Shareholders' Equity - Shares Repurchased (Details) - USD ($)	1 Months Ended	3 Months Ended					4 Months Ended	12 Months Ended			40 Months Ended
	May. 04, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	May. 04, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	May. 04, 2016	Feb. 24, 2016
Equity, Class of Treasury Stock [Line Items]
Repurchases of common stock		$ 75,000,000				$ 152,000,000
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Shares repurchased		3,038,928	5,035,637	5,362,103	4,737,388	5,860,291		20,995,419	24,413,781	12,512,759
Repurchases of common stock		$ 75,000,000	$ 135,000,000	$ 135,000,000	$ 133,000,000	$ 152,000,000		$ 555,000,000	$ 590,000,000	$ 264,000,000
Average price paid (in dollars per share)		$ 24.69	$ 26.81	$ 25.17	$ 28.13	$ 25.87		$ 26.43	$ 24.17	$ 21.12
Authorized repurchase amount		$ 230,000,000										$ 250,000,000
Subsequent Event [Member] | Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Shares repurchased	793,672						3,832,600				61,754,559
Repurchases of common stock	$ 20,000,000						$ 95,000,000				$ 1,504,000,000
Average price paid (in dollars per share)	$ 25.20						$ 24.80				$ 24.36
Remaining capacity of shares repurchase program	$ 210,000,000						$ 210,000,000				$ 210,000,000