Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock	Retained Earnings	Accumulated Other Comprehensive Loss
Shares outstanding, beginning (in shares) at Dec. 31, 2015		49,800,000
Shareholders' equity, beginning at Dec. 31, 2015	$ 617.2	$ 134.0	$ 483.8	$ (0.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	108.1		108.1
Issuance of common stock (in shares)		300,000
Issuance of common stock	$ 2.6	$ 2.6
Repurchase of common stock (in shares)	(635,370)	(800,000)
Repurchase of common stock	$ (39.0)	$ (39.0)
Issuance of restricted stock awards, net of forfeitures (in shares)		200,000
Issuance of restricted stock awards, net of forfeitures	0.0
Stock-based compensation	18.9	$ 18.9
Cash dividends	(22.2)		(22.2)
Foreign currency translation, net of tax	0.2			0.2
Change in pension benefits, net of tax	(0.8)			(0.8)
Shares outstanding, ending (in shares) at Dec. 31, 2016		49,500,000
Shareholders' equity, ending at Dec. 31, 2016	685.0	$ 116.5	569.7	(1.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	140.5		140.5
Issuance of common stock (in shares)		100,000
Issuance of common stock	2.1	$ 2.1
Repurchase of common stock (in shares)		(3,600,000)
Repurchase of common stock	(190.9)	$ (138.4)	(52.5)
Issuance of restricted stock awards, net of forfeitures (in shares)		200,000
Issuance of restricted stock awards, net of forfeitures	0.0
Stock-based compensation	27.1	$ 27.1
Cash dividends	(23.4)		(23.4)
Foreign currency translation, net of tax	(0.1)			(0.1)
Change in pension benefits, net of tax	0.0
Shares outstanding, ending (in shares) at Dec. 31, 2017		46,200,000
Shareholders' equity, ending at Dec. 31, 2017	640.3	$ 7.3	634.3	(1.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	352.8		352.8
Issuance of common stock (in shares)		300,000
Issuance of common stock	1.5	$ 1.5
Repurchase of common stock (in shares)		(6,200,000)
Repurchase of common stock	(549.5)	$ (29.9)	(519.6)
Issuance of restricted stock awards, net of forfeitures (in shares)		100,000
Issuance of restricted stock awards, net of forfeitures	0.0
Stock-based compensation	21.1	$ 21.1
Cash dividends	(23.0)		(23.0)
Foreign currency translation, net of tax	0.6			0.6
Change in pension benefits, net of tax	(0.2)			0.2
Shares outstanding, ending (in shares) at Dec. 31, 2018		40,400,000
Shareholders' equity, ending at Dec. 31, 2018	$ 473.3	$ 0.0	$ 474.2	$ (0.9)