Income Taxes	3 Months Ended
Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes At the end of each interim period, we estimate our annual effective tax rate and apply that rate to our interim earnings. We also record the tax impact of certain unusual or infrequently occurring items, including changes in judgment about valuation allowances and the effects of changes in tax laws or rates, in the interim period in which they occur. The computation of the annual estimated effective tax rate at each interim period requires certain estimates and significant judgment including, but not limited to, the expected operating income for the year, projections of the proportion of income earned and taxed in state and foreign jurisdictions, permanent and temporary differences between book and taxable income, and the likelihood of recovering deferred tax assets generated in the current year. The accounting estimates used to compute the provision for income taxes may change as new events occur, additional information is obtained or as the tax environment changes. Our effective income tax rate for the three months ended March 31, 2018 was 24.7%, compared to 33.9% for the comparable prior year period. The decrease was primarily attributable to the reduction of the U.S. federal statutory income tax rate from 35% to 21% as a result of the enactment of the Tax Act in December 2017. The effective tax rate also reflects the impact of favorable discrete items of approximately $3 million for each of the three months ended March 31, 2018 and 2017 for excess tax benefits from stock-based payments. The quarter over quarter change in these amounts increased the effective tax rate by 0.2% compared to the prior year. The Tax Act introduced broad and complex changes to the U.S. tax code, including the aforementioned reduction in the U.S. corporate tax rate, a one-time transition tax on the historical unremitted earnings of foreign subsidiaries, and a new minimum tax on foreign earnings (Global Intangible Low-Taxed Income, “GILTI”). On December 22, 2017, the SEC staff issued Staff Accounting Bulletin 118 ("SAB 118"), which provides guidance on accounting for the tax effects of the Tax Act. SAB 118 provides a measurement period that should not extend beyond one year from the Tax Act enactment date for companies to complete the related accounting for provisional amounts under ASC 740, "Accounting for Income Taxes." As a result of the Tax Act, in 2017, we recognized a provisional tax liability of $51 million related to the one-time transition tax on historical foreign earnings, payable over a period of eight years. We also recorded a provisional decrease to net U.S. deferred tax liabilities of $73 million. For a description of the impact of the Tax Act for the year ended December 31, 2017, refer to Note 13, "Income Taxes" of our financial statements as of and for the year ended December 31, 2017 included in the 2017 Form 10-K. During the three-month period ended March 31, 2018, there were no changes made to the provisional amounts recognized in 2017. We continue to gather the information necessary to finalize those provisional amounts. Our estimates could be affected as we gain a more thorough understanding of the Tax Act from additional guidance issued by the U.S. tax authorities. Changes to the provisional estimates of the tax effect of the Tax Act will be recorded as a discrete item in the interim period the amounts are considered complete. The Company has included the estimated 2018 impact of the GILTI Tax as a period cost and included it as part of the estimated annual effective tax rate. The 2018 estimated annual effective tax rate also includes the impact of all other U.S. tax reform provisions that were effective on January 1, 2018. These estimates are subject to change as additional guidance on the tax reform provisions is issued.

Income Taxes

At the end of each interim period, we estimate our annual effective tax rate and apply that rate to our interim earnings. We also record the tax impact of certain unusual or infrequently occurring items, including changes in judgment about valuation allowances and the effects of changes in tax laws or rates, in the interim period in which they occur.

The computation of the annual estimated effective tax rate at each interim period requires certain estimates and significant judgment including, but not limited to, the expected operating income for the year, projections of the proportion of income earned and taxed in state and foreign jurisdictions, permanent and temporary differences between book and taxable income, and the likelihood of recovering deferred tax assets generated in the current year. The accounting estimates used to compute the provision for income taxes may change as new events occur, additional information is obtained or as the tax environment changes.

Our effective income tax rate for the three months ended March 31, 2018 was 24.7%, compared to 33.9% for the comparable prior year period. The decrease was primarily attributable to the reduction of the U.S. federal statutory income tax rate from 35% to 21% as a result of the enactment of the Tax Act in December 2017. The effective tax rate also reflects the impact of favorable discrete items of approximately $3 million for each of the three months ended March 31, 2018 and 2017 for excess tax benefits from stock-based payments. The quarter over quarter change in these amounts increased the effective tax rate by 0.2% compared to the prior year.

The Tax Act introduced broad and complex changes to the U.S. tax code, including the aforementioned reduction in the U.S. corporate tax rate, a one-time transition tax on the historical unremitted earnings of foreign subsidiaries, and a new minimum tax on foreign earnings (Global Intangible Low-Taxed Income, “GILTI”). On December 22, 2017, the SEC staff issued Staff Accounting Bulletin 118 ("SAB 118"), which provides guidance on accounting for the tax effects of the Tax Act. SAB 118 provides a measurement period that should not extend beyond one year from the Tax Act enactment date for companies to complete the related accounting for provisional amounts under ASC 740, "Accounting for Income Taxes."

As a result of the Tax Act, in 2017, we recognized a provisional tax liability of $51 million related to the one-time transition tax on historical foreign earnings, payable over a period of eight years. We also recorded a provisional decrease to net U.S. deferred tax liabilities of $73 million. For a description of the impact of the Tax Act for the year ended December 31, 2017, refer to Note 13, "Income Taxes" of our financial statements as of and for the year ended December 31, 2017 included in the 2017 Form 10-K. During the three-month period ended March 31, 2018, there were no changes made to the provisional amounts recognized in 2017. We continue to gather the information necessary to finalize those provisional amounts. Our estimates could be affected as we gain a more thorough understanding of the Tax Act from additional guidance issued by the U.S. tax authorities. Changes to the provisional estimates of the tax effect of the Tax Act will be recorded as a discrete item in the interim period the amounts are considered complete.

The Company has included the estimated 2018 impact of the GILTI Tax as a period cost and included it as part of the estimated annual effective tax rate. The 2018 estimated annual effective tax rate also includes the impact of all other U.S. tax reform provisions that were effective on January 1, 2018. These estimates are subject to change as additional guidance on the tax reform provisions is issued.