Summary of Significant Accounting Policies - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Net income	$ 1,245	$ 1,193	$ 1,092
Weighted average common shares outstanding – basic	300.4	293.0	292.9
Add: Incremental shares attributable to effect of potentially dilutive securities	1.5	1.4	1.1
Adjusted weighted average common shares outstanding – diluted	301.9	294.4	294.0
Net income per common share — basic (dollars per share)	$ 4.15	$ 4.07	$ 3.73
Net income per common share — diluted (dollars per share)	$ 4.12	$ 4.05	$ 3.71