Earnings Per Share (Details) - USD ($)	3 Months Ended		9 Months Ended		21 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Jun. 30, 2014
Earnings Per Share [Abstract]
Stock Repurchase Program, Authorized Amount						$ 250,000,000
Stock Repurchased During Period, Shares			2,100,000.0	1,900,000.0	4,000,000.0
Treasury Stock, Value, Acquired, Cost Method			$ 111,000,000	$ 124,600,000	$ 235,600,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 14,400,000		14,400,000		$ 14,400,000
Numerator:
Net income	$ 39,500,000	$ 51,300,000	$ 147,100,000	$ 146,500,000
Denominator:
Denominator for basic earnings per share — weighted average shares	42,800,000	44,400,000	43,500,000	45,300,000
Effect of dilutive securities:
Equity compensation plans	600,000	800,000	600,000	800,000
Denominator for diluted earnings per share — adjusted weighted average and assumed conversion	43,400,000	45,200,000	44,100,000	46,100,000
Basic earnings per share (in dollars per share)	$ 0.92	$ 1.16	$ 3.38	$ 3.24
Diluted earnings per share (in dollars per share)	$ 0.91	$ 1.14	$ 3.33	$ 3.18