Summary of Significant Accounting Policies - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accounting Policies [Abstract]
Net income	$ 428	$ 388
Weighted average common shares outstanding – basic	310.4	305.1
Add: Incremental shares attributable to effect of potentially dilutive securities	1.2	1.2
Adjusted weighted average common shares outstanding – diluted	311.6	306.3
Net income per common share-basic (dollars per share)	$ 1.38	$ 1.27
Net income per common share-diluted (dollars per share)	$ 1.37	$ 1.27