Income taxes

3 Months Ended

Mar. 31, 2013

Income Tax Disclosure [Abstract]

Income taxes

Income taxes

Federal income taxes for the interim periods presented have been included in the accompanying financial statements on the basis of an estimated annual effective tax rate. As of March 31, 2013, the estimated annual effective tax rate for 2013 is about 22 percent, which differs from the 35 percent statutory corporate tax rate primarily due to lower statutory tax rates applicable to our operations in many of the jurisdictions in which we operate and from U.S. tax benefits. The first-quarter 2013 tax provision includes a $65 million discrete tax benefit from the reinstatement of the federal research tax credit retroactive to the beginning of 2012.