Income Taxes	3 Months Ended
Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	14. Income Taxes The effective tax rate was 21.5 percent and 32.0 percent for the three months ended March 31, 2018 and 2017, respectively. The change in tax rates primarily reflects the reduction in the U.S. federal statutory tax rate as a result of the Tax Act. The tax rates in both periods also reflect the resolution of certain prior years’ tax items. In 2017, we recorded an estimated net discrete tax charge of $2.6 billion related to the Tax Act that was accounted for as a provisional charge under SAB 118. There were no material changes to these estimates for the current period. We continue to analyze the impacts of the Tax Act; therefore, the 2017 charge continues to be provisional. We are under continuous examination by the Internal Revenue Service (IRS) and tax authorities in other countries and states in which we have significant business operations. The tax years under examination and open for examination vary by jurisdiction. The resolution of such matters did not have a material impact on our effective tax rate. We are currently under examination with the IRS for tax years 2008 through 2014. We believe it is reasonably possible that our unrecognized tax benefits could decrease within the next 12 months by as much as $319 million, principally as a result of potential resolutions of prior years’ tax items with various taxing authorities. The prior years’ tax items include unrecognized tax benefits relating to the deductibility of certain expenses or losses and the attribution of taxable income to a particular jurisdiction or jurisdictions. Of the $319 million of unrecognized tax benefits, approximately $291 million relates to amounts that, if recognized, would impact the effective tax rate in a future period.

14. Income Taxes

The effective tax rate was 21.5 percent and 32.0 percent for the three months ended March 31, 2018 and 2017, respectively. The change in tax rates primarily reflects the reduction in the U.S. federal statutory tax rate as a result of the Tax Act. The tax rates in both periods also reflect the resolution of certain prior years’ tax items. In 2017, we recorded an estimated net discrete tax charge of $2.6 billion related to the Tax Act that was accounted for as a provisional charge under SAB 118. There were no material changes to these estimates for the current period. We continue to analyze the impacts of the Tax Act; therefore, the 2017 charge continues to be provisional.

We are under continuous examination by the Internal Revenue Service (IRS) and tax authorities in other countries and states in which we have significant business operations. The tax years under examination and open for examination vary by jurisdiction. The resolution of such matters did not have a material impact on our effective tax rate. We are currently under examination with the IRS for tax years 2008 through 2014.

We believe it is reasonably possible that our unrecognized tax benefits could decrease within the next 12 months by as much as $319 million, principally as a result of potential resolutions of prior years’ tax items with various taxing authorities. The prior years’ tax items include unrecognized tax benefits relating to the deductibility of certain expenses or losses and the attribution of taxable income to a particular jurisdiction or jurisdictions. Of the $319 million of unrecognized tax benefits, approximately $291 million relates to amounts that, if recognized, would impact the effective tax rate in a future period.