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Debt (Details 1) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Total $ 259,146 $ 259,122
Senior Notes [Member]    
Total Principal 260,000 260,000
Unamortized debt issuance costs (854) (878)
Total 259,146 259,122
Senior Notes [Member] | 2025 Senior Unsecured Notes [Member]    
Debt Instrument, Face Amount 50,000 50,000
Senior Notes [Member] | 2027 Senior Unsecured Notes [Member]    
Debt Instrument, Face Amount 50,000 50,000
Senior Notes [Member] | 2028 Senior Unsecured Notes [Member]    
Debt Instrument, Face Amount 60,000 60,000
Senior Notes [Member] | 2029 Senior Unsecured Notes [Member]    
Debt Instrument, Face Amount $ 100,000 $ 100,000