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, 2014		17,500,000
Shareholders' equity, beginning at Dec. 31, 2014	$ 700.1	$ 262.3	$ 437.9	$ (0.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	65.2		65.2
Issuance of common stock (in shares)		0
Issuance of common stock	3.5	$ 3.5
Tax windfall from stock-based compensation	5.5	$ 5.5
Repurchase of common stock (in shares)		(1,100,000)
Repurchase of common stock	(151.1)	$ (151.1)
Issuance of restricted stock awards, net of forfeitures (in shares)		200,000
Issuance of restricted stock awards, net of forfeitures	0.0	$ 0.0
Stock-based compensation	13.8	$ 13.8
Cash & stock unit dividends	(19.3)		(19.3)
Foreign currency translation, net of tax	(0.5)			(0.5)
Change in pension benefits, net of tax	0.0
Shares outstanding, ending (in shares) at Dec. 31, 2015		16,600,000
Shareholders' equity, ending 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)		100,000
Issuance of common stock	$ 2.6	$ 2.6
Repurchase of common stock (in shares)	(211,790)	(300,000)
Repurchase of common stock	$ (39.0)	$ (39.0)
Issuance of restricted stock awards, net of forfeitures (in shares)		100,000
Issuance of restricted stock awards, net of forfeitures	0.0	$ 0.0
Stock-based compensation	18.9	$ 18.9
Cash & stock unit 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		16,500,000
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)		0
Issuance of common stock	2.1	$ 2.1
Repurchase of common stock (in shares)		(1,200,000)
Repurchase of common stock	(190.9)	$ (138.4)	(52.5)
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	27.1	$ 27.1
Cash & stock unit 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		15,400,000
Shareholders' equity, ending at Dec. 31, 2017	$ 640.3	$ 7.3	$ 634.3	$ (1.3)