Quarterly Results (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 155,572	$ 156,836	$ 130,309	$ 92,571	$ 87,285	$ 85,212	$ 85,701	$ 84,287	$ 535,288	$ 342,485	$ 326,946
Operating income (loss)	12,189	(3,929)	(7,046)	21,312	21,721	15,089	18,750	18,526	22,526	74,086	61,943
Net income (loss)	$ (184)	$ (7,596)	$ (6,995)	$ 13,880	$ 12,108	$ 7,996	$ 10,474	$ 10,286	$ (895)	$ 40,864	$ 33,883
Basic income (loss) per share (in dollars per share)	$ 0.00	$ (0.16)	$ (0.14)	$ 0.29	$ 0.24	$ 0.17	$ 0.22	$ 0.21	$ (0.02)	$ 0.84	$ 0.70
Diluted income (loss) per share (in dollars per share)	$ 0.00	$ (0.16)	$ (0.14)	$ 0.28	$ 0.24	$ 0.17	$ 0.21	$ 0.21	$ (0.02)	$ 0.83	$ 0.70