Statement of Consolidated Stockholders' Equity (USD $) In Millions, except Share data, unless otherwise specified	Total	Common Stock	Additional Paid in Capital	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Loss
Beginning balance at Dec. 31, 2012	$ 1,070.8	$ 0.2	$ 1,017.7	$ 151.2	$ (72.3)	$ (26.0)
Beginning balance, shares at Dec. 31, 2012	19,313,235	19,313,235
Stockholders' Equity [Roll Forward]
Net income	33.5			33.5
Other comprehensive income, net of tax	1.2					1.2
Issuance of non-vested shares to employees		57,190
Issuance of common shares to employees upon vesting of restricted stock units and performance shares		34,623
Cancellation of shares to cover employees’ tax withholdings upon vesting of non-vested shares, shares		(35,838)
Cancellation of shares to cover employees’ tax withholdings upon vesting of non-vested shares	(2.2)		(2.2)
Repurchase of common stock, shares	(259,596)	(259,596)
Repurchase of common stock	(16.2)				(16.2)
Cash dividends on common stock ($0.30 per share)	(5.9)			(5.9)
Excess tax benefit upon vesting of non-vested shares and dividend payment on unvested shares expected to vest	0.8		0.8
Amortization of unearned equity compensation	2.6		2.6
Dividends on unvested equity awards that canceled	0.2			0.2
Ending balance at Mar. 31, 2013	$ 1,084.8	$ 0.2	$ 1,018.9	$ 179.0	$ (88.5)	$ (24.8)
Ending balance, shares at Mar. 31, 2013	19,109,614	19,109,614