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
	Mar. 31, 2020	Mar. 31, 2019
Net income per share - basic
Net income attributable to common stockholders	$ 89.3	$ 5.3
Weighted average common shares outstanding (in shares)	30,235	32,206
Earnings per common share (in dollars per share)	$ 2.95	$ 0.16
Net income per share - diluted
Net income attributable to common stockholders	$ 89.3	$ 5.3
Weighted average common shares outstanding (in shares)	30,235	32,206
Common equivalent shares:
Dilutive share-based awards (in shares)	306	214
Weighted average common shares outstanding - assuming dilution (in shares)	30,541	32,420
Earnings per common share assuming dilution (in dollars per share)	$ 2.92	$ 0.16