Quarterly Financial 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 financial data and earnings per share [Abstract]
Revenues	$ 348,255	$ 439,893	$ 600,558	$ 555,573	$ 849,736	$ 1,173,970	$ 1,159,931	$ 949,189	$ 1,944,279	$ 4,132,826	$ 3,945,969
Earnings (Loss) from continuing operations	$ (8,404)	$ (308)	$ 4,340	$ 3,097	$ (6,974)	$ 3,855	$ 3,975	$ 5,363	$ (1,275)	$ 6,219	$ 22,121
Earnings (Loss) from continuing operations, Per Share (in dollars per share)	$ (1.99)	$ (0.07)	$ 1.03	$ 0.73	$ (1.65)	$ 0.92	$ 0.94	$ 1.27	$ (0.30)	$ 1.48	$ 5.24
Net Earnings (Loss), Amount	$ (8,404)	$ (308)	$ 4,340	$ 3,097	$ (6,670)	$ 3,855	$ 3,975	$ 5,363	$ (1,275)	$ 6,523	$ 21,610
Net Earnings (Loss), Per Share (in dollars per share)	$ (1.99)	$ (0.07)	$ 1.03	$ 0.73	$ (1.58)	$ 0.92	$ 0.94	$ 1.27	$ (0.30)	$ 1.55	$ 5.12
Dividends	$ 928	$ 928	$ 928	$ 928	$ 927	$ 928	$ 928	$ 928	$ 3,712	$ 3,711	$ 2,783
Dividends, Per share (in dollars per share)	$ 0.22	$ 0.22	$ 0.22	$ 0.22	$ 0.22	$ 0.22	$ 0.22	$ 0.22	$ 0.88	$ 0.88