Income Taxes	3 Months Ended
Mar. 31, 2016
Income Tax Disclosure [Abstract]
Income taxes	4. 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, 2016, the estimated annual effective tax rate for 2016 is about 30 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.

Income Taxes

3 Months Ended

Mar. 31, 2016

4. 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, 2016, the estimated annual effective tax rate for 2016 is about 30 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.