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						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Net income per share - basic
Net income attributable to common stockholders	$ 66.9	$ 168.9	$ 89.3	$ 69.2	$ 32.7	$ 5.3	$ 325.1	$ 107.2
Weighted average common shares outstanding (in shares)	29,111			31,447			29,546	31,961
Earnings per common share (in dollars per share)	$ 2.30			$ 2.20			$ 11.00	$ 3.35
Net income per share - diluted
Net income attributable to common stockholders	$ 66.9	$ 168.9	$ 89.3	$ 69.2	$ 32.7	$ 5.3	$ 325.1	$ 107.2
Weighted average common shares outstanding (in shares)	29,111			31,447			29,546	31,961
Common equivalent shares:
Dilutive share-based awards (in shares)	388			257			341	228
Weighted average common shares outstanding - assuming dilution (in shares)	29,499			31,704			29,887	32,189
Earnings per common share assuming dilution (in dollars per share)	$ 2.27			$ 2.18			$ 10.88	$ 3.33