Quarterly Results (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 151,617	$ 152,382	$ 153,867	$ 154,125	$ 155,572	$ 156,836	$ 130,309	$ 92,571	$ 611,991	$ 535,288	$ 342,485
Operating income (loss)	18,110	9,475	8,252	10,673	12,189	(3,929)	(7,046)	21,312	46,510	22,526	74,086
Net income (loss)	$ 60,595	$ 3,534	$ (80)	$ 2,341	$ (184)	$ (7,596)	$ (6,995)	$ 13,880	$ 66,390	$ (895)	$ 40,864
Net income (loss) per share - basic (in dollars per share)	$ 1.23	$ 0.07	$ 0.00	$ 0.05	$ 0.00	$ (0.16)	$ (0.14)	$ 0.29	$ 1.35	$ (0.02)	$ 0.84
Net income (loss) per share - diluted (in dollars per share)	$ 1.21	$ 0.07	$ 0.00	$ 0.05	$ 0.00	$ (0.16)	$ (0.14)	$ 0.28	$ 1.33	$ (0.02)	$ 0.83