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Stock-Based Compensation Plans (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	May 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares approved for issuance (in shares)					5,485,000
Shares available for future grants (in shares)	976,000		976,000
Number of stock options granted			0	0
Stock-based compensation expense	$ 3.2	$ 1.3	$ 9.4	$ (1.0)
Aggregate intrinsic value of options exercised			7.9	2.7
Cash received from option exercises			$ 10.6	$ 4.1
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Term			7 years
Stock options \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			3 years
Stock options \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			5 years
Restricted shares \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			1 year
Restricted shares \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			5 years
LTIP awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			3 years
Term			7 years
LTIP awards \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance based award payouts			0.00%
LTIP awards \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance based award payouts			150.00%
Performance-based stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			3 years
Performance period			3 years