Selected Quarterly Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Selected Quarterly Data (Unaudited)
Revenues	$ 129,815	$ 121,968	$ 122,345	$ 121,173	$ 146,518	$ 134,121	$ 140,310	$ 132,727	$ 495,301	$ 553,676	$ 559,609
Operating income (loss)	6,792	(205,438)	8,057	3,015	14,656	10,540	10,987	4,579	(187,575)	40,762	42,661
Net income (loss)	$ 2,543	$ (170,914)	$ (4,172)	$ 1,615	$ 10,089	$ 6,420	$ 5,637	$ 1,845	$ (170,928)	$ 23,991	$ 13,370
Basic earnings (loss) per common share (in dollars per share)	$ 0.04	$ (2.77)	$ (0.07)	$ 0.03	$ 0.16	$ 0.10	$ 0.09	$ 0.03	$ (2.77)	$ 0.38	$ 0.21
Diluted earnings (loss) per common share (in dollars per share)	$ 0.04	$ (2.77)	$ (0.07)	$ 0.03	$ 0.16	$ 0.10	$ 0.09	$ 0.03	$ (2.77)	$ 0.38	$ 0.21