Property and Equipment (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Impairment of oil and natural gas properties	$ 0	$ 170,621	$ 0	$ 389,612
Cumulative capitalization of general and administrative costs incurred and capitalized to the full cost pool	146,600		146,600
Capitalized general and administrative costs	8,300	$ 7,900	16,700	$ 15,000
Capitalized costs of oil and natural gas properties excluded from amortization	$ 3,109,143		$ 3,109,143		$ 1,580,305
Non-producing leases, extension term			5 years
Minimum
Property, Plant and Equipment [Line Items]
Expected number of years amortization will commence			3 years
Maximum
Property, Plant and Equipment [Line Items]
Expected number of years amortization will commence			5 years