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Stock-Based Compensation Plans (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	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)	2,800,000		2,800,000
Number of stock options granted			0	0
Stock-based compensation expense	$ 3.0	$ 3.4	$ 6.3	$ 7.1
Aggregate intrinsic value of options exercised			3.3	10.9
Cash received from option exercises			$ 3.6	$ 9.7
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum 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 \| 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
Awards vested and issued in excess of target (in shares)			33,950