Derivatives (Gross Current and Noncurrent Assets and Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Derivatives, Fair Value [Line Items]
Commodity derivative assets	$ 68,244	$ 17,252	$ (12,835)
Commodity derivative liabilities	0	0	0
Commodity derivative assets - current	54,259	30,702	11,619
Commodity derivative assets - noncurrent	1,008	310	1,191
Commodity derivative liabilities - current	(85,160)	(29,651)	(18,104)
Commodity derivative liabilities - noncurrent	(3,202)	(825)	(334)
Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	125,601	37,730	31,708
Commodity derivative liabilities	(105,808)	(38,545)	(52,788)
Cash collateral	(52,888)	1,351	15,452
Total	(33,095)	536	(5,628)
Commodity Derivative Assets - Current \| Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	123,917	36,929	26,101
Commodity derivative liabilities	(16,770)	(7,578)	(29,934)
Cash collateral	(52,888)	1,351	15,452
Commodity derivative assets - current	54,259	30,702	11,619
Commodity Derivative Assets - Noncurrent \| Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	1,022	311	1,201
Commodity derivative liabilities	(14)	(1)	(10)
Cash collateral	0	0	0
Commodity derivative assets - noncurrent	1,008	310	1,191
Commodity Derivative Liabilities - Current \| Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	626	489	4,404
Commodity derivative liabilities	(85,786)	(30,140)	(22,508)
Cash collateral	0	0	0
Commodity derivative liabilities - current	(85,160)	(29,651)	(18,104)
Commodity Derivative Liabilities - Noncurrent \| Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	36	1	2
Commodity derivative liabilities	(3,238)	(826)	(336)
Cash collateral	0	0	0
Commodity derivative liabilities - noncurrent	$ (3,202)	$ (825)	$ (334)

Carrying amount as of the balance sheet date of the asset arising from commodity contracts such as futures contracts tied to the movement of a particular commodity, which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer.

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Carrying amount as of the balance sheet date of the asset arising from commodity contracts such as futures contracts tied to the movement of a particular commodity, which are expected to be converted into cash or otherwise disposed of after a year or beyond the normal operating cycle, if longer.

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Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.

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The net amount as of the balance sheet date of the fair value of derivative assets and derivative liabilities that in accordance with the entity's accounting policy were offset against collateral under a master netting arrangement.

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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