Quarterly Financial Information (unaudited) (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
Quarterly Financial Information Disclosure [Abstract]
Total operating revenues	$ 174,109	$ 163,295	$ 155,540	$ 141,497	$ 171,208	$ 130,123	$ 103,622	$ 110,192	$ 634,441	$ 515,145	$ 383,659
Total operating expenses	153,146	158,394	145,370	136,642	123,590	108,929	92,989	95,651	593,552	421,159	370,990
Operating income	20,963	4,901	10,170	4,855	47,618	21,194	10,633	14,541	40,889	93,986	12,669
Income from continuing operations	14,034	5,521	24,807	(13,916)					30,446	18,826	8,525
Income from discontinued operations, net of income tax	30	(1,237)	2,705	(182)					1,316
Net income (loss)	$ 14,064	$ 4,284	$ 27,512	$ (14,098)	$ 36,490	$ 3,998	$ (19,036)	$ (2,626)	$ 31,762	$ 18,826	$ 8,525
Income (loss) per common share:
Basic and diluted from continuing operations	$ 0.39	$ 0.15	$ 0.69	$ (0.54)
Basic and diluted from discontinued operations		(0.03)	0.08	(0.01)
Basic and diluted	$ 0.39	$ 0.12	$ 0.77	$ (0.55)
Basic					$ 1.81	$ 0.20	$ (0.95)	$ (0.13)	$ 0.96	$ 0.94	$ 0.49
Diluted					$ 1.80	$ 0.20	$ (0.95)	$ (0.13)	$ 0.95	$ 0.94	$ 0.48