Net Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Numerator:
Net earnings	$ 107.7	$ 134.0	$ 325.9	$ 360.7
Distributed and allocated undistributed net earnings to unvested restricted stockholders	0.0	0.0	0.0	(0.1)
Net earnings available to common stockholders	$ 107.7	$ 134.0	$ 325.9	$ 360.6
Denominator:
Weighted average number of common shares outstanding - basic (in shares)	149.9	155.5	151.8	155.2
Basic net earnings per common share:
Basic net earnings per common share (in dollars per share)	$ 0.72	$ 0.86	$ 2.15	$ 2.32
Numerator:
Net earnings available to common stockholders	$ 107.7	$ 134.0	$ 325.9	$ 360.6
Denominator:
Weighted average number of common shares outstanding - basic (in shares)	149.9	155.5	151.8	155.2
Effect of unvested restricted stock units (in shares)	1.5	0.6	1.4	0.6
Weighted average number of common shares outstanding - diluted under treasury stock (in shares)	151.4	156.1	153.2	155.8
Diluted net earnings per common share (in dollars per share)	$ 0.71	$ 0.86	$ 2.13	$ 2.31