Income Taxes
 3 Months Ended
Mar. 31, 2018
Income Tax Disclosure [Abstract]  
Income Taxes
Income Taxes
The combined federal, state and foreign income tax rate was 9 percent and 29 percent for the three months ended March 31, 2018 and 2017, respectively. The effective tax rate for the three months ended March 31, 2018 was less than the U.S. statutory rate of 21 percent primarily due to certain permanent tax differences related to net income attributable to noncontrolling interest, state and local deferred tax benefits primarily resulting from the February 1 dropdown and GP/IDR Exchange and equity compensation. The effective tax rate for the three months ended March 31, 2017 was less than the U.S. statutory rate of 35 percent primarily due to certain permanent tax differences related to equity compensation, net income attributable to noncontrolling interest and the domestic manufacturing deduction offset by state and local tax expense.
We are continuously undergoing examination of our income tax returns, which have been completed through the 2007 tax year for state returns and the 2009 tax year for our U.S. federal return. As of March 31, 2018, we had $19 million of unrecognized tax benefits.
Prior to its spin-off on June 30, 2011, Marathon Petroleum Corporation was included in the Marathon Oil Corporation (“Marathon Oil”) federal income tax returns for all applicable years. During the third quarter 2017, Marathon Oil received a notice of Final Partnership Administrative Adjustment (“FPAA”) from the IRS for taxable year 2010, relating to certain pre-spinoff transactions. Marathon Oil filed a U.S. Tax Court petition disputing these adjustments during the fourth quarter of 2017. We received an FPAA for taxable years 2011-2014 for items resulting from this matter and filed a U.S. Tax Court petition for tax years 2011-2014 to dispute these corollary adjustments in the fourth quarter of 2017. We continue to believe that the issue in dispute is more likely than not to be fully sustained and therefore, no liability has been accrued for this matter.
Pursuant to our tax sharing agreement with Marathon Oil, the unrecognized tax benefits related to pre-spinoff operations for which Marathon Oil was the taxpayer remain the responsibility of Marathon Oil and we have indemnified Marathon Oil accordingly. See Note 22 for indemnification information.