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Marketable Securities Marketable Securities Table (Details) - USD ($)
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Gross Unrealized Gain (Loss) $ 4,000    
Available-for-sale Securities, Gross Unrealized Loss 93,000 $ 220,000  
Available-for-sale Securities, Gross Unrealized Gain 37,000    
Available-for-sale Securities, Current 50,976,000 30,301,000 $ 39,495,000
Available-for-sale Securities, Noncurrent 33,839,000 90,436,000 70,559,000
Bonds [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Current 14,378,000   11,527,000
Available-for-sale Securities, Noncurrent 33,839,000   70,559,000
Certificates of Deposit [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Current 36,598,000   27,968,000
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 $ 29,000 22,000  
Available-for-sale Securities, Gross Unrealized Gain   $ 592,000