Quarterly Results (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								6 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	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total operating revenue	$ 161,484	$ 158,731	$ 156,501	$ 154,138	$ 151,617	$ 152,382	$ 153,867	$ 154,125		$ 630,854	$ 611,991	$ 535,288
Operating income (loss)	26,994	28,329	21,169	16,754	18,110	9,475	8,252	10,673		93,246	46,510	22,526
Net income (loss)	$ 14,852	$ 15,534	$ 9,626	$ 6,583	$ 60,595	$ 3,534	$ (80)	$ 2,341	$ 16,209	$ 46,595	$ 66,390	$ (895)
Net income (loss) per share - basic (in dollars per share)	$ 0.31	$ 0.31	$ 0.19	$ 0.13	$ 1.23	$ 0.07	$ 0.00	$ 0.05	$ 0.33	$ 0.94	$ 1.35	$ (0.02)
Net income (loss) per share - diluted (in dollars per share)	$ 0.30	$ 0.31	$ 0.19	$ 0.13	$ 1.21	$ 0.07	$ 0.00	$ 0.05	$ 0.32	$ 0.93	$ 1.33	$ (0.02)