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Derivatives (Disclosure of Fair Value of Derivatives) (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Derivative [Line Items]    
Derivative Assets $ 18,879 $ 55,550
Derivative Liabilities (31,089) (57,259)
Effect of counterparty netting in derivative assets (17,801) (50,042)
Effect of counterparty netting in derivative liabilities 17,801 50,042
Net derivative assets as classified in the balance sheet 1,078 5,508
Net derivative liabilities as classified in the balance sheet (13,288) (7,217)
Net derivatives as classified in the balance sheet (12,210) (1,709)
Derivatives Not Designated as Hedging Instruments    
Derivative [Line Items]    
Derivative Assets 16,537 54,608
Derivative Liabilities (30,284) (55,113)
Derivatives Not Designated as Hedging Instruments | Coal held for trading purposes, exchange traded swaps and futures    
Derivative [Line Items]    
Derivative Assets 10,329 44,088
Derivative Liabilities (10,701) (45,221)
Coal | Derivatives Designated as Hedging Instruments    
Derivative [Line Items]    
Derivative Assets 2,342 942
Derivative Liabilities (805) (2,146)
Coal | Derivatives Not Designated as Hedging Instruments    
Derivative [Line Items]    
Derivative Assets 5,672 5,139
Derivative Liabilities (19,579) (9,892)
Heating oil -- diesel purchases | Derivatives Not Designated as Hedging Instruments    
Derivative [Line Items]    
Derivative Assets 532 5,354
Derivative Liabilities 0 0
Natural gas | Derivatives Not Designated as Hedging Instruments    
Derivative [Line Items]    
Derivative Assets 4 27
Derivative Liabilities $ (4) $ 0