Net Income Per Common Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Net income from continuing operations	$ 108.3	$ 103.2	$ 120.2	$ 117.3
Net (loss) income from discontinued operations	(1.2)	(0.1)	(1.5)	167.8
Numerator for basic net income per common share	107.1	103.1	118.7	285.1
Numerator for diluted net income from continuing operations per common share	108.3	103.2	120.2	117.3
Numerator for diluted net income per common share:	$ 107.1	$ 103.1	$ 118.7	$ 285.1
Earnings Per Share, Basic and Diluted [Abstract]
Basic (in shares)	40.1	40.7	40.3	42.0
Plus dilutive effect of stock awards (in shares)	0.6	0.2	0.4	0.2
Diluted (in shares)	40.7	40.9	40.7	42.2
Basic
Continuing operations (in dollars per share)	$ 2.69	$ 2.54	$ 2.99	$ 2.80
Discontinued operations (in dollars per share)	(0.03)	0	(0.04)	3.99
Net income per common share data - basic (in dollars per share)	2.66	2.54	2.95	6.79
Diluted
Continuing operations (in dollars per share)	2.66	2.52	2.96	2.78
Discontinued operations (in dollars per share)	(0.03)	0	(0.04)	3.97
Net income per common share data - diluted (in dollars per share)	$ 2.63	$ 2.52	$ 2.92	$ 6.75