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Share-based Compensation (Narrative) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$ 2,100		$ 1,200		$ 4,243	$ 3,415
Proceeds from stock options exercised					$ 1,424	$ 279
Shares withheld for employee taxes					0.0	0.0
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares withheld for employee taxes		5,000		1,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	1,000		700		$ 2,500	$ 2,200
Total income tax benefit recognized related to restricted stock awards	300		200		$ 900	1,500
Compensation cost recognition weighted average period					1 year 9 months 18 days
Share-based compensation arrangement, fair value of awards vested					$ 3,400	$ 4,100
Shares granted in period					43,000	28,000
Shares vested in period					30,000	32,000
Unrecognized compensation costs	4,600				$ 4,600
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				$ 200
Compensation cost recognition weighted average period					2 years 2 months 12 days
Weighted-average remaining contractual life of options outstanding					6 years
Aggregate intrinsic value of options outstanding	7,800				$ 7,800
Weighted-average remaining contractual life of options exercisable					5 years 6 months
Aggregate intrinsic value of options exercisable	$ 5,900				$ 5,900
Intrinsic value of options exercised					$ 1,100	$ 1,000
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost recognition weighted average period					2 years
Share-based compensation arrangement shares to be issued	44,162				44,162
Shares vested in period							273,300
Unrecognized compensation costs	$ 4,800				$ 4,800
Performance Shares [Member] \| Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation			200			$ 500
Shares granted in period						210,000
Performance Shares [Member] \| Certain Key Executive [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation						$ 200
Performance Shares [Member] \| Key Executives [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation			100
Shares granted in period						63,300
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	500		$ 200		$ 800	$ 500
Shares granted in period					16,637	17,780
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					$ 900
Shares granted in period					27,525
Granted in 2020 [Member] \| Performance Shares [Member] \| Key Executives [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$ 600
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