Quarterly Financial Data (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
Selected quarterly financial data and earnings per share [Abstract]
Revenues	$ 408,460	$ 295,311	$ 315,202	$ 303,087	$ 298,969	$ 256,877	$ 293,163	$ 250,531	$ 1,322,060	$ 1,099,540	$ 1,944,279
Operating (losses) earnings	3,757	(1,290)	619	(1,584)	(64)	(1,822)	5,601	2,339
Earnings (losses) from continuing operations	3,693	(3,033)	(282)	(860)	(168)	(983)	3,540	1,554	(482)	3,943	(1,275)
Net (losses) earnings	$ 3,693	$ (3,033)	$ (282)	$ (860)	$ (168)	$ (2,153)	$ 3,404	$ 1,430	$ (482)	$ 2,513	$ (1,275)
Earnings (losses) per share:
From continuing operations (in dollars per share)	$ 0.88	$ (0.72)	$ (0.07)	$ (0.20)	$ (0.04)	$ (0.23)	$ 0.84	$ 0.37	$ (0.11)	$ 0.94	$ (0.30)
From investment in unconsolidated affiliate (in dollars per share)	0.00	0.00	0.00	0.00	0.00	(0.28)	(0.03)	(0.03)	0.00	(0.34)	0.00
Basic and diluted net (losses) earnings per common share (in dollars per share)	$ 0.88	$ (0.72)	$ (0.07)	$ (0.20)	$ (0.04)	$ (0.51)	$ 0.81	$ 0.34	$ (0.11)	$ 0.60	$ (0.30)