Risk Management Activities (Details)		6 Months Ended
		Jun. 30, 2018 USD ($) MMBTU $ / bbl $ / MMBTU bbl	Dec. 31, 2017 USD ($)
Derivatives, Fair Value [Line Items]
Assets from Price Risk Management Activities		$ 1,840,000	$ 7,699,000
Liabilities from Price Risk Management Activities		195,457,000	50,429,000
Derivative Collateral [Abstract]
Collateral Had on Derivative		0	0
Collateral Held on Derivative		0	0
Other Assets [Member] | Price Risk Derivative [Member]
Derivatives, Fair Value [Line Items]
Other Assets		17,000,000	0
Liabilities From Price Risk Management Activities [Member]
Derivatives, Fair Value [Line Items]
Assets from Price Risk Management Activities		66,000,000	5,000,000
Liabilities from Price Risk Management Activities		261,000,000	55,000,000
Liabilities From Price Risk Management Activities [Member] | Price Risk Derivative [Member]
Derivatives, Fair Value [Line Items]
Liabilities from Price Risk Management Activities	[1]	195,000,000	50,000,000
Other Liabilities [Member] | Price Risk Derivative [Member]
Derivatives, Fair Value [Line Items]
Other Liabilities		0	7,000,000
Assets From Price Risk Management Activities [Member] | Price Risk Derivative [Member]
Derivatives, Fair Value [Line Items]
Assets from Price Risk Management Activities		$ 2,000,000	$ 8,000,000
Crude Oil [Member] | Midland Differential Basis Swaps [Member] | Derivative Contracts - January through July (closed) [Member]
Derivatives, Fair Value [Line Items]
Volume (Bbld) | bbl		15,000
Derivative, Swap Type, Average Fixed Price | $ / bbl		1.063
Crude Oil [Member] | Midland Differential Basis Swaps [Member] | Derivative Contracts - August through December [Member]
Derivatives, Fair Value [Line Items]
Volume (Bbld) | bbl		15,000
Derivative, Swap Type, Average Fixed Price | $ / bbl		1.063
Crude Oil [Member] | Midland Differential Basis Swaps [Member] | Derivative Contracts Year Two - January through December [Member]
Derivatives, Fair Value [Line Items]
Volume (Bbld) | bbl		20,000
Derivative, Swap Type, Average Fixed Price | $ / bbl		1.075
Crude Oil [Member] | Gulf Coast Differential Basis Swap [Member] | Derivative Contracts - January through July (closed) [Member]
Derivatives, Fair Value [Line Items]
Volume (Bbld) | bbl		37,000
Derivative, Swap Type, Average Fixed Price | $ / bbl		3.818
Crude Oil [Member] | Gulf Coast Differential Basis Swap [Member] | Derivative Contracts - August through December [Member]
Derivatives, Fair Value [Line Items]
Volume (Bbld) | bbl		37,000
Derivative, Swap Type, Average Fixed Price | $ / bbl		3.818
Crude Oil [Member] | Gulf Coast Differential Basis Swap [Member] | Derivative Contracts Year Two - January through December [Member]
Derivatives, Fair Value [Line Items]
Volume (Bbld) | bbl		8,000
Derivative, Swap Type, Average Fixed Price | $ / bbl		5.660
Crude Oil [Member] | Price Swaps [Member] | Derivative Contracts - January through June (closed) [Member]
Derivatives, Fair Value [Line Items]
Volume (Bbld) | bbl		134,000
Derivative, Swap Type, Average Fixed Price | $ / bbl		60.04
Crude Oil [Member] | Price Swaps [Member] | Derivative Contracts - July through December [Member]
Derivatives, Fair Value [Line Items]
Volume (Bbld) | bbl		134,000
Derivative, Swap Type, Average Fixed Price | $ / bbl		60.04
Natural Gas [Member] | Price Swaps [Member] | Derivative Contracts - March through July (closed) [Member]
Derivatives, Fair Value [Line Items]
Derivative, Swap Type, Average Fixed Price | $ / MMBTU		3.00
Volume (MMBtud) | MMBTU		35,000
Natural Gas [Member] | Price Swaps [Member] | Derivative Contracts - August through November [Member]
Derivatives, Fair Value [Line Items]
Derivative, Swap Type, Average Fixed Price | $ / MMBTU		3.00
Volume (MMBtud) | MMBTU		35,000
Natural Gas [Member] | Call Option [Member] | Derivative Contracts - March through July (closed) [Member]
Derivatives, Fair Value [Line Items]
Volume (MMBtud) | MMBTU		120,000
Derivative, Average Price Risk Option Strike Price | $ / MMBTU		3.38
Natural Gas [Member] | Call Option [Member] | Derivative Contracts - August through November [Member]
Derivatives, Fair Value [Line Items]
Volume (MMBtud) | MMBTU		120,000
Derivative, Average Price Risk Option Strike Price | $ / MMBTU		3.38
Natural Gas [Member] | Put Option [Member] | Derivative Contracts - March through July (closed) [Member]
Derivatives, Fair Value [Line Items]
Volume (MMBtud) | MMBTU		96,000
Derivative, Average Price Risk Option Strike Price | $ / MMBTU		2.94
Natural Gas [Member] | Put Option [Member] | Derivative Contracts - August through November [Member]
Derivatives, Fair Value [Line Items]
Volume (MMBtud) | MMBTU		96,000
Derivative, Average Price Risk Option Strike Price | $ / MMBTU		2.94

[1] The current portion of Liabilities from Price Risk Management Activities consists of gross liabilities of $261 million, partially offset by gross assets of $66 million at June 30, 2018, and gross liabilities of $55 million, partially offset by gross assets of $5 million at December 31, 2017.