Quarterly Financial Data - Schedule of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure Quarterly Financial Data Schedule Of Quarterly Financial Information [Abstract]
Revenues	$ 131,973	$ 92,626	$ 94,541	$ 160,464	$ 134,214	$ 93,400	$ 93,872	$ 168,831	$ 479,604	$ 490,316	$ 449,646
Operating Income	29,642	14,358	18,164	44,123	28,873	12,879	12,238	40,853	106,287	94,843	89,730
Net Income	22,562	5,622	8,304	28,665	17,801	5,538	6,385	26,856	65,153	56,580	58,124
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	17,169	6,246	8,913	28,814	17,796	6,090	5,705	27,271	61,142	56,862	60,326
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	(9)	(624)	(609)	(149)	$ 5	$ (552)	$ 680	$ (415)	(1,391)	$ (282)	$ (2,202)
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax	$ 5,402	$ 0	$ 0	$ 0					$ 5,402
Income (Loss) from Continuing Operations, Per Basic Share	$ 1.05	$ 0.38	$ 0.54	$ 1.76	$ 1.09	$ 0.37	$ 0.35	$ 1.67	$ 3.73	$ 3.48	$ 3.69
Earnings per share:
Basic (in usd per share)	1.38	0.34	0.51	1.75	1.09	0.34	0.39	1.64	3.97	3.46	3.56
Income (Loss) from Continuing Operations, Per Diluted Share	1.37	0.38	0.54	1.75	1.09	0.37	0.35	1.66	3.72	3.47	3.68
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax, Per Diluted Share	1.04	(0.04)	(0.04)	(0.01)	0	(0.04)	0.04	(0.02)	0.24	(0.02)	(0.13)
Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Basic Share	0.33	(0.04)	(0.03)	(0.01)	0	(0.03)	0.04	(0.03)	0.24	(0.02)	(0.13)
Diluted (in usd per share)	$ 0.33	$ 0.34	$ 0.50	$ 1.74	$ 1.09	$ 0.33	$ 0.39	$ 1.64	$ 3.96	$ 3.45	$ 3.55