Derivatives (Gross Current and Noncurrent Assets and Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Derivatives, Fair Value [Line Items]
Commodity derivative assets	$ 10,199	$ 22,285	$ 36,653
Commodity derivative liabilities	0	0	0
Commodity derivative assets - current	120,510	51,421	76,861
Commodity derivative assets - noncurrent	1,943	480	766
Commodity derivative liabilities - current	(67,606)	(32,647)	(91,403)
Commodity derivative liabilities - noncurrent	(1,960)	(889)	(2,548)
Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	130,195	44,658	71,493
Commodity derivative liabilities	(101,305)	(41,237)	(102,759)
Cash collateral paid (received)	23,997	14,944
Cash collateral (received)			14,942
Total	52,887	18,365	(16,324)
Commodity \| Commodity Derivative Assets - Current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	122,462	43,463	69,957
Commodity derivative liabilities	(29,566)	(6,986)	(8,038)
Cash collateral paid (received)	27,614	14,944
Cash collateral (received)			14,942
Commodity derivative assets - current	120,510	51,421	76,861
Commodity \| Commodity Derivative Assets - Noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	1,951	484	767
Commodity derivative liabilities	(8)	(4)	(1)
Cash collateral paid (received)	0	0
Cash collateral (received)			0
Commodity derivative assets - noncurrent	1,943	480	766
Commodity \| Commodity Derivative Liabilities - Current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	5,715	706	731
Commodity derivative liabilities	(69,704)	(33,353)	(92,134)
Cash collateral paid (received)	(3,617)	0
Cash collateral (received)			0
Commodity derivative liabilities - current	(67,606)	(32,647)	(91,403)
Commodity \| Commodity Derivative Liabilities - Noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	67	5	38
Commodity derivative liabilities	(2,027)	(894)	(2,586)
Cash collateral paid (received)	0	0
Cash collateral (received)			0
Commodity derivative liabilities - noncurrent	$ (1,960)	$ (889)	$ (2,548)

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 obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

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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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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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