Derivatives (Gross Current and Noncurrent Assets and Liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Derivatives, Fair Value [Line Items]
Commodity derivative assets	$ 38,580	$ 22,285	$ (18,874)
Commodity derivative liabilities	0	0	0
Commodity derivative assets - current	158,277	51,421	43,810
Commodity derivative assets - noncurrent	3,757	480	1,739
Commodity derivative liabilities - current	(66,623)	(32,647)	(15,424)
Commodity derivative liabilities - noncurrent	(3,821)	(889)	(1,414)
Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	146,801	44,658	34,874
Commodity derivative liabilities	(76,962)	(41,237)	(60,925)
Cash collateral paid (received)	21,751	14,944	54,762
Total	91,590	18,365	28,711
Commodity \| Commodity Derivative Assets - Current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	142,262	43,463	29,861
Commodity derivative liabilities	(5,736)	(6,986)	(40,813)
Cash collateral paid (received)	21,751	14,944	54,762
Commodity derivative assets - current	158,277	51,421	43,810
Commodity \| Commodity Derivative Assets - Noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	3,781	484	1,851
Commodity derivative liabilities	(24)	(4)	(112)
Cash collateral paid (received)	0	0	0
Commodity derivative assets - noncurrent	3,757	480	1,739
Commodity \| Commodity Derivative Liabilities - Current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	665	706	3,115
Commodity derivative liabilities	(67,288)	(33,353)	(18,539)
Cash collateral paid (received)	0	0	0
Commodity derivative liabilities - current	(66,623)	(32,647)	(15,424)
Commodity \| Commodity Derivative Liabilities - Noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	93	5	47
Commodity derivative liabilities	(3,914)	(894)	(1,461)
Cash collateral paid (received)	0	0	0
Commodity derivative liabilities - noncurrent	$ (3,821)	$ (889)	$ (1,414)

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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