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 (Unaudited)
Operating revenues	$ 91,598	$ 91,013	$ 91,298	$ 94,510	$ 96,780	$ 97,848	$ 99,541	$ 98,794	$ 368,419	$ 392,963	$ 393,413
Operating Income (Loss)	(333)	(1,088)	2,767	655	3,700	3,703	6,889	4,500	2,001	18,792	19,262
Net Income (Loss)	$ (9,094)	$ (92,719)	$ (3,476)	$ (1,952)	$ (214)	$ (279)	$ 1,445	$ 154	$ (107,241)	$ 1,106	$ 1,100
Earnings (Loss) per share - basic (in dollars per share)	$ (0.780)	$ (8.000)	$ (0.300)	$ (0.170)	$ (0.020)	$ (0.020)	$ 0.130	$ 0.010	$ (9.270)	$ 0.100	$ 0.10
Earnings (Loss) per share - diluted (in dollars per share)	$ (0.780)	$ (8.000)	$ (0.300)	$ (0.170)	$ (0.020)	$ (0.020)	$ 0.130	$ 0.010	$ (9.270)	$ 0.100	$ 0.10
Loss on early extinguishment of debt									$ (4,826)
Pension settlement losses	$ 600	$ 1,100	$ 300	$ 2,000	$ 800	$ 500
Change in valuation allowance		$ 89,400							$ 67,523	$ (15)	$ (132)