Description of Business and Accounting Policies - Calculation of Diluted Shares (Details) - shares shares in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Shares used in computation of basic earnings per share	483	481	479	477	476	474	473	471	480	474	467
Total dilutive effect of outstanding stock awards									13	10	10
Shares used in computation of diluted earnings per share	496	494	492	490	486	485	483	481	493	484	477