Income Taxes	9 Months Ended
Sep. 30, 2017
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 nine months ended September 30, 2017 was 33.5%, compared to 32.5% for the comparable prior year period. The increase in our effective tax rate primarily reflected the impact of favorable discrete items of $7 million and $11 million for the nine months ended September 30, 2017 and 2016, respectively, for excess tax benefits from stock-based payments; the year over year decrease in the tax benefits resulted in an increase to the effective tax rate compared to the prior year.

Income Taxes

9 Months Ended

Sep. 30, 2017

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 nine months ended September 30, 2017 was 33.5%, compared to 32.5% for the comparable prior year period. The increase in our effective tax rate primarily reflected the impact of favorable discrete items of $7 million and $11 million for the nine months ended September 30, 2017 and 2016, respectively, for excess tax benefits from stock-based payments; the year over year decrease in the tax benefits resulted in an increase to the effective tax rate compared to the prior year.