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		$ 60.6
Repurchase of common stock (shares)		(806,307)
Repurchase of common stock		$ (64.9)
Ending balance at Sep. 30, 2017							$ (32.7)
Stockholders' Equity [Roll Forward]
Net income		19.9
Ending balance at Sep. 30, 2017							(32.7)
Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment	[1]	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		68.1			68.1
Other comprehensive loss, net of tax		(2.7)					(2.7)
Issuance of non-vested shares to non-employee directors (shares)			9,009
Issuance of common shares to non-employee directors (shares)			1,954
Issuance of common shares to non-employee directors		0.2		0.2
Issuance of common shares to employees upon vesting of restricted stock units and performance shares (shares)			135,294
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares (shares)			(68,166)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares		$ (6.9)		(6.9)
Repurchase of common stock (shares)		(304,440)	(304,440)
Repurchase of common stock		$ (32.3)				(32.3)
Cash dividends on common stock and restricted shares and dividend equivalents on restricted stock units and performance shares		(28.5)			(28.5)
Amortization of unearned equity compensation		$ 8.2		8.2
Ending balance (shares) at Sep. 30, 2018		16,547,237	16,547,237
Ending balance at Sep. 30, 2018		$ 762.5	$ 0.2	1,057.4	135.6	(390.9)	(39.8)
Stockholders' Equity [Roll Forward]
Net income		$ 21.7
Ending balance (shares) at Sep. 30, 2018		16,547,237	16,547,237
Ending balance at Sep. 30, 2018		$ 762.5	$ 0.2	$ 1,057.4	$ 135.6	$ (390.9)	$ (39.8)

[1] See Note 1 for discussion of our adoption of ASC 606 and ASU 2016-01 (as defined in Note 1).