Net Income Per Common Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income from continuing operations									$ 182.6	$ 122.4	$ 96.7
Net income from continuing operations allocated to participating securities									0.0	(0.1)	(1.0)
Net income from discontinued operations	$ 4.1	$ (1.7)	$ (0.1)	$ 167.9	$ 3.8	$ 3.8	$ 5.4	$ 5.1	170.2	18.1	11.4
Numerator for basic net income per common share									352.8	140.4	107.1
Numerator for diluted net income from continuing operations per common share									182.6	122.4	96.7
Numerator for diluted net income per common share									$ 352.8	$ 140.5	$ 108.1
Denominator for net income per common share:
Basic (in shares)									41.3	47.2	49.3
Plus dilutive effect of stock awards (in shares)									0.3	0.6	0.6
Plus dilutive effect of participating securities (in shares)									0.0	0.2	0.6
Diluted (in shares)									41.6	48.0	50.5
Earnings Per Share, Basic [Abstract]
Continuing operations (in dollars per share)	$ 0.18	$ 1.43	$ 2.54	$ 0.33	$ 0.75	$ 0.28	$ 1.50	$ 0.04	$ 4.42	$ 2.59	$ 1.94
Discontinued operations (in dollars per share)	0.10	(0.04)	0.00	3.88	0.08	0.08	0.11	0.11	4.12	0.38	0.23
Net income per common share - basic (in dollars per share)	0.28	1.39	2.54	4.21	0.83	0.36	1.61	0.15	8.54	2.97	2.17
Earnings Per Share, Diluted [Abstract]
Continuing operations (in dollars per share)	0.18	1.42	2.52	0.32	0.74	0.28	1.49	0.04	4.39	2.55	1.92
Discontinued operations (in dollars per share)	0.10	(0.04)	0.00	3.86	0.08	0.08	0.11	0.10	4.09	0.37	0.22
Net income per common share - diluted (in dollars per share)	$ 0.28	$ 1.38	$ 2.52	$ 4.18	$ 0.82	$ 0.36	$ 1.60	$ 0.14	$ 8.48	$ 2.92	$ 2.14