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Investments - Schedule of Investments Classified by Contractual Maturity Date (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]    
Amortized cost, Due in less than one year $ 30,042  
Amortized cost, Due after one year but within five years 196,229  
Amortized cost, Due after five years but within ten years 94,540  
Amortized cost, Due after ten years 84,605  
Amortized cost, Total 405,416 $ 388,520
Estimated fair value, Due in less than one year 29,939  
Estimated fair value, Due after one year but within five years 192,202  
Estimated fair value, Due after five years but within ten years 92,727  
Estimated fair value, Due after ten years 83,269  
Estimated fair value, Total $ 398,137 $ 387,983