Stockholders' Equity Basis and Diluted Shares Reconciliation (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Earnings Per Share [Abstract]
Net income	$ 47.6	$ 9.0	$ 19.8	$ 16.2	$ 13.7	$ 18.2	$ 14.8	$ 18.6	$ 92.6	$ 65.3	$ 35.6
Less: Dividends paid to participating securities									(0.1)	0	(0.3)
Less: Undistributed earnings available to participating securities									(1.1)	(1.4)	0
Undistributed and distributed earnings available to common shareholders									$ 91.4	$ 63.9	$ 35.3
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Average number of common shares outstanding									16,615,100	17,686,700	15,550,100
Effect of dilutive stock-based compensation									128,300	362,700	453,400
Average number of common and potential common shares outstanding									16,743,400	18,049,400	16,003,500
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount											368,300
Antidilutive Shares Excluded from Computation, Weighted-Average Share Price of Options Exceeded the Average Market Price									$ 28.02