Annual report pursuant to Section 13 and 15(d)

Derivative Instruments and Hedging Activities (Schedule of Derivative Instruments in Statement of Financial Position, Fair Value) (Details)

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Derivative Instruments and Hedging Activities (Schedule of Derivative Instruments in Statement of Financial Position, Fair Value) (Details) - Not Designated as Hedging Instrument - USD ($)
Dec. 31, 2019
Dec. 31, 2018
Fair value of derivatives - Current    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting $ 43,567,000 $ 78,091,000
Effects of Netting (17,511,000) (12,977,000)
As Presented with Effects of Netting 26,056,000 65,114,000
Fair value of derivatives - Current | Contingent consideration arrangements    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting 16,718,000  
Effects of Netting 0  
As Presented with Effects of Netting 16,718,000  
Fair value of derivatives - Current | Commodity derivative instruments    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting 26,849,000  
Effects of Netting (17,511,000)  
As Presented with Effects of Netting 9,338,000  
Fair value of derivatives - Non-current    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting 9,216,000  
Effects of Netting 0  
As Presented with Effects of Netting 9,216,000  
Fair value of derivatives - Non-current | Contingent consideration arrangements    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting 9,216,000  
Effects of Netting 0  
As Presented with Effects of Netting 9,216,000  
Fair value of derivatives - Non-current | Commodity derivative instruments    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting 0  
Effects of Netting 0  
As Presented with Effects of Netting 0  
Fair value of derivatives - Current    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting (88,708,000) (23,457,000)
Effects of Netting 17,511,000 12,977,000
As Presented with Effects of Netting (71,197,000) (10,480,000)
Fair value of derivatives - Current | Contingent consideration arrangements    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting (50,000,000)  
Effects of Netting 0  
As Presented with Effects of Netting (50,000,000)  
Fair value of derivatives - Current | Commodity derivative instruments    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting (38,708,000)  
Effects of Netting 17,511,000  
As Presented with Effects of Netting (21,197,000)  
Fair value of derivatives - Non-current    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting (32,695,000) (7,440,000)
Effects of Netting 0 0
As Presented with Effects of Netting (32,695,000) $ (7,440,000)
Fair value of derivatives - Non-current | Contingent consideration arrangements    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting (19,760,000)  
Effects of Netting 0  
As Presented with Effects of Netting (19,760,000)  
Fair value of derivatives - Non-current | Commodity derivative instruments    
Offsetting Assets and Liabilities [Line Items]    
Presented without Effects of Netting (12,935,000)  
Effects of Netting 0  
As Presented with Effects of Netting $ (12,935,000)