Summary of Significant Accounting Policies - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Net income	$ 1,525	$ 1,245	$ 1,193
Weighted average common shares outstanding – basic	307.1	300.4	293.0
Add: Incremental shares attributable to effect of potentially dilutive securities	1.7	1.5	1.4
Adjusted weighted average common shares outstanding – diluted	308.8	301.9	294.4
Net income per common share — basic (dollars per share)	$ 4.97	$ 4.15	$ 4.07
Net income per common share — diluted (dollars per share)	$ 4.94	$ 4.12	$ 4.05