Stockholders' Equity - Changes in Equity (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 1,377	$ 1,948	$ 1,413	$ 1,948
Total comprehensive income	112	36	143	79
Preferred stock dividends	(2)	(7)	(5)	(15)
Common stock dividends	(9)	(7)	(16)	(15)
Distributions to noncontrolling interests	(6)	(4)	(7)	(5)
Share conversion			3	0
Common stock share repurchases	(49)	(62)	(113)	(86)
Adjustments to paid-in capital for purchase of noncontrolling interests			0	6
Adjustments to other comprehensive income for purchase of noncontrolling interests			0	(3)
Purchase of noncontrolling interests			0	(9)
Stock compensation	4	9	10	15
Stock withheld for employee taxes	(1)	(2)	(2)	(4)
Ending Balance	1,426	1,911	1,426	1,911
Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	1,272	1,840	1,309	1,836
Total comprehensive income	107	35	136	72
Preferred stock dividends	(2)	(7)	(5)	(15)
Common stock dividends	(9)	(7)	(16)	(15)
Share conversion			3
Common stock share repurchases	(49)	(62)	(113)	(86)
Adjustments to paid-in capital for purchase of noncontrolling interests				6
Adjustments to other comprehensive income for purchase of noncontrolling interests				(3)
Stock compensation	4	9	10	15
Stock withheld for employee taxes	(1)	(2)	(2)	(4)
Ending Balance	1,322	1,806	1,322	1,806
Attributable to Non-Controlling Interests [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	105	108	104	112
Total comprehensive income	5	1	7	7
Distributions to noncontrolling interests	(6)	(4)	(7)	(5)
Purchase of noncontrolling interests				(9)
Ending Balance	$ 104	$ 105	$ 104	$ 105