Net Earnings per Common Share - Calculation of Basic and Diluted Net Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator
Net earnings	$ 502.9	$ 263.0	$ 193.1
Distributed and allocated undistributed net earnings to unvested restricted stockholders	(0.2)	(0.5)	(0.9)
Distributed and allocated undistributed net earnings	502.7	262.5	192.2
Distributed net earnings - dividends paid to common stockholders	(99.4)	(98.7)	(101.7)
Allocation of undistributed net earnings to common stockholders	$ 403.3	$ 163.8	$ 90.5
Denominator
Weighted average number of common shares outstanding - basic (in shares)	155.2	154.3	159.4
Basic net earnings per common share
Distributed net earnings (in dollars per share)	$ 0.64	$ 0.64	$ 0.64
Allocated undistributed net earnings to common stockholders (in dollars per share)	2.60	1.06	0.57
Net earnings per common share - basic (in dollars per share)	$ 3.24	$ 1.70	$ 1.21
Numerator
Distributed and allocated undistributed net earnings	$ 502.7	$ 262.5	$ 192.2
Add: Allocated undistributed net earnings to unvested restricted stockholders	0.1	0.4	0.5
Less: Undistributed net earnings reallocated to unvested restricted stockholders	(0.1)	(0.4)	(0.5)
Net earnings available to common stockholders - diluted	$ 502.7	$ 262.5	$ 192.2
Denominator
Weighted average number of common shares outstanding - basic (in shares)	155.2	154.3	159.4
Effect of contingently issuable shares (in shares)	0.2	0.2	0.1
Weighted average number of common shares outstanding - diluted under two-class (in shares)	156.0	154.8	159.8
Effect of unvested restricted stock - participating security (in shares)	0.0	0.4	0.4
Weighted average number of common shares outstanding - diluted under treasury stock (in shares)	156.0	155.2	160.2
Diluted net earnings per common share - (in dollars per share)	$ 3.22	$ 1.69	$ 1.20
Restricted Stock-Nonparticipating Security
Denominator
Effect of unvested restricted stock units (in shares)	0.1	0.0	0.0
Restricted Stock Units
Denominator
Effect of unvested restricted stock units (in shares)	0.5	0.3	0.3