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General - Investments by Maturity Date (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Amortized Cost    
Investments amortized cost maturity dates 2018 $ 65,167  
Investments amortized cost maturity dates 2019 313,016  
Investment amortized cost maturity dates 2020 1,067  
Amortized Cost 379,250 $ 328,364
Estimated Fair Value    
Investments fair value maturity dates 2018 65,132  
Investments fair value maturity dates 2019 312,563  
Investments fair value maturity dates 2020 1,064  
Estimated Fair Value $ 378,759 $ 327,865