Net Earnings Per Common Share (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:
Net earnings	$ 108.1	$ 100.1	$ 218.2	$ 226.7
Distributed and allocated undistributed net earnings to unvested restricted stockholders	0.0	0.0	0.0	(0.1)
Net earnings available to common stockholders	$ 108.1	$ 100.1	$ 218.2	$ 226.6
Denominator:
Weighted average number of common shares outstanding - basic (in shares)	151.4	155.6	152.8	155.1
Basic net earnings per common share:
Basic net earnings per common share (in dollars per share)	$ 0.71	$ 0.64	$ 1.43	$ 1.46
Numerator:
Net earnings available to common stockholders	$ 108.1	$ 100.1	$ 218.2	$ 226.6
Denominator:
Weighted average number of common shares outstanding - basic (in shares)	151.4	155.6	152.8	155.1
Effect of unvested restricted stock units (in shares)	1.3	0.3	1.2	0.3
Weighted average number of common shares outstanding - diluted under treasury stock (in shares)	152.7	155.9	154.0	155.4
Diluted net earnings per common share (in dollars per share)	$ 0.71	$ 0.64	$ 1.42	$ 1.46