Selected Quarterly Financial Information (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]
Revenues	$ 43,770	$ 43,636	$ 42,325	$ 41,444	$ 44,184	$ 45,355	$ 44,635	$ 45,388	$ 171,175	$ 179,561	$ 189,628
Operating income	3,589	3,325	2,410	1,382	4,703	6,029	5,620	5,800	10,706	22,153	25,100
Net income (loss)	$ (21,384)	$ 3,727	$ 1,498	$ 854	$ 3,026	$ 4,058	$ 3,451	$ 3,444	$ (15,306)	$ 13,979	$ 80,246
Basic and diluted net (loss) income per common share (usd per share)	$ (1.07)	$ 0.19	$ 0.07	$ 0.04	$ 0.15	$ 0.20	$ 0.17	$ 0.17	$ (0.76)	$ 0.68	$ 3.74
Reduction of DTA	$ 24,200