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Cash and Cash Equivalents and Investments in Marketable Securities (Narrative) (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Investment Income, Interest $ 819 $ 75
Maximum term to maturity from date of purchase (years) 1 year  
Available-for-sale Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Investment Income, Interest $ 300