Calculation of Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Income (Loss) from Continuing Operations, Per Basic Share	$ 0.56	$ 0.38	$ 2.97	$ 2.68
Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Basic Share	$ 0	$ (0.04)	$ 0.01	$ (0.08)
Calculation of Basic Earnings Per Share:
Net Income	$ 9,261	$ 5,621	$ 49,146	$ 42,589
Weighted shares outstanding (shares)	16,533,748	16,403,776	16,466,106	16,396,646
Basic Earnings Per Share (in dollars per share)	$ 0.56	$ 0.34	$ 2.98	$ 2.60
Reconciliation of Numerator:
Net Income	$ 9,261	$ 5,621	$ 49,146	$ 42,589
Reconciliation of Denominator:
Weighted shares outstanding - Basic (shares)	16,533,748	16,403,776	16,466,106	16,396,646
Effect of dilutive securities:
Share-based compensation (shares)	59,094	50,091	57,094	47,585
Adjusted denominator-Diluted (shares)	16,592,842	16,453,867	16,523,200	16,444,231
Income (Loss) from Continuing Operations, Per Diluted Share	$ 0.56	$ 0.38	$ 2.96	$ 2.67
Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Diluted Share	0	(0.04)	0.01	(0.08)
Diluted Earnings Per Share (in dollars per share)	$ 0.56	$ 0.34	$ 2.97	$ 2.59
Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, Net of Tax	$ (19)	$ (630)	$ 165	$ (1,388)
Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, Net of Tax	(19)	(630)	165	(1,388)
Net Income	$ 9,261	$ 5,621	$ 49,146	$ 42,589
Basic (shares)	16,533,748	16,403,776	16,466,106	16,396,646
Income (Loss) from Continuing Operations, Per Basic Share	$ 0.56	$ 0.38	$ 2.97	$ 2.68
Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Basic Share	0	(0.04)	0.01	(0.08)
Basic (in dollars per share)	$ 0.56	$ 0.34	$ 2.98	$ 2.60
Share-based compensation (shares)	59,094	50,091	57,094	47,585
Diluted (shares)	16,592,842	16,453,867	16,523,200	16,444,231
Income (Loss) from Continuing Operations, Per Diluted Share	$ 0.56	$ 0.38	$ 2.96	$ 2.67
Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Diluted Share	$ 0	$ (0.04)	$ 0.01	$ (0.08)
Retained Earnings [Member]
Earnings Per Share [Abstract]
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	$ 9,280	$ 6,251	$ 48,981	$ 43,977
Calculation of Basic Earnings Per Share:
Net Income	9,261	5,621	49,146	42,589
Reconciliation of Numerator:
Net Income	9,261	5,621	49,146	42,589
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	9,280	6,251	48,981	43,977
Net Income	$ 9,261	$ 5,621	$ 49,146	$ 42,589