Summary of Significant Accounting Policies - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Sep. 30, 2016
Accounting Policies [Abstract]
Net income	$ 457	$ 175	$ 388	$ 497	$ 232	$ 310	$ 1,020	$ 1,039
Weighted average common shares outstanding – basic	307.8			304.5			306.2	299.1
Add: Incremental shares attributable to effect of potentially dilutive securities	1.5			1.4			1.5	1.4
Adjusted weighted average common shares outstanding – diluted	309.3			305.9			307.7	300.5
Net income per common share-basic (dollars per share)	$ 1.48			$ 1.63			$ 3.33	$ 3.47
Net income per common share-diluted (dollars per share)	$ 1.48			$ 1.62			$ 3.31	$ 3.46