Summary of Significant Accounting Policies - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accounting Policies [Abstract]
Net income	$ 188	$ 175	$ 616	$ 563
Weighted average common shares outstanding – basic	310.8	305.4	310.6	305.3
Add: Incremental shares attributable to effect of potentially dilutive securities	1.1	1.4	1.1	1.4
Adjusted weighted average common shares outstanding – diluted	311.9	306.8	311.7	306.7
Net income per common share-basic (dollars per share)	$ 0.60	$ 0.57	$ 1.98	$ 1.84
Net income per common share-diluted (dollars per share)	$ 0.60	$ 0.57	$ 1.98	$ 1.84