Derivatives (Effect of Derivative Instruments Recognized in Income Statements, Derivative Instruments) (Details) - USD ($) $ in Thousands	1 Months Ended	2 Months Ended	3 Months Ended	8 Months Ended	9 Months Ended
Sep. 30, 2020	Aug. 31, 2020	Sep. 30, 2019	Aug. 31, 2020	Sep. 30, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	$ 3,939	$ (4,250)	$ 4,237	$ (10,704)	$ 5,232
Cash receipts on derivatives settled	(1,418)	(3,552)	6,515	(4,244)	11,829
Gain (loss) on derivatives [Member] \| Commodity Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivatives	$ 3,939	$ (4,250)	$ 4,237	$ (10,704)	$ 5,232

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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