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Debt (Details Textual) - USD ($)
1 Months Ended 3 Months Ended
Sep. 30, 2017
May 31, 2017
Mar. 31, 2018
Dec. 31, 2017
Senior Notes due 2024        
Debt Instrument [Line Items]        
Net unamortized debt issuance costs     $ 4,700,000 $ 4,900,000
Debt Instrument, Face Amount   $ 250,000,000    
Proceeds from Debt   $ 245,000,000    
Stated interest rate   5.875%    
Silvertip Debt Obligation [Member]        
Debt Instrument [Line Items]        
Letters of Credit Outstanding, Amount       15,000,000
Repayments of Debt     12,600,000  
Letter of Credit [Member]        
Debt Instrument [Line Items]        
Letters of Credit Outstanding, Amount     12,000,000  
Revolving Credit Facility        
Debt Instrument [Line Items]        
Net unamortized debt issuance costs     $ 1,800,000 1,900,000
Line of Credit Facility, Maximum Borrowing Capacity $ 200,000,000.0      
Line of Credit Facility, Additional Borrowing Capacity $ 50,000,000.0      
Debt Instrument, Term 4 years      
Line of Credit Facility, Current Borrowing Capacity       73,000,000
Stated interest rate     4.10%  
Silvertip [Member]        
Debt Instrument [Line Items]        
Letters of Credit Outstanding, Amount       $ 100,000,000
Base Rate [Member] | Minimum | Revolving Credit Facility        
Debt Instrument [Line Items]        
Stated interest rate 1.00%      
Base Rate [Member] | Maximum | Revolving Credit Facility        
Debt Instrument [Line Items]        
Stated interest rate 1.75%      
London Interbank Offered Rate (LIBOR) [Member] | Minimum | Revolving Credit Facility        
Debt Instrument [Line Items]        
Stated interest rate 2.00%      
London Interbank Offered Rate (LIBOR) [Member] | Maximum | Revolving Credit Facility        
Debt Instrument [Line Items]        
Stated interest rate 2.75%