Shareholders' Equity - Shares Repurchased (Details) (USD $)	3 Months Ended					12 Months Ended		1 Months Ended	4 Months Ended	28 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	May 07, 2015	May 07, 2015	May 07, 2015	Aug. 06, 2014	May 08, 2015	May 06, 2015
Equity, Class of Treasury Stock [Line Items]
Repurchases of common stock	$ (152,000,000)	$ (35,000,000)
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Shares repurchased	5,860,291	1,350,443	6,388,187	9,623,309	7,051,842	24,413,781	12,512,759
Repurchases of common stock	(152,000,000)	(35,000,000)	(152,000,000)	(226,000,000)	(177,000,000)	(590,000,000)	(264,000,000)
Average price paid (in dollars per share)	$ 25.87	$ 25.92	$ 23.83	$ 23.47	$ 25.14	$ 24.17	$ 21.12
Authorized repurchase amount											400,000,000
Subsequent Event [Member] | Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Shares repurchased								2,058,927	7,919,218	44,845,758
Repurchases of common stock								(56,000,000)	(208,000,000)	(1,062,000,000)
Average price paid (in dollars per share)								$ 27.36	$ 26.26	$ 23.69
Remaining capacity of shares repurchase program												2,000,000
Authorized repurchase amount													$ 400,000,000