Net Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator:
Net earnings	$ 110.1	$ 126.6
Distributed and allocated undistributed net earnings to unvested restricted stockholders	0.0	(0.1)
Net earnings available to common stockholders	$ 110.1	$ 126.5
Denominator:
Weighted average number of common shares outstanding - basic (in shares)	154.1	154.5
Basic net earnings per common share:
Basic net earnings per common share (in dollars per share)	$ 0.71	$ 0.82
Numerator:
Net earnings available to common stockholders	$ 110.1	$ 126.5
Denominator:
Weighted average number of common shares outstanding - basic (in shares)	154.1	154.5
Effect of unvested restricted stock units (in shares)	1.3	0.3
Weighted average number of common shares outstanding - diluted under treasury stock (in shares)	155.4	154.8
Diluted net earnings per common share (in dollars per share)	$ 0.71	$ 0.82