Oil and Gas Properties and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Proven Properties [Abstract]
Acquisition, equipment, exploration, drilling and environmental costs	$ 10,672,172	$ 10,480,165
Capitalized Costs, Accumulated Depreciation, Depletion, Amortization and Valuation Allowance for Relating to Oil and Gas Producing Activities	(9,713,408)	(9,629,020)
Proved	$ 958,764	$ 851,145