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Derivatives - Fair Values (Gross and Net) of Outstanding Derivatives (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Liabilities:    
Total derivatives $ 9,861 $ 91,885
Commodity Contracts | Current assets    
Assets:    
Gross Amounts Recognized at Fair Value 21,987 89,981
Gross Amounts Offset in the Balance Sheet (5,542) (1,385)
Net Fair Value Presented on the Balance Sheet 16,445 88,596
Commodity Contracts | Non-current assets    
Assets:    
Gross Amounts Recognized at Fair Value 18 3,289
Gross Amounts Offset in the Balance Sheet 0 0
Net Fair Value Presented on the Balance Sheet 18 3,289
Commodity Contracts | Current liabilities    
Liabilities:    
Gross Amounts Recognized at Fair Value (12,144) (1,385)
Gross Amounts Offset in the Balance Sheet 5,542 1,385
Net Fair Value Presented on the Balance Sheet $ (6,602) $ 0