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Share-based Compensation (Narrative) (Detail) - USD ($)	3 Months Ended		12 Months Ended
Share-based Compensation (Narrative) (Detail) - USD ($)	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$ 2,198,000	$ 981,000
Proceeds from stock options exercised	$ 481,000
Shares withheld for employee taxes	6,814	4,718
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares withheld for employee taxes	7,000	5,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$ 900,000	$ 700,000
Total income tax benefit recognized related to restricted stock awards	$ 1,200,000	200,000
Compensation cost recognition weighted average period	2 years 4 months 24 days
Share-based compensation arrangement, fair value of awards vested	$ 3,900,000	$ 2,200,000
Shares granted in period	17,000	29,000
Shares vested in period	14,000	21,000
Unrecognized compensation costs	$ 6,800,000
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee service share based compensation nonvested awards total compensation cost not yet recognized	$ 200,000
Compensation cost recognition weighted average period	1 year 10 months 24 days
Weighted-average remaining contractual life of options outstanding	5 years 7 months 6 days
Aggregate intrinsic value of options outstanding	$ 8,300,000
Weighted-average remaining contractual life of options exercisable	5 years 2 months 12 days
Aggregate intrinsic value of options exercisable	$ 6,800,000
Intrinsic value of options exercised	$ 2,300,000	$ 0
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost recognition weighted average period	2 years 1 month 6 days
Share-based compensation arrangement shares to be issued	58,394
Unrecognized compensation costs	$ 8,300,000
Granted in 2019 [Member] \| Performance Shares [Member] \| Certain Key Executive [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	700,000	$ 200,000
Shares granted in period			16,637
Granted in 2020 [Member] \| Performance Shares [Member] \| Certain Key Executive [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$ 100,000
Shares granted in period	27,525	25,738
Granted in 2020 [Member] \| Performance Shares [Member] \| Key Executives [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$ 500,000
Granted in 2021 [Member] \| Performance Shares [Member] \| Certain Key Executive [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$ 100,000
Shares granted in period	14,232
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, shares authorized under stock option plans, exercise price range, lower range limit	$ 26.52
Minimum [Member] \| Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, shares authorized under stock option plans, exercise price range, upper range limit	$ 171.68
Maximum [Member] \| Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	5 years
Maximum [Member] \| Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period	10 years