Summary of Significant Accounting Policies - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accounting Policies [Abstract]
Net income	$ 388	$ 310
Weighted average common shares outstanding – basic	305.1	293.7
Add: Incremental shares attributable to effect of potentially dilutive securities	1.2	1.1
Adjusted weighted average common shares outstanding – diluted	306.3	294.8
Net income per common share-basic (dollars per share)	$ 1.27	$ 1.05
Net income per common share-diluted (dollars per share)	$ 1.27	$ 1.05