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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accounting Policies [Abstract]
Net income	$ 435	$ 457	$ 1,051	$ 1,020
Weighted average common shares outstanding – basic	311.1	307.8	310.8	306.2
Add: Incremental shares attributable to effect of potentially dilutive securities	1.2	1.5	1.1	1.5
Adjusted weighted average common shares outstanding – diluted	312.3	309.3	311.9	307.7
Net income per common share - basic (dollars per share)	$ 1.40	$ 1.48	$ 3.38	$ 3.33
Net income per common share - diluted (dollars per share)	$ 1.39	$ 1.48	$ 3.37	$ 3.31