Statement of Consolidated Stockholders' Equity (Unaudited) - USD ($) $ in Millions	Total	Common Stock	Additional Paid in Capital	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Loss
Stockholders' Equity [Roll Forward]
Net income	$ 52.3
Repurchase of common stock (shares)	(109,033)
Repurchase of common stock	$ (8.6)
Ending balance at Jun. 30, 2016						$ (29.9)
Stockholders' Equity [Roll Forward]
Net income	$ 26.0
Ending balance at Jun. 30, 2016						(29.9)
Beginning balance, shares at Dec. 31, 2016	17,651,461	17,651,461
Beginning balance at Dec. 31, 2016	$ 804.7	$ 0.2	$ 1,047.4	$ 75.2	$ (281.4)	(36.7)
Stockholders' Equity [Roll Forward]
Net income	40.7			40.7
Other comprehensive income, net of tax	2.4					2.4
Issuance of non-vested shares to non-employee directors (shares)		11,817
Issuance of common shares to non-employee directors (shares)		2,282
Issuance of common shares to non-employee directors	0.2		0.2
Issuance of common shares to employees upon option exercises and vesting of restricted stock units and performance shares (shares)		102,737
Cancellation of employee non-vested shares (shares)		(378)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares (shares)		(56,195)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares	$ (4.5)		(4.5)
Repurchase of common stock (shares)	(787,986)	(787,986)
Repurchase of common stock	$ (63.2)				(63.2)
Cancellation of treasury stock			(0.2)	(0.4)	0.6
Cash dividends on common stock and restricted shares and dividend equivalents on restricted stock units and performance shares	(17.9)			(17.9)
Amortization of unearned equity compensation	$ 6.4		6.4
Ending balance, shares at Jun. 30, 2017	16,923,738	16,923,738
Ending balance at Jun. 30, 2017	$ 768.8	$ 0.2	1,049.3	97.6	(344.0)	(34.3)
Stockholders' Equity [Roll Forward]
Net income	$ 4.7
Ending balance, shares at Jun. 30, 2017	16,923,738	16,923,738
Ending balance at Jun. 30, 2017	$ 768.8	$ 0.2	$ 1,049.3	$ 97.6	$ (344.0)	$ (34.3)