Net Income Per Common Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income from continuing operations									$ 122.4	$ 96.7	$ 70.8
Net income from continuing operations allocated to participating securities									(0.1)	(1.0)	(0.6)
Income (loss) from discontinued operations, net of tax	$ 3.8	$ 3.8	$ 5.4	$ 5.1	$ 7.4	$ 5.3	$ 3.0	$ (4.3)	18.1	11.4	(5.6)
Numerator for basic net income per common share									140.4	107.1	64.6
Numerator for diluted net income from continuing operations per common share									122.4	96.7	70.8
Numerator for diluted net income per common share									$ 140.5	$ 108.1	$ 65.2
Denominator for net income per common share:
Basic (in shares)									15.7	16.4	17.2
Plus dilutive effect of stock awards (in shares)									0.2	0.2	0.1
Plus dilutive effect of participating securities (in shares)									0.1	0.2	0.3
Diluted (in shares)									16.0	16.8	17.6
Earnings Per Share, Basic [Abstract]
Continuing operations (in dollars per share)	$ 2.25	$ 0.85	$ 4.52	$ 0.13	$ 1.17	$ 0.20	$ 3.97	$ 0.43	$ 7.76	$ 5.83	$ 4.08
Discontinued operations (in dollars per share)	0.25	0.24	0.34	0.31	0.45	0.32	0.19	(0.26)	1.15	0.69	(0.33)
Net income per common share - basic (in dollars per share)	2.50	1.09	4.86	0.44	1.62	0.52	4.16	0.17	8.91	6.52	3.75
Earnings Per Share, Diluted [Abstract]
Continuing operations (in dollars per share)	2.22	0.84	4.47	0.13	1.16	0.20	3.93	0.42	7.64	5.74	4.03
Discontinued operations (in dollars per share)	0.24	0.24	0.34	0.31	0.44	0.32	0.18	(0.26)	1.13	0.68	(0.32)
Net income per common share - diluted (in dollars per share)	$ 2.46	$ 1.08	$ 4.81	$ 0.44	$ 1.60	$ 0.52	$ 4.11	$ 0.16	$ 8.77	$ 6.42	$ 3.71