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Derivative Instruments - Narrative (Details)
1 Months Ended 3 Months Ended 6 Months Ended
Mar. 31, 2020
USD ($)
Jun. 30, 2019
USD ($)
Jun. 30, 2021
USD ($)
Jun. 30, 2020
USD ($)
BTU
Jun. 30, 2021
USD ($)
BTU
Jun. 30, 2020
USD ($)
Derivative [Line Items]            
Cash received (paid) in settlement of commodity derivative instruments     $ (10,359,000) $ 142,256,000 $ (7,954,000) $ 293,417,000
Interest Rate Swap on Line of Credit | Long            
Derivative [Line Items]            
Put option, basis points (as a percent) 0.25%          
Interest Rate Swap on Revolving Credit Facility | Long            
Derivative [Line Items]            
Put option, basis points (as a percent) 0.00%          
Term of derivative contract 4 years          
Derivative notional amount $ 250,000,000          
NYMEX Natural Gas Hedges | Natural Gas            
Derivative [Line Items]            
Cash received (paid) in settlement of commodity derivative instruments       $ 29,015,000   29,015,000
Quantity of natural gas contract terminated (in Btus) | BTU       39,000,000,000,000 39,000,000,000,000  
NYMEX Commodity Swaps | Natural Gas            
Derivative [Line Items]            
Cash received (paid) in settlement of commodity derivative instruments           $ 54,982,000
Line of Credit            
Derivative [Line Items]            
Credit facility, face amount $ 175,000,000          
Revolving Credit Facility            
Derivative [Line Items]            
Credit facility, face amount   $ 160,000,000        
Credit facility, modification period   3 years