Earnings per Share ("EPS") - Schedule of Earnings Per Share Calculations, Basic and Diluted (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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 basic common share
Income from continuing operations	$ 18.9	$ 18.8	$ 25.0	$ 17.6	$ 16.4	$ 16.4	$ 21.4	$ 19.2	$ 80.3	$ 73.4	$ 60.5
Amounts attributable to participating securities									(0.6)	(0.7)	(0.6)
Income from continuing operations available to common stockholders									79.7	72.7	59.9
Income (loss) from discontinued operations, net of income taxes									0.0	(0.4)	(9.4)
Amounts attributable to participating securities									0.0	0.0	0.1
Net income available to common stockholders									$ 79.7	$ 72.3	$ 50.6
Weighted-average basic shares outstanding (in shares)									16,805	16,773	16,754
Continuing operations (in dollars per share)	$ 1.11	$ 1.11	$ 1.47	$ 1.04	$ 0.97	$ 0.97	$ 1.26	$ 1.13	$ 4.74	$ 4.33	$ 3.58
Discontinued operations (in dollars per share)									0.00	(0.02)	(0.56)
Basic (in dollars per share)									$ 4.74	$ 4.31	$ 3.02
Earnings per diluted common share
Income from continuing operations	$ 18.9	$ 18.8	$ 25.0	$ 17.6	$ 16.4	$ 16.4	$ 21.4	$ 19.2	$ 80.3	$ 73.4	$ 60.5
Amounts attributable to participating securities									(0.5)	(0.6)	(0.5)
Income from continuing operations available to common stockholders									79.8	72.8	60.0
Income (loss) from discontinued operations, net of income taxes									0.0	(0.4)	(9.4)
Amounts attributable to participating securities									0.0	0.0	0.1
Net income available to common stockholders									$ 79.8	$ 72.4	$ 50.7
Weighted-average basic shares outstanding (in shares)									16,805	16,773	16,754
Add: Assumed incremental shares under stock-based compensation plans (shares)									247	314	258
Weighted average diluted shares (in shares)									17,052	17,087	17,012
Continuing operations (in dollars per share)	$ 1.10	$ 1.10	$ 1.46	$ 1.03	$ 0.95	$ 0.95	$ 1.24	$ 1.11	$ 4.68	$ 4.26	$ 3.53
Discontinued operations (in dollars per share)									0.00	(0.02)	(0.55)
Diluted (in dollars per share)									$ 4.68	$ 4.24	$ 2.98