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Marketable Securities - Maturities (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Amortized Cost    
Maturing in one year or less $ 141,371 $ 115,748
Maturing after one year through five years 238,906 274,632
Total amortized cost 380,277 390,380
Estimate Fair Value    
Maturing in one year or less 141,225 115,608
Maturing after one year through five years 237,865 273,340
Total estimated fair value $ 379,090 $ 388,948