Share-Based Compensation Plans - Share-Based Compensation Amounts Included in Net Income (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation expense	$ 4,829	$ 4,279	$ 3,410
Less: tax benefit	(1,254)	(1,117)	(934)
Share-Based Compensation amounts included in net income	3,575	3,162	2,476
Share-based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount	3,900
SICP Awards to Non-employee directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation expense	733	620	539
SICP Awards to Key Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Nonvested	20,200	15,100	10,700
Total compensation expense	$ 4,096	$ 3,659	$ 2,871

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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-Publisher FASB
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