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Marketable Securities Marketable Securities Table (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Gross Unrealized Gain (Loss) $ 113    
Available-for-sale Securities, Gross Unrealized Loss   $ 93  
Available-for-sale Securities, Gross Unrealized Gain   4  
Available-for-sale Securities, Current 57,896 50,976 $ 64,027
Available-for-sale Securities, Noncurrent 0 33,839 29,523
Bonds [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Current 26,451   11,979
Available-for-sale Securities, Noncurrent 0   29,523
Certificates of Deposit [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Current 31,445   52,048
Available-for-sale Securities, Noncurrent $ 0   $ 0
Other Long-term Investments [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Gross Unrealized Loss   29  
Available-for-sale Securities, Gross Unrealized Gain   $ 37