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Bank Debt (Narrative) (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2017
Mar. 18, 2016
Mar. 17, 2016
Debt Instrument [Line Items]      
Closing costs of debt $ 9.1    
Loan initiation amortization period 5 years    
Credit agreement, amount outstanding $ 225.0    
Credit facility, maximum borrowing capacity   $ 200.0 $ 250.0
Debt, unused borrowing capacity 63.0    
Liquidity $ 79.0    
Maximum leverage ratio 276.00%    
Debt service coverage ratio 215.00%    
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] | 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 $ 137.0    
Credit Agreement [Member]      
Debt Instrument [Line Items]      
Debt maturity date Aug. 01, 2019    
Debt instrument, interest rate, effective percentage 4.54%    
Term Loan [Member]      
Debt Instrument [Line Items]      
Credit agreement, amount outstanding $ 88.0    
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