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Marketable Securities Marketable Securities (Details 1) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities $ 142,258 $ 121,151
Cash, fair value 41,374 51,031
Cash, Cash Equivalents and Short Term Investments, Fair Value Disclosure 183,632 172,182
Cash Equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities 140,971 84,263
Short-term Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities $ 1,287 $ 36,888