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Marketable Securities - Maturities (Details)
$ in Thousands
Dec. 31, 2017
USD ($)
Amortized Cost  
Maturing in one year or less $ 208,200
Maturing after one year 140,399
Total amortized cost 348,599
Estimate Fair Value  
Maturing in one year or less 207,603
Maturing after one year 139,197
Total estimated fair value $ 346,800