Quarterly Financial Data - Schedule of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure Quarterly Financial Data Schedule Of Quarterly Financial Information [Abstract]
Revenues	$ 137,038	$ 101,419	$ 97,051	$ 152,690	$ 131,974	$ 92,626	$ 94,542	$ 160,464	$ 488,198	$ 479,605	$ 490,316
Operating Income	35,206	17,406	17,977	42,134	29,641	14,357	18,165	44,122	112,723	106,285	94,844
Net Income	22,352	9,261	10,956	28,930	22,564	5,621	8,304	28,663	71,498	65,153	56,580
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	21,661	9,280	10,661	29,041	17,123	6,251	8,914	28,811	70,642	61,100	56,968
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	$ 691	$ (19)	$ 125	$ (111)	39	$ (630)	$ (610)	$ (148)	686	(1,349)	$ (388)
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax					$ 5,402				$ 170	$ 5,402
Income (Loss) from Continuing Operations, Per Basic Share	$ 1.24	$ 0.56	$ 0.65	$ 1.77	$ 1.05	$ 0.38	$ 0.54	$ 1.76	$ 4.23	$ 3.73	$ 3.48
Earnings per share:
Basic (in usd per share)	1.28	0.56	0.67	1.76	1.38	0.34	0.51	1.75	4.28	3.97	3.46
Income (Loss) from Continuing Operations, Per Diluted Share	1.24	0.56	0.64	1.77	1.04	0.38	0.54	1.75	4.21	3.72	3.47
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax, Per Diluted Share	0.04	0	0.02	(0.01)	0.33	(0.04)	(0.04)	(0.01)	0.05	0.24	(0.02)
Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Basic Share	0.04	0	0.02	(0.01)	0.33	(0.04)	(0.03)	(0.01)	0.05	0.24	(0.02)
Diluted (in usd per share)	$ 1.28	$ 0.56	$ 0.66	$ 1.76	$ 1.37	$ 0.34	$ 0.50	$ 1.74	$ 4.26	$ 3.96	$ 3.45