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	$ 36.0
Repurchase of common stock (shares)	(415,346)
Repurchase of common stock	$ (32.7)
Ending balance at Mar. 31, 2017						$ (35.4)
Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment	10.1			$ 10.5	$ 0.0	(0.4)
Adjusted balance	$ 756.4	$ 0.2	$ 1,055.9	96.0	(358.6)	(37.1)
Beginning balance, shares at Dec. 31, 2017	16,773,586	16,773,586
Beginning balance at Dec. 31, 2017	$ 746.3	$ 0.2	1,055.9	85.5	(358.6)	(36.7)
Stockholders' Equity [Roll Forward]
Net income	25.7			25.7
Other comprehensive loss, net of tax	(8.3)					(8.3)
Issuance of common shares to employees upon option exercises and vesting of restricted stock units and performance shares (shares)		135,134
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares (shares)		(68,029)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares	$ (6.9)		(6.9)
Repurchase of common stock (shares)	(58,155)	(58,155)
Repurchase of common stock	$ (6.1)				(6.1)
Cash dividends on common stock and restricted shares and dividend equivalents on restricted stock units and performance shares	(10.0)			(10.0)
Amortization of unearned equity compensation	$ 2.8		2.8
Ending balance, shares at Mar. 31, 2018	16,782,536	16,782,536
Ending balance at Mar. 31, 2018	$ 753.6	$ 0.2	$ 1,051.8	$ 111.7	$ (364.7)	$ (45.4)