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Debt Commitments and Contingencies - Debt Obligations and Letters of Credit Outstanding (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]    
Total debt $ 390,000 $ 365,000
Less: Current portion of debt (10,482) (62,050)
Long-term debt 379,518 302,950
Credit Facility [Member] | Unsecured Revolving Credit Facility [Member]    
Debt Instrument [Line Items]    
Total debt 305,000 350,000
Credit Facility [Member] | Letter of Credit Subfacility [Member]    
Debt Instrument [Line Items]    
Outstanding letters of credit under unsecured revolving Credit Facility - contingent obligation 36,267 36,266
Senior Unsecured Promissory Notes [Member] | 2.00% Senior Unsecured Promissory Note Payable [Member]    
Debt Instrument [Line Items]    
Total debt 40,000 0
Senior Unsecured Promissory Notes [Member] | 2.45% Senior Unsecured Promissory Note Payable [Member]    
Debt Instrument [Line Items]    
Total debt 35,000 0
Note Payable Under Asset Purchase Agreement [Member]    
Debt Instrument [Line Items]    
Total debt $ 10,000 $ 15,000