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Debt - Credit Facility (Details) - Long-term Debt [Member] - USD ($)
 12 Months Ended
Dec. 31, 2017
Jun. 26, 2015
U.S revolving credit facility [Member]    
Line of Credit Facility [Line Items]    
Credit facility face amount $ 150,000,000  
Outstanding borrowings 0  
Credit facility maximum amount 150,000,000  
Borrowing availability 147,706,000  
Borrowing availability, net of outstanding letters of credit and borrowings 109,849,000  
Letters of credit [Member]    
Line of Credit Facility [Line Items]    
Letter of credit sub-facility amount   $ 110,000,000
Outstanding borrowings 0  
Outstanding letters of credit issued 37,857,000  
Iceland revolving credit facility [Member]    
Line of Credit Facility [Line Items]    
Outstanding borrowings 0  
Credit facility maximum amount 50,000,000  
Borrowing availability 50,000,000  
Outstanding letters of credit issued 0  
Borrowing availability, net of outstanding letters of credit and borrowings $ 50,000,000  
Prepayment terms Any outstanding borrowings may be prepaid without penalty or premium (except incurred breakage costs) in whole or in part.