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Summary of Significant Accounting Policies Investments by Maturity Date (Details) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Amortized Cost    
Investments amortized cost maturity dates 2016 $ 322,747  
Investments amortized cost maturity dates 2017 3,842  
Amortized Cost 326,589 $ 267,891
Estimated Fair Value    
Investments fair value maturity dates 2016 322,339  
Investments fair value maturity dates 2017 3,826  
Estimated Fair Value $ 326,165 $ 267,654