Income taxes

6 Months Ended

Jun. 30, 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 June 30, 2013, the estimated annual effective tax rate for 2013 is about 24 percent, which differs from the 35 percent statutory corporate tax rate 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. These lower tax rates are generally statutory in nature, without expiration and available to companies that operate in those taxing jurisdictions. The first quarter 2013 tax provision included a $65 million discrete tax benefit from the reinstatement of the federal research tax credit retroactive to the beginning of 2012.