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Investments - Schedule of Investments Classified by Contractual Maturity Date (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]    
Amortized cost, Due in less than one year $ 36,142  
Amortized cost, Due after one year but within five years 171,631  
Amortized cost, Due after five years but within ten years 97,947  
Amortized cost, Due after ten years 80,113  
Amortized cost, Total 385,833 $ 388,520
Estimated fair value, Due in less than one year 36,019  
Estimated fair value, Due after one year but within five years 166,705  
Estimated fair value, Due after five years but within ten years 95,324  
Estimated fair value, Due after ten years 77,987  
Estimated fair value, Total $ 376,039 $ 387,983