Net Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net earnings	$ 134.0	$ 68.0	$ 360.7	$ 158.7
Distributed and allocated undistributed net earnings to unvested restricted stockholders	0.0	(0.1)	(0.1)	(0.3)
Distributed and allocated undistributed net earnings	134.0	67.9	360.6	158.4
Distributed net earnings - dividends paid to common stockholders	(24.9)	(24.6)	(74.6)	(74.1)
Allocation of undistributed net earnings to common stockholders	$ 109.1	$ 43.3	$ 286.0	$ 84.3
Denominator:
Weighted average number of common shares outstanding - basic (in shares)	155.5	154.0	155.2	154.4
Basic net earnings per common share:
Distributed net earnings (in dollars per share)	$ 0.16	$ 0.16	$ 0.48	$ 0.48
Allocated undistributed net earnings (loss) to common stockholders (in dollars per share)	0.70	0.28	1.84	0.55
Net earnings per common share - basic (in dollars per share)	$ 0.86	$ 0.44	$ 2.32	$ 1.03
Numerator:
Distributed and allocated undistributed net earnings	$ 134.0	$ 67.9	$ 360.6	$ 158.4
Add: Allocated undistributed net earnings to unvested restricted stockholders	0.0	0.1	0.1	0.2
Less: Undistributed net earnings reallocated to unvested restricted stockholders	0.0	(0.1)	(0.1)	(0.2)
Net earnings available to common stockholders - diluted	$ 134.0	$ 67.9	$ 360.6	$ 158.4
Denominator:
Weighted average number of common shares outstanding - basic (in shares)	155.5	154.0	155.2	154.4
Effect of contingently issuable shares (in shares)	0.2	0.2	0.2	0.2
Weighted average number of common shares outstanding - diluted under two-class (in shares)	156.1	154.5	155.8	154.9
Effect of unvested restricted stock - participating security (in shares)	0.0	0.3	0.0	0.3
Weighted average number of common shares outstanding - diluted under treasury stock (in shares)	156.1	154.8	155.8	155.2
Diluted net earnings per common share (in dollars per share)	$ 0.86	$ 0.44	$ 2.31	$ 1.02
Restricted Stock
Denominator:
Effect of unvested restricted stock units (in shares)	0.1	0.0	0.1	0.0
Restricted Stock Units
Denominator:
Effect of unvested restricted stock units (in shares)	0.3	0.3	0.3	0.3