Derivatives (Effect of Derivative Instruments Recognized in Income Statements, Derivative Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	$ 483	$ (6,932)
Cash receipts on derivatives settled	551	2,656
Gain (loss) on derivatives [Member] \| Commodity Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	$ 483	$ (6,932)

Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4CC
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109998890-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration