Stockholders' Equity - Changes in Equity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 884	$ 1,097	$ 831	$ 1,180
Total comprehensive income	25	84	115	76
Common stock dividends	(9)	(10)	(18)	(18)
Distributions to noncontrolling interests	(13)	(5)	(14)	(6)
Common stock share repurchases	(53)	(63)	(81)	(126)
Derecognition of noncontrolling interests			0	(5)
Stock compensation	4	6	6	10
Stock withheld for employee taxes			(1)	(2)
Ending Balance	838	1,109	838	1,109
Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	778	991	728	1,080
Total comprehensive income	23	82	109	62
Common stock dividends	(9)	(10)	(18)	(18)
Common stock share repurchases	(53)	(63)	(81)	(126)
Stock compensation	4	6	6	10
Stock withheld for employee taxes			(1)	(2)
Ending Balance	743	1,006	743	1,006
Attributable to Non-Controlling Interests [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	106	106	103	100
Total comprehensive income	2	2	6	14
Distributions to noncontrolling interests	(13)	(5)	(14)	(6)
Derecognition of noncontrolling interests				(5)
Ending Balance	$ 95	$ 103	$ 95	$ 103