Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 14.6	$ 13.7	$ 12.0	$ 2.4	$ 15.0	$ 12.3	$ 11.7	$ 2.6	$ 42.7	[1]	$ 41.6	[1]	$ 33.9	[1]
Weighted-average shares outstanding, basic EPS	23.1	23.1	23.1	23.0	23.0	23.0	23.0	23.0	23.1		23.0		23.0
Net income per common share, basic EPS (in dollars per share)	$ 0.63	$ 0.59	$ 0.52	$ 0.11	$ 0.65	$ 0.54	$ 0.51	$ 0.11	$ 1.85		$ 1.81		$ 1.48
Effect of dilutive common share equivalents: [Abstract]
Net income, diluted									$ 42.7		$ 41.6		$ 33.9
Weighted-average shares outstanding, diluted EPS	23.4	23.3	23.2	23.2	23.2	23.2	23.2	23.2	23.3		23.2		23.2
Net income per common share, diluted EPS (in dollars per share)	$ 0.62	$ 0.59	$ 0.52	$ 0.10	$ 0.65	$ 0.53	$ 0.51	$ 0.11	$ 1.83		$ 1.79		$ 1.46
Restricted stock units (RSUs) [Member]
Effect of dilutive common share equivalents: [Abstract]
Incremental common shares attributable to share-based payment arrangements									0.1		0		0.2
Net income per common share, share-based payment arrangements (in dollars per share)									$ (0.01)		$ (0.01)		$ (0.01)
Options [Member]
Effect of dilutive common share equivalents: [Abstract]
Incremental common shares attributable to share-based payment arrangements									0.1		0.2		0
Net income per common share, share-based payment arrangements (in dollars per share)									$ (0.01)		$ (0.01)		$ (0.01)

[1] Amounts have been rounded for presentation purposes and might differ from unrounded results.