determined by deducting the carrying amount of the liability component from the par value of the notes. The difference represents the debt discount, recorded as a reduction of the convertible senior notes on our condensed consolidated balance sheets, and is amortized to interest expense over the term of the notes using the effective interest rate method. The equity component is not remeasured as long as it continues to meet the conditions for equity classification. In accounting for the issuance costs related to the notes, we allocated the total amount of issuance costs incurred to liability and equity components based on their relative values. Issuance costs attributable to the liability component are being amortized on a straight-line basis, which approximates the effective interest rate method, to interest expense over the term of the notes. The issuance costs attributable to the equity component are recorded as a reduction of the equity component within additional paid-in capital.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with generally accepted accounting principles in the United States (U.S. GAAP) requires management to make estimates, judgments, and assumptions that affect the reported amounts of assets and liabilities; the disclosure of contingent liabilities at the date of the financial statements; and the reported amounts of revenues and expenses during the reporting periods. Significant estimates, assumptions, and judgments are used for, but not limited to: revenue recognition, recoverability of accounts receivable, restructuring charges, share-based compensation expense including estimated forfeitures, accounting for income taxes, useful lives assigned to long-lived assets for depreciation and amortization, impairment of goodwill and long-lived assets, the valuation of acquired intangible assets, the valuation of our convertible senior notes, and operating lease ROU assets and operating lease liabilities. We base our estimates on historical experience, knowledge of current business conditions, and various other factors we believe to be reasonable under the circumstances. These estimates are based on management’s knowledge about current events and expectations about actions we may undertake in the future. Actual results could differ from these estimates, and such differences could be material to our financial position and results of operations.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Issued Accounting Pronouncements Not Yet Adopted In May 2019, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2019-05, Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief. ASU 2019-05 provides entities with an option to irrevocably elect the fair value option for eligible instruments. In April 2019, the FASB issued ASU 2019-04, Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments. ASU 2019-04 provides codification updates to ASU 2016-01 and ASU 2016-13. In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. ASU 2016-13 replaces the existing incurred loss impairment model for trade receivables with an expected loss model which requires the use of forward-looking information to calculate expected credit loss estimates. Additionally, the concept of other-than-temporary impairment for available-for-sale investments is eliminated and instead ASU 2016-13 requires an entity to focus on determining whether any impairment is a result of a credit loss or other factors. It also requires credit losses related to available-for-sale debt securities to be recorded through an allowance for credit losses rather than as a reduction to the amortized cost basis. These changes may result in earlier recognition of credit losses. Early adoption is permitted and these guidance updates require a modified retrospective method of adoption, though a prospective method of adoption is required for available-for-sale debt securities for which an other-than-temporary impairment had been recognized before the effective date. These guidance updates are effective for annual periods beginning after December 15, 2019. We plan to adopt this guidance on January 1, 2020 and we are currently in the process of evaluating the impact of these guidance updates. In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with existing guidance contained within subtopic 350-40 to develop or obtain internal-use software. Early adoption is permitted and the guidance allows for a retrospective or prospective application. The guidance is effective for annual periods beginning after December 15, 2019. We plan to adopt this guidance on January 1, 2020 under the prospective application and we are currently in the process of evaluating the impact of this guidance. In July 2018, the FASB issued ASU 2018-09, Codification Improvements. ASU 2018-09 provides updates for technical corrections, clarifications, and other minor improvements to a wide variety of topics in the ASC. The transition method of adoption is dependent on the ASC topic impacted by this guidance. Additionally, some of the ASC topic updates are effective upon issuance of ASU 2018-09 and some of the ASC topic updates are effective at a future date. The ASC topic updates effective upon issuance of ASU 2018-09 do not impact our accounting for the respective ASC topics. For those ASC topic updates effective at a future date, we are currently in the process of evaluating the impact of this guidance update. Recently Adopted Accounting Pronouncements The FASB has issued four ASUs related to Accounting Standards Codification (ASC) 842. In March 2019, the FASB issued ASU 2019-01, Leases (Topic 842): Codification Improvements. In July 2018, the FASB issued ASU 2018-11, Leases (Topic 842): Targeted Improvements and ASU 2018-10, Codification Improvements to Topic 842, Leases. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). ASC 842 requires an entity to recognize a ROU asset and lease liability for all leases with terms of more than 12 months. Recognition, measurement, and presentation of expenses will depend on classification as a finance or operating lease. The amendments in this update also require certain quantitative and qualitative disclosures about leasing arrangements. We have adopted ASC 842 on January 1, 2019 and have elected the transition method of adoption that allows for a modified retrospective adoption with a cumulative-effect adjustment to the opening balance of accumulated deficit and recorded an immaterial decrease to our opening balance of accumulated deficit. As a result, we have not changed previously disclosed amounts or provided additional disclosures for comparative periods. We initially recorded ROU assets of $17.2 million and lease liabilities of $21.1 million on our condensed consolidated balance sheet. ASC 842 does not have a material impact to our condensed consolidated statements of operations. We have elected a package of transition practical expedients which include not reassessing whether any expired or existing contracts are or contain leases, not reassessing the lease classification of expired or existing leases, and not reassessing initial direct costs for existing leases. We have also elected a practical expedient to not separate lease and non-lease components. We did not elect the practical expedient to use hindsight in determining our lease terms or assessing impairment of our ROU assets. See Note 8. Leases for more information.
Revenue Recognition	Revenue Recognition Revenues are recognized when control of the promised goods or services is transferred to our customers, in an amount that reflects the consideration we expect to be entitled to in exchange for those goods or services. The majority of our revenues are recognized over time as services are performed, with certain revenues, most significantly the revenue share we earn from our print textbook partners, being recognized at the point in time when print textbooks are shipped to students.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is computed by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period. Diluted net loss per share is computed by giving effect to all potential shares of common stock, including stock options, restricted stock units (RSUs), performance-based restricted stock units (PSUs), and shares related to convertible senior notes, to the extent dilutive. Basic and diluted net loss per share was the same for each period presented as the inclusion of all potential common shares outstanding would have been anti-dilutive.

Background and Basis of Presentation (Policies)

9 Months Ended

Sep. 30, 2019

Organization, Consolidation and Presentation of Financial Statements [Abstract]

The accompanying condensed consolidated balance sheet as of September 30, 2019, the condensed consolidated statements of operations, the condensed consolidated statements of comprehensive loss, and the condensed consolidated statements of stockholder's equity for the three and nine months ended September 30, 2019 and 2018, the condensed consolidated statements of cash flows for the nine months ended September 30, 2019 and 2018 and the related footnote disclosures are unaudited. In the opinion of management, the accompanying unaudited condensed consolidated financial statements contain all adjustments, including normal recurring adjustments, necessary to present fairly our financial position as of September 30, 2019, our results of operations, results of comprehensive loss, and stockholder's equity for the three and nine months ended September 30, 2019 and 2018 and cash flows for the nine months ended September 30, 2019 and 2018. Our results of operations, stockholder's equity, and cash flows for the nine months ended September 30, 2019 are not necessarily indicative of the results to be expected for the full year.

We operate in a single segment. Our fiscal year ends on December 31 and in this report we refer to the year ended December 31, 2018 as 2018.

The condensed consolidated financial statements and related financial information should be read in conjunction with the audited consolidated financial statements and the related notes thereto that are included in our Annual Report on Form 10-K for the year ended December 31, 2018 (the Annual Report on Form 10-K) filed with the U.S. Securities and Exchange Commission (SEC).

We have changed the captions on our condensed consolidated statements of cash flows from “purchases of marketable securities” to “purchases of investments” and from “maturities of marketable securities” to “maturities of investments.” This change does not impact any current or previously reported results.

Except for our policies on leases and convertible senior notes, there have been no material changes to our significant accounting policies as compared to the significant accounting policies described in our Annual Report on Form 10-K.

Leases

We determine if an arrangement is a lease at inception. Operating leases are included in operating lease right of use (ROU) assets, operating lease liabilities within current liabilities, and operating lease liabilities within long-term liabilities on our condensed consolidated balance sheet. Operating lease ROU assets and operating lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term at commencement date. Our leases do not provide an implicit rate and therefore we use our incremental borrowing rate based on the information available at commencement date in determining the present value of future minimum lease payments. Our incremental borrowing rate is estimated based on the estimated rate incurred to borrow, on a collateralized basis over a similar term as our leases, an amount equal to the lease payments in a similar economic environment. Our lease terms may include options to extend or terminate the lease when it is reasonably certain that we will exercise such options. We do not record leases on our condensed consolidated balance sheet with a term of one year or less. We do not separate lease and non-lease components but rather account for each separate component as a single lease component for all underlying classes of assets. Some of our leases include payments that are dependent on an index, such as the Consumer Price Index (CPI), and our minimum lease payments include payments based on the index at inception with any future changes in such indices recognized as an expense in the period of change. Where leases contain escalation clauses, rent abatements, or concessions, such as rent holidays and landlord or tenant incentives or allowances, we apply them in the determination of straight-line operating lease cost over the lease term.

Convertible Senior Notes, net

In March 2019, we issued $700 million in aggregate principal amount of 0.125% convertible senior notes due in 2025 (2025 notes) and in April 2019, the initial purchasers fully exercised their option to purchase $100 million of additional 2025 notes for aggregate total gross proceeds of $800 million. In April 2018, we issued $345 million in aggregate principal amount of 0.25% convertible senior notes due in 2023 (2023 notes). Collectively, the 2025 notes and the 2023 notes are referred to as the “notes.” In accounting for their issuance, we separated the notes into liability and equity components. The carrying amount of the liability component was calculated by measuring the fair value of similar liabilities that do not have an associated convertible feature. The carrying amount of the equity component representing the conversion option was determined by deducting the carrying amount of the liability component from the par value of the notes. The difference represents the debt discount, recorded as a reduction of the convertible senior notes on our condensed consolidated balance sheets, and is amortized to interest expense over the term of the notes using the effective interest rate method. The equity component is not remeasured as long as it continues to meet the conditions for equity classification. In accounting for the issuance costs related to the notes, we allocated the total amount of issuance costs incurred to liability and equity components based on their relative values. Issuance costs attributable to the liability component are being amortized on a straight-line basis, which approximates the effective interest rate method, to interest expense over the term of the notes. The issuance costs attributable to the equity component are recorded as a reduction of the equity component within additional paid-in capital.

Use of Estimates

The preparation of financial statements in conformity with generally accepted accounting principles in the United States (U.S. GAAP) requires management to make estimates, judgments, and assumptions that affect the reported amounts of assets and liabilities; the disclosure of contingent liabilities at the date of the financial statements; and the reported amounts of revenues and expenses during the reporting periods. Significant estimates, assumptions, and judgments are used for, but not limited to: revenue recognition, recoverability of accounts receivable, restructuring charges, share-based compensation expense including estimated forfeitures, accounting for income taxes, useful lives assigned to long-lived assets for depreciation and amortization, impairment of goodwill and long-lived assets, the valuation of acquired intangible assets, the valuation of our convertible senior notes, and operating lease ROU assets and operating lease liabilities. We base our estimates on historical experience, knowledge of current business conditions, and various other factors we believe to be reasonable under the circumstances. These estimates are based on management’s knowledge about current events and expectations about actions we may undertake in the future. Actual results could differ from these estimates, and such differences could be material to our financial position and results of operations.

Recent Accounting Pronouncements

Recently Issued Accounting Pronouncements Not Yet Adopted

In May 2019, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2019-05, Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief. ASU 2019-05 provides entities with an option to irrevocably elect the fair value option for eligible instruments. In April 2019, the FASB issued ASU 2019-04, Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments. ASU 2019-04 provides codification updates to ASU 2016-01 and ASU 2016-13. In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. ASU 2016-13 replaces the existing incurred loss impairment model for trade receivables with an expected loss model which requires the use of forward-looking information to calculate expected credit loss estimates. Additionally, the concept of other-than-temporary impairment for available-for-sale investments is eliminated and instead ASU 2016-13 requires an entity to focus on determining whether any impairment is a result of a credit loss or other factors. It also requires credit losses related to available-for-sale debt securities to be recorded through an allowance for credit losses rather than as a reduction to the amortized cost basis. These changes may result in earlier recognition of credit losses. Early adoption is permitted and these guidance updates require a modified retrospective method of adoption, though a prospective method of adoption is required for available-for-sale debt securities for which an other-than-temporary impairment had been recognized before the effective date. These guidance updates are effective for annual periods beginning after December 15, 2019. We plan to adopt this guidance on January 1, 2020 and we are currently in the process of evaluating the impact of these guidance updates.

In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is

a service contract with existing guidance contained within subtopic 350-40 to develop or obtain internal-use software. Early adoption is permitted and the guidance allows for a retrospective or prospective application. The guidance is effective for annual periods beginning after December 15, 2019. We plan to adopt this guidance on January 1, 2020 under the prospective application and we are currently in the process of evaluating the impact of this guidance.

In July 2018, the FASB issued ASU 2018-09, Codification Improvements. ASU 2018-09 provides updates for technical corrections, clarifications, and other minor improvements to a wide variety of topics in the ASC. The transition method of adoption is dependent on the ASC topic impacted by this guidance. Additionally, some of the ASC topic updates are effective upon issuance of ASU 2018-09 and some of the ASC topic updates are effective at a future date. The ASC topic updates effective upon issuance of ASU 2018-09 do not impact our accounting for the respective ASC topics. For those ASC topic updates effective at a future date, we are currently in the process of evaluating the impact of this guidance update.

Recently Adopted Accounting Pronouncements

The FASB has issued four ASUs related to Accounting Standards Codification (ASC) 842. In March 2019, the FASB issued ASU 2019-01, Leases (Topic 842): Codification Improvements. In July 2018, the FASB issued ASU 2018-11, Leases (Topic 842): Targeted Improvements and ASU 2018-10, Codification Improvements to Topic 842, Leases. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). ASC 842 requires an entity to recognize a ROU asset and lease liability for all leases with terms of more than 12 months. Recognition, measurement, and presentation of expenses will depend on classification as a finance or operating lease. The amendments in this update also require certain quantitative and qualitative disclosures about leasing arrangements.

We have adopted ASC 842 on January 1, 2019 and have elected the transition method of adoption that allows for a modified retrospective adoption with a cumulative-effect adjustment to the opening balance of accumulated deficit and recorded an immaterial decrease to our opening balance of accumulated deficit. As a result, we have not changed previously disclosed amounts or provided additional disclosures for comparative periods. We initially recorded ROU assets of $17.2 million and lease liabilities of $21.1 million on our condensed consolidated balance sheet. ASC 842 does not have a material impact to our condensed consolidated statements of operations. We have elected a package of transition practical expedients which include not reassessing whether any expired or existing contracts are or contain leases, not reassessing the lease classification of expired or existing leases, and not reassessing initial direct costs for existing leases. We have also elected a practical expedient to not separate lease and non-lease components. We did not elect the practical expedient to use hindsight in determining our lease terms or assessing impairment of our ROU assets. See Note 8. Leases for more information.

Revenue Recognition

Revenues are recognized when control of the promised goods or services is transferred to our customers, in an amount that reflects the consideration we expect to be entitled to in exchange for those goods or services. The majority of our revenues are recognized over time as services are performed, with certain revenues, most significantly the revenue share we earn from our print textbook partners, being recognized at the point in time when print textbooks are shipped to students.

Net Loss Per Share

Basic net loss per share is computed by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period. Diluted net loss per share is computed by giving effect to all potential shares of common stock, including stock options, restricted stock units (RSUs), performance-based restricted stock units (PSUs), and shares related to convertible senior notes, to the extent dilutive. Basic and diluted net loss per share was the same for each period presented as the inclusion of all potential common shares outstanding would have been anti-dilutive.