Quarterly Financial Information (unaudited) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total operating revenues	$ 171,208	$ 130,123	$ 103,622	$ 110,192	$ 104,056	$ 91,549	$ 86,191	$ 101,863	$ 515,145	$ 383,659	$ 235,034
Total operating expenses	123,590	108,929	92,989	95,651	108,431	94,338	80,589	87,632	421,159	370,990	352,436
Operating income (loss)	47,618	21,194	10,633	14,541	(4,375)	(2,789)	5,602	14,231	93,986	12,669	(117,402)
Net income (loss)	$ 36,490	$ 3,998	$ (19,036)	$ (2,626)	$ (13,122)	$ (16,694)	$ 11,494	$ 26,847	$ 18,826	$ 8,525	$ (206,735)
Income (loss) per common share:
Basic	$ 1.81	$ 0.20	$ (0.95)	$ (0.13)	$ (0.75)	$ (0.95)	$ 0.66	$ 1.54	$ 0.94	$ 0.49	$ (12.84)
Diluted	$ 1.80	$ 0.20	$ (0.95)	$ (0.13)	$ (0.74)	$ (0.95)	$ 0.65	$ 1.52	$ 0.94	$ 0.48	$ (12.84)