Earnings Per Share Earnings Per Share - Schedule of Earnings per Share (Details) - USD ($) shares in Millions, $ 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
Earnings Per Share [Abstract]
Net income attributable to common shareholders	$ 1,871	$ 824	$ 80	$ 995	$ 204	$ 490	$ 267	$ 173	$ 3,770	$ 1,134	$ 2,269
Weighted average common shares outstanding — basic	964	962	934	928	925	925	924	923	947	924	890
Assumed exercise and/or distributions of stock-based awards									2	3	3
Weighted average common shares outstanding — diluted	967	965	936	930	928	927	926	925	949	927	893