Earnings Per Share, Calculation of EPS (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Numerator:
Net income	$ 33.5		$ 26.5
Denominator — Weighted-average common shares outstanding (in thousands):
Basic	19,143	[1]	19,059	[1]
Diluted	19,366	[1]	19,161	[1]
Earnings per common share, Basic:
Net income per share	$ 1.75		$ 1.39
Earnings per common share, Diluted:
Net income per share	$ 1.73		$ 1.38

[1] The basic weighted-average number of common shares outstanding during the period excludes unvested share-based payment awards. The diluted weighted-average number of common shares outstanding during the period is calculated using the treasury method.