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Marketable Securities Marketable Securities (Details 1) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities $ 257,570 $ 261,208
Cash, fair value 40,770 68,168
Cash, Cash Equivalents and Short Term Investments, Fair Value Disclosure 298,340 329,376
Cash Equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities 207,514 157,676
Short-term Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities $ 50,056 $ 103,532