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Bank Debt (Narrative) (Details) - USD ($)
$ in Millions
1 Months Ended 9 Months Ended
Nov. 30, 2017
Sep. 30, 2017
Mar. 18, 2016
Mar. 17, 2016
Debt Instrument [Line Items]        
Closing costs of debt   $ 9.1    
Credit agreement, amount outstanding   $ 211.0    
Credit facility, maximum borrowing capacity     $ 200.0 $ 250.0
Maximum leverage ratio   242.00%    
Debt service coverage ratio   219.00%    
Minimum [Member]        
Debt Instrument [Line Items]        
Fixed charge coverage ratio   125.00%    
London Interbank Offered Rate (LIBOR) [Member]        
Debt Instrument [Line Items]        
Debt, interest rate spread on variable rate   3.50%    
London Interbank Offered Rate (LIBOR) [Member] | Scenario, Forecast [Member]        
Debt Instrument [Line Items]        
Debt, interest rate spread on variable rate 3.00%      
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]        
Debt Instrument [Line Items]        
Debt, interest rate spread on variable rate   2.25%    
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]        
Debt Instrument [Line Items]        
Debt, interest rate spread on variable rate   4.00%    
Revolving Credit Facility [Member]        
Debt Instrument [Line Items]        
Credit agreement, amount outstanding   $ 131.0    
Credit Agreement [Member]        
Debt Instrument [Line Items]        
Debt, unused borrowing capacity   69.0    
Liquidity   $ 83.0    
Debt maturity date   Aug. 01, 2019    
Debt instrument, interest rate, effective percentage   4.74%    
Term Loan [Member]        
Debt Instrument [Line Items]        
Credit agreement, amount outstanding   $ 80.0    
Sunrise Coal [Member] | PNC Bank [Member]        
Debt Instrument [Line Items]        
Percentage decrease in amount of debt with maximum interest rate per quarter after commencement date   10.00%    
Sunrise Coal [Member] | Maximum [Member] | PNC Bank [Member]        
Debt Instrument [Line Items]        
Interest rate maximum   5.00%    
Sunrise Coal [Member] | Revolving Credit Facility [Member] | PNC Bank [Member]        
Debt Instrument [Line Items]        
Amount of debt with maximum interest rate   $ 100.0