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, 2017	Mar. 31, 2017	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016
Accounting Policies [Abstract]
Net income	$ 175	$ 388	$ 232	$ 310	$ 563	$ 542
Weighted average common shares outstanding – basic	305.4		299.1		305.3	296.7
Add: Incremental shares attributable to effect of potentially dilutive securities	1.4		1.3		1.4	1.3
Adjusted weighted average common shares outstanding – diluted	306.8		300.4		306.7	298.0
Net income per common share-basic (dollars per share)	$ 0.57		$ 0.78		$ 1.84	$ 1.83
Net income per common share-diluted (dollars per share)	$ 0.57		$ 0.77		$ 1.84	$ 1.82