Earnings Per Share (Details) - USD ($)	3 Months Ended		6 Months Ended		18 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015
Earnings Per Share [Abstract]
Stock Repurchase Program, Authorized Amount		$ 250,000,000		$ 250,000,000
Stock Repurchased During Period, Shares			1,200,000.0		3,100,000.0
Treasury Stock, Value, Acquired, Cost Method			$ 68,000,000		$ 192,500,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 57,500,000		57,500,000		$ 57,500,000
Numerator:
Net income	$ 45,400,000	$ 53,100,000	$ 107,600,000	$ 95,200,000
Denominator:
Denominator for basic earnings per share — weighted average shares	43,500,000	45,500,000	43,800,000	45,700,000
Effect of dilutive securities:
Equity compensation plans	700,000	800,000	700,000	800,000
Denominator for diluted earnings per share — adjusted weighted average and assumed conversion	44,200,000	46,300,000	44,500,000	46,500,000
Basic earnings per share (in dollars per share)	$ 1.04	$ 1.17	$ 2.46	$ 2.08
Diluted earnings per share (in dollars per share)	$ 1.03	$ 1.15	$ 2.42	$ 2.05