Earnings Per Share (Reconciliation of Basic and Diluted Earnings Per Share Computations) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Net income per share - basic
Net income attributable to common stockholders	$ 168.9	$ 89.3	$ 32.7	$ 5.3	$ 258.2	$ 38.0
Weighted average common shares outstanding (in shares)	29,181		32,112		29,708	32,159
Earnings per common share (in dollars per share)	$ 5.79		$ 1.02		$ 8.69	$ 1.18
Net income per share - diluted
Net income attributable to common stockholders	$ 168.9	$ 89.3	$ 32.7	$ 5.3	$ 258.2	$ 38.0
Weighted average common shares outstanding (in shares)	29,181		32,112		29,708	32,159
Common equivalent shares:
Dilutive share-based awards (in shares)	314		216		310	213
Weighted average common shares outstanding - assuming dilution (in shares)	29,495		32,328		30,018	32,372
Earnings per common share assuming dilution (in dollars per share)	$ 5.73		$ 1.01		$ 8.60	$ 1.18