Stock Repurchase (Details) - USD ($) $ in Millions	3 Months Ended	88 Months Ended
	Sep. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Jan. 18, 2016	May 15, 2015	Dec. 11, 2007
Equity [Abstract]
Stock repurchase program, number of shares authorized to be repurchased				15,000,000.0	20,000,000	10,000,000.0
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased					11,400,000	2,500,000
Repurchase of common stock	8,600,000	7,500,000
Value of shares repurchased	$ 363.8	$ 234.7
Treasury stock, number of shares held			22,500,000