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Investments - Below investment grade securities (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities    
Debt securities, amortized cost $ 526,678  
Available-for-sale securities 529,240 $ 547,749
Private label residential mortgage backed securities    
Schedule of Available-for-sale Securities    
Debt securities, amortized cost 1,723 $ 1,807
Below investment grade | Private label residential mortgage backed securities    
Schedule of Available-for-sale Securities    
Debt securities, amortized cost $ 1,578