Net Income Per Common Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Numerator for basic net income (loss) per common share:
Net income (loss) from continuing operations	$ 108.3	$ (23.6)	$ 144.4	$ (46.2)
Net loss attributable to noncontrolling interest	0.0	0.0	0.0	(0.1)
Net income (loss) from continuing operations, net of loss attributable to noncontrolling interests	108.3	(23.6)	144.4	(46.1)
Net loss from discontinued operations	0.0	(95.2)	0.0	(96.1)
Numerator for basic net income (loss) per common share	108.3	(118.8)	144.4	(142.2)
Numerator for diluted net income (loss) from continuing operations per common share	108.3	(23.6)	144.4	(46.1)
Numerator for diluted net income (loss) per common share	$ 108.3	$ (118.8)	$ 144.4	$ (142.2)
Denominator for net income (loss) per common share:
Basic (in shares)	38.7	39.5	38.8	39.6
Plus dilutive effect of stock awards (in shares)	0.6	0.0	0.6	0.0
Diluted (in shares)	39.3	39.5	39.4	39.6
Net income (loss) per common share data: Basic
Continuing operations (in dollars per share)	$ 2.80	$ (0.59)	$ 3.72	$ (1.16)
Discontinued operations (in dollars per share)	0	(2.41)	0	(2.43)
Net income (loss) per common share - basic (in dollars per share)	2.80	(3.00)	3.72	(3.59)
Net income (loss) per common share data: Diluted
Continuing operations (in dollars per share)	2.76	(0.59)	3.66	(1.16)
Discontinued operations (in dollars per share)	0	(2.41)	0	(2.43)
Net income (loss) per common share - diluted (in dollars per share)	$ 2.76	$ (3.00)	$ 3.66	$ (3.59)
Anti-dilutive stock awards excluded from the calculation of diluted shares (in shares)	0.0	0.5	0.0	0.5