Significant Accounting Policies	3 Months Ended
Mar. 31, 2018
Accounting Policies [Abstract]
Significant Accounting Policies	NOTE 2: SIGNIFICANT ACCOUNTING POLICIES New Accounting Pronouncements Not Yet Adopted In June 2016, the Financial Accounting Standard Board (“FASB”) issued new accounting guidance on the measurement of credit losses for financial assets measured at amortized cost, which includes accounts receivable, and available-for-sale debt securities. For financial assets measured at amortized cost, this new guidance requires an entity to: (1) estimate its lifetime expected credit losses upon recognition of the financial assets and establish an allowance to present the net amount expected to be collected; (2) recognize this allowance and changes in the allowance during subsequent periods through net income; and (3) consider relevant information about past events, current conditions and reasonable and supportable forecasts in assessing the lifetime expected credit losses. For available-for-sale debt securities, this new guidance made several targeted amendments to the existing other-than-temporary impairment model, including: (1) requiring disclosure of the allowance for credit losses; (2) allowing reversals of the previously recognized credit losses until the entity has the intent to sell, is more-likely-than-not required to sell the securities or the maturity of the securities; (3) limiting impairment to the difference between the amortized cost basis and fair value; and (4) not allowing entities to consider the length of time that fair value has been less than amortized cost as a factor in evaluating whether a credit loss exists. This guidance is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted, including interim periods within those fiscal years beginning after December 15, 2018. We are currently considering our timing of adoption and in the process of evaluating the impact of adopting this guidance on our consolidated financial statements and related disclosures. In February 2016, the FASB issued new guidance related to accounting for leases. The new standard requires the recognition of assets (right-of-use-assets) and liabilities arising from lease transactions on the balance sheet and the disclosure of key information about leasing arrangements. Accordingly, a lessee will recognize a lease asset for its right to use the underlying asset and a lease liability for the corresponding lease obligation. Both the asset and liability will initially be measured at the present value of the future minimum lease payments over the lease term. The new guidance will classify leases as either finance or operating leases, with classification determining the presentation of expenses and cash flows on our consolidated financial statements. Initial costs directly attributable to negotiating and arranging the lease will be included in the asset. For leases with a term of 12 months or less, a lessee can make an accounting policy election by class of underlying asset to not recognize an asset and corresponding liability. The transition guidance also provides specific guidance for sale and leaseback transactions, build-to-suit leases and amounts previously recognized in accordance with the business combinations guidance for leases. We will also be required to provide additional qualitative and quantitative disclosures regarding the amount, timing and uncertainty of cash flows arising from leases which include, among other things, the computation and disclosure of our weighted average remaining lease term and discount rate, cash paid for amounts included in the measurement of lease liabilities, and supplemental non-cash information on lease liabilities arising from obtaining the right-of-use assets. These disclosures are intended to provide supplemental information to the amounts recorded in the financial statements so that users can better understand the nature of an entity’s leasing activities. This guidance is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018, with early adoption permitted, which will require the recognition and measurement of leases at the beginning of the earliest comparative period presented in the financial statements using a modified retrospective approach. We anticipate adopting this new guidance in the first quarter of 2019. We continue to evaluate the new lease guidance and are in the process of updating accounting policies, accounting position memos, and evaluating our existing population of contracts to ensure all contracts that meet the definition of a lease contract under the new standard are identified. We are also in the process of implementing additional lease software to support our accounting and reporting process, including the new quantitative and qualitative financial disclosure requirements. In addition, we are evaluating the impact of the system implementation and new accounting guidance on our internal controls. We will continue to provide updates on our assessment of the effect that this new lease guidance will have on our consolidated financial statements, disclosures, systems and related internal controls, and will disclose material effects, if any, when known. Recently Adopted Accounting Pronouncements In May 2017, the FASB issued new accounting guidance that clarifies when changes to the terms or conditions of a share-based payment award must be accounted for as modifications which will reduce diversity in practice. Under the new guidance, an entity will not apply modification accounting to a share-based payment award if the award’s fair value (or calculated value or intrinsic value, if those measurement methods are used), the award’s vesting conditions, and the award’s classification as an equity or liability instrument are the same immediately before and after the change. The guidance also states that an entity is not required to estimate the value of the award immediately before and after the change if the change does not affect any of the inputs to the model used to value the award. We adopted this guidance prospectively in the first quarter of 2018. We believe the new guidance will likely result in fewer changes to the terms of an award being accounted for as modifications. In January 2017, the FASB issued new accounting guidance to clarify the definition of a business and provide additional guidance to assist entities with evaluating whether transactions should be accounted for as asset acquisitions (or asset disposals) or business combinations (or disposals of a business). Under this new guidance, an entity first determines whether substantially all of the fair value of the assets acquired is concentrated in a single identifiable asset or a group of similar identifiable assets. If this criterion is met, the transaction should be accounted for as an asset acquisition as opposed to a business combination. This distinction is important because the accounting for an asset acquisition significantly differs from the accounting for a business combination. This new guidance eliminates the requirement to evaluate whether a market participant could replace missing elements (e.g. inputs or processes), narrows the definition of outputs and requires that a business include, at a minimum, an input and a substantive process that together significantly contribute to the ability to create outputs. We adopted this guidance in the first quarter of 2018 and it will be applied prospectively to any transactions occurring within and after the adoption date. In November 2016, the FASB issued new accounting guidance on the classification and presentation of restricted cash in the statement of cash flows to address the diversity in practice. This new guidance requires entities to show changes in cash, cash equivalents and restricted cash on a combined basis in the statement of cash flows. In addition, this accounting guidance requires a reconciliation of the total cash, cash equivalent and restricted cash in the statement of cash flows to the related captions in the balance sheet if cash, cash equivalents and restricted cash are presented in more than one line item in the balance sheet. We adopted this guidance in the first quarter of 2018 and applied it retrospectively to all prior periods presented in the financial statements as required under the new guidance. The adoption did not have a material impact on our consolidated financial statements and related disclosures. In October 2016, the FASB issued new accounting guidance on income tax accounting associated with intra-entity transfers of assets other than inventory. This accounting update, which is part of the FASB's simplification initiative, is intended to reduce diversity in practice and the complexity of tax accounting, particularly for those transfers involving intellectual property. This new guidance requires an entity to recognize the income tax consequences of an intra-entity transfer of an asset other than inventory when the transfer occurs. We adopted this new guidance in the first quarter of 2018 on a modified retrospective basis. Accordingly, we recognized the cumulative effect of initial application of this new guidance as an adjustment to the opening balance of retained earnings, which was not material to our consolidated financial statements. In August 2016, the FASB issued new accounting guidance which clarifies how companies present and classify certain cash receipts and cash payments in the statement of cash flows. The new guidance specifically addresses the following cash flow topics in an effort to reduce diversity in practice: (1) debt prepayment or debt extinguishment costs; (2) settlement of zero-coupon bonds; (3) contingent consideration payments made after a business combination; (4) proceeds from the settlement of insurance claims; (5) proceeds from the settlement of corporate-owned life insurance policies, including bank-owned life insurance policies; (6) distributions received from equity method investees; (7) beneficial interests in securitization transactions; and (8) separately identifiable cash flows and application of the predominance principle. We adopted this new guidance in the first quarter of 2018 retrospectively and the adoption did not have an impact on our consolidated financial statements and related disclosures. In January 2016, the FASB issued new accounting guidance which amends the standard on the recognition and measurement of financial instruments. The FASB clarified certain aspects of this guidance by issuing an update for technical corrections and improvements related to this guidance in February 2018. The guidance (1) requires an entity to measure equity investments (except those accounted for under the equity method or those that result in consolidation of the investee) at fair value with changes in fair value recognized in net income rather than accumulated other comprehensive income on the balance sheet; (2) allows an entity to elect to measure the equity investments that do not have a readily determinable fair value using a new measurement alternative which measure these equity investments at cost less impairment, if any, plus or minus changes resulting from observable price changes in orderly transactions for the identical or a similar investment of the same issuer; (3) simplifies the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment to identify impairment; and (4) clarifies that an entity should evaluate the need for a valuation allowance on a deferred tax asset related to available-for-sale securities in combination with the entity’s evaluation of their other deferred tax assets. We adopted this guidance in the first quarter of 2018 and elected to prospectively account for our investments in equity securities of privately-held companies that do not have a readily determinable fair value using the measurement alternative. The adoption did not have a material impact on our consolidated financial statements and related disclosures. In May 2014, the FASB issued new accounting guidance on revenue from contracts with customers, or ASC 606, Revenue from Contracts with Customers (“ASC 606”), which replaced numerous requirements in GAAP, and provides companies with a single model for recognizing revenue from contracts with customers. The core principle of the new standard is that a company should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. This guidance also requires additional disclosures about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. In addition, the FASB has also issued several amendments to the standard, which clarifies certain aspects of the guidance, including principal versus agent considerations and identifying performance obligations. In the first quarter of 2018, we adopted ASC 606 under the modified retrospective method for all contracts that were not completed as of January 1, 2018. Results for reporting periods beginning after January 1, 2018 are presented under the new revenue guidance, while prior period amounts are not adjusted and continue to be reported in accordance with our previous accounting policies under the historical revenue guidance, or ASC 605, Revenue Recognition. We evaluated each of our revenue streams and applied ASC 606 as further discussed in “Note 3: Revenue Recognition.” As a result of adoption of the new revenue guidance, certain revenue streams, such as the instant booking revenue recorded under the consumption model which we previously recorded upon completion of the traveler stay, is now recognized upon booking. The amount of the recognized transaction price is recorded as revenue net of the impact of estimated cancellations. We also recorded an adjustment to capitalize certain costs to obtain contracts for existing arrangements as of the implementation date. We expect the adoption of this new revenue standard will not have a material impact, either on an annual or quarterly basis, to our consolidated financial statements on an ongoing basis. Our systems and internal controls were not significantly impacted as a result of the accounting changes and we have made the necessary changes to our accounting policies and internal processes to support the new revenue recognition standard, including the related disclosures. We recognized the cumulative effect of initial application of ASC 606 as an adjustment to the opening balance of retained earnings. We recorded a net increase in opening retained earnings of $4 million as of January 1, 2018 due to the cumulative impact of adoption of the new revenue guidance and all other accounts were not materially impacted. There have been no changes to our significant accounting policies since December 31, 2017, other than noted above. See “Note 3: Revenue Recognition” below for further discussion about our revenue recognition policies under ASC 606. For additional information about our significant accounting policies and estimates, refer to “Note 2: Significant Accounting Policies”, in the notes to consolidated financial statements in Item 8 of our Annual Report on Form 10-K for the year ended December 31, 2017.

NOTE 2: SIGNIFICANT ACCOUNTING POLICIES

New Accounting Pronouncements Not Yet Adopted

In June 2016, the Financial Accounting Standard Board (“FASB”) issued new accounting guidance on the measurement of credit losses for financial assets measured at amortized cost, which includes accounts receivable, and available-for-sale debt securities. For financial assets measured at amortized cost, this new guidance requires an entity to: (1) estimate its lifetime expected credit losses upon recognition of the financial assets and establish an allowance to present the net amount expected to be collected; (2) recognize this allowance and changes in the allowance during subsequent periods through net income; and (3) consider relevant information about past events, current conditions and reasonable and supportable forecasts in assessing the lifetime expected credit losses. For available-for-sale debt securities, this new guidance made several targeted amendments to the existing other-than-temporary impairment model, including: (1) requiring disclosure of the allowance for credit losses; (2) allowing reversals of the previously recognized credit losses until the entity has the intent to sell, is more-likely-than-not required to sell the securities or the maturity of the securities; (3) limiting impairment to the difference between the amortized cost basis and fair value; and (4) not allowing entities to consider the length of time that fair value has been less than amortized cost as a factor in evaluating whether a credit loss exists. This guidance is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted, including interim periods within those fiscal years beginning after December 15, 2018. We are currently considering our timing of adoption and in the process of evaluating the impact of adopting this guidance on our consolidated financial statements and related disclosures.

In February 2016, the FASB issued new guidance related to accounting for leases. The new standard requires the recognition of assets (right-of-use-assets) and liabilities arising from lease transactions on the balance sheet and the disclosure of key information about leasing arrangements. Accordingly, a lessee will recognize a lease asset for its right to use the underlying asset and a lease liability for the corresponding lease obligation. Both the asset and liability will initially be measured at the present value of the future minimum lease payments over the lease term. The new guidance will classify leases as either finance or operating leases, with classification determining the presentation of expenses and cash flows on our consolidated financial statements. Initial costs directly attributable to negotiating and arranging the lease will be included in the asset. For leases with a term of 12 months or less, a lessee can make an accounting policy election by class of underlying asset to not recognize an asset and corresponding liability. The transition guidance also provides specific guidance for sale and leaseback transactions, build-to-suit leases and amounts previously recognized in accordance with the business combinations guidance for leases. We will also be required to provide additional qualitative and quantitative disclosures regarding the amount, timing and uncertainty of cash flows arising from leases which include, among other things, the computation and disclosure of our weighted average remaining lease term and discount rate, cash paid for amounts included in the measurement of lease liabilities, and supplemental non-cash information on lease liabilities arising from obtaining the right-of-use assets. These disclosures are intended to provide supplemental information to the amounts recorded in the financial statements so that users can better understand the nature of an entity’s leasing activities. This guidance is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018, with early adoption permitted, which will require the recognition and measurement of leases at the beginning of the earliest comparative period presented in the financial statements using a modified retrospective approach. We anticipate adopting this new guidance in the first quarter of 2019.

We continue to evaluate the new lease guidance and are in the process of updating accounting policies, accounting position memos, and evaluating our existing population of contracts to ensure all contracts that meet the definition of a lease contract under the new standard are identified. We are also in the process of implementing additional lease software to support our accounting and reporting process, including the new quantitative and qualitative financial disclosure requirements. In addition, we are evaluating the impact of the system implementation and new accounting guidance on our internal controls. We will continue to provide updates on our assessment of the effect that this new lease guidance will have on our consolidated financial statements, disclosures, systems and related internal controls, and will disclose material effects, if any, when known.

Recently Adopted Accounting Pronouncements

In May 2017, the FASB issued new accounting guidance that clarifies when changes to the terms or conditions of a share-based payment award must be accounted for as modifications which will reduce diversity in practice. Under the new guidance, an entity will not apply modification accounting to a share-based payment award if the award’s fair value (or calculated value or intrinsic value, if those measurement methods are used), the award’s vesting conditions, and the award’s classification as an equity or liability instrument are the same immediately before and after the change. The guidance also states that an entity is not required to estimate the value of the award immediately before and after the change if the change does not affect any of the inputs to the model used to value the award. We adopted this guidance prospectively in the first quarter of 2018. We believe the new guidance will likely result in fewer changes to the terms of an award being accounted for as modifications.

In January 2017, the FASB issued new accounting guidance to clarify the definition of a business and provide additional guidance to assist entities with evaluating whether transactions should be accounted for as asset acquisitions (or asset disposals) or business combinations (or disposals of a business). Under this new guidance, an entity first determines whether substantially all of the fair value of the assets acquired is concentrated in a single identifiable asset or a group of similar identifiable assets. If this criterion is met, the transaction should be accounted for as an asset acquisition as opposed to a business combination. This distinction is important because the accounting for an asset acquisition significantly differs from the accounting for a business combination. This new guidance eliminates the requirement to evaluate whether a market participant could replace missing elements (e.g. inputs or processes), narrows the definition of outputs and requires that a business include, at a minimum, an input and a substantive process that together significantly contribute to the ability to create outputs. We adopted this guidance in the first quarter of 2018 and it will be applied prospectively to any transactions occurring within and after the adoption date.

In November 2016, the FASB issued new accounting guidance on the classification and presentation of restricted cash in the statement of cash flows to address the diversity in practice. This new guidance requires entities to show changes in cash, cash equivalents and restricted cash on a combined basis in the statement of cash flows. In addition, this accounting guidance requires a reconciliation of the total cash, cash equivalent and restricted cash in the statement of cash flows to the related captions in the balance sheet if cash, cash equivalents and restricted cash are presented in more than one line item in the balance sheet. We adopted this guidance in the first quarter of 2018 and applied it retrospectively to all prior periods presented in the financial statements as required under the new guidance. The adoption did not have a material impact on our consolidated financial statements and related disclosures.

In October 2016, the FASB issued new accounting guidance on income tax accounting associated with intra-entity transfers of assets other than inventory. This accounting update, which is part of the FASB's simplification initiative, is intended to reduce diversity in practice and the complexity of tax accounting, particularly for those transfers involving intellectual property. This new guidance requires an entity to recognize the income tax consequences of an intra-entity transfer of an asset other than inventory when the transfer occurs. We adopted this new guidance in the first quarter of 2018 on a modified retrospective basis. Accordingly, we recognized the cumulative effect of initial application of this new guidance as an adjustment to the opening balance of retained earnings, which was not material to our consolidated financial statements.

In August 2016, the FASB issued new accounting guidance which clarifies how companies present and classify certain cash receipts and cash payments in the statement of cash flows. The new guidance specifically addresses the following cash flow topics in an effort to reduce diversity in practice: (1) debt prepayment or debt extinguishment costs; (2) settlement of zero-coupon bonds; (3) contingent consideration payments made after a business combination; (4) proceeds from the settlement of insurance claims; (5) proceeds from the settlement of corporate-owned life insurance policies, including bank-owned life insurance policies; (6) distributions received from equity method investees; (7) beneficial interests in securitization transactions; and (8) separately identifiable cash flows and application of the predominance principle. We adopted this new guidance in the first quarter of 2018 retrospectively and the adoption did not have an impact on our consolidated financial statements and related disclosures.

In January 2016, the FASB issued new accounting guidance which amends the standard on the recognition and measurement of financial instruments. The FASB clarified certain aspects of this guidance by issuing an update for technical corrections and improvements related to this guidance in February 2018. The guidance (1) requires an entity to measure equity investments (except those accounted for under the equity method or those that result in consolidation of the investee) at fair value with changes in fair value recognized in net income rather than accumulated other comprehensive income on the balance sheet; (2) allows an entity to elect to measure the equity investments that do not have a readily determinable fair value using a new measurement alternative which measure these equity investments at cost less impairment, if any, plus or minus changes resulting from observable price changes in orderly transactions for the identical or a similar investment of the same issuer; (3) simplifies the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment to identify impairment; and (4) clarifies that an entity should evaluate the need for a valuation allowance on a deferred tax asset related to available-for-sale securities in combination with the entity’s evaluation of their other deferred tax assets. We adopted this guidance in the first quarter of 2018 and elected to prospectively account for our investments in equity securities of privately-held companies that do not have a readily determinable fair value using the measurement alternative. The adoption did not have a material impact on our consolidated financial statements and related disclosures.

In May 2014, the FASB issued new accounting guidance on revenue from contracts with customers, or ASC 606, Revenue from Contracts with Customers (“ASC 606”), which replaced numerous requirements in GAAP, and provides companies with a single model for recognizing revenue from contracts with customers. The core principle of the new standard is that a company should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. This guidance also requires additional disclosures about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. In addition, the FASB has also issued several amendments to the standard, which clarifies certain aspects of the guidance, including principal versus agent considerations and identifying performance obligations.

In the first quarter of 2018, we adopted ASC 606 under the modified retrospective method for all contracts that were not completed as of January 1, 2018. Results for reporting periods beginning after January 1, 2018 are presented under the new revenue guidance, while prior period amounts are not adjusted and continue to be reported in accordance with our previous accounting policies under the historical revenue guidance, or ASC 605, Revenue Recognition.

We evaluated each of our revenue streams and applied ASC 606 as further discussed in “Note 3: Revenue Recognition.” As a result of adoption of the new revenue guidance, certain revenue streams, such as the instant booking revenue recorded under the consumption model which we previously recorded upon completion of the traveler stay, is now recognized upon booking. The amount of the recognized transaction price is recorded as revenue net of the impact of estimated cancellations. We also recorded an adjustment to capitalize certain costs to obtain contracts for existing arrangements as of the implementation date. We expect the adoption of this new revenue standard will not have a material impact, either on an annual or quarterly basis, to our consolidated financial statements on an ongoing basis. Our systems and internal controls were not significantly impacted as a result of the accounting changes and we have made the necessary changes to our accounting policies and internal processes to support the new revenue recognition standard, including the related disclosures.

We recognized the cumulative effect of initial application of ASC 606 as an adjustment to the opening balance of retained earnings. We recorded a net increase in opening retained earnings of $4 million as of January 1, 2018 due to the cumulative impact of adoption of the new revenue guidance and all other accounts were not materially impacted.

There have been no changes to our significant accounting policies since December 31, 2017, other than noted above. See “Note 3: Revenue Recognition” below for further discussion about our revenue recognition policies under ASC 606. For additional information about our significant accounting policies and estimates, refer to “Note 2: Significant Accounting Policies”, in the notes to consolidated financial statements in Item 8 of our Annual Report on Form 10-K for the year ended December 31, 2017.