Selected Quarterly Data (Unaudited) (Details) - 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
Quarterly Financial Data [Abstract]
Operating revenues	$ 70,209	$ 63,588	$ 69,633	$ 81,198	$ 99,866	$ 94,424	$ 94,722	$ 94,894	$ 284,628	$ 383,906
Operating income (loss)	(4,338)	(10,353)	(6,308)	(5,035)	(33,682)	950	(1,791)	(6,342)	(26,034)	(40,865)
Income (loss) before income taxes	(5,673)	(11,421)	(7,318)	23,849	(35,942)	(2,098)	(4,852)	(8,862)	(562)	(51,754)
Net income (loss)	$ 1,639	$ (9,984)	$ (6,734)	$ 32,629	$ (29,341)	$ (2,480)	$ (2,653)	$ (7,386)	$ 17,550	$ (41,860)
Basic earnings (loss) per common share (in dollars per share)	$ 0.21	$ (1.62)	$ (1.10)	$ 5.24	$ (4.73)	$ (0.40)	$ (0.43)	$ (1.20)	$ 2.39	$ (6.76)
Diluted earnings (loss) per common share (in dollars per share)	$ 0.21	$ (1.62)	$ (1.10)	$ 4.67	$ (4.73)	$ (0.40)	$ (0.43)	$ (1.20)	$ 2.38	$ (6.76)