Fair Value Measurements Proved and Unproved Oil and Gas Properties and Other Property and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of Proved Oil and Gas Properties	$ 0	$ 0	$ 0	$ 0
Abandonment and impairment of unproved oil and gas properties	9,055	$ 0	26,615	157
Properties held for sale, net	5,040		5,040		$ 111,700
Properties Held-for-sale, Fair Value	5,040		5,040		0
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Properties held for sale, net	$ 5,040		$ 5,040		$ 111,700
Divide County Disposal Group [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down				$ 526,500