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Summary of Significant Accounting Policies - Investments by Maturity Date (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Amortized Cost    
Investments amortized cost maturity dates 2017 $ 297,748  
Investments amortized cost maturity dates 2018 7,792  
Amortized Cost 305,540 $ 326,589
Estimated Fair Value    
Investments fair value maturity dates 2017 297,493  
Investments fair value maturity dates 2018 7,760  
Estimated Fair Value $ 305,253 $ 326,165