Stockholders' Equity - Changes in Equity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 1,097	$ 1,377	$ 1,180	$ 1,413
Total comprehensive income	84	112	76	143
Preferred stock dividends	0	(2)	0	(5)
Common stock dividends	(10)	(9)	(18)	(16)
Distributions to noncontrolling interests	(5)	(6)	(6)	(7)
Share conversion			0	3
Common stock share repurchases	(63)	(49)	(126)	(113)
Derecognition of noncontrolling interests			(5)	0
Stock compensation	6	4	10	10
Stock withheld for employee taxes	0	(1)	(2)	(2)
Ending Balance	1,109	1,426	1,109	1,426
Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	991	1,272	1,080	1,309
Total comprehensive income	$ 82	107	$ 62	136
Preferred stock dividends		(2)		(5)
Common stock dividends	$ (10)	(9)	$ (18)	(16)
Share conversion				3
Common stock share repurchases	(63)	(49)	$ (126)	(113)
Stock compensation	$ 6	4	10	10
Stock withheld for employee taxes		(1)	(2)	(2)
Ending Balance	$ 1,006	1,322	1,006	1,322
Attributable to Non-Controlling Interests [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	106	105	100	104
Total comprehensive income	2	5	14	7
Distributions to noncontrolling interests	(5)	(6)	(6)	(7)
Derecognition of noncontrolling interests			(5)
Ending Balance	$ 103	$ 104	$ 103	$ 104