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Marketable Securities Marketable Securities Table (Details) - USD ($)
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Gross Unrealized Gain (Loss) $ 1,000    
Available-for-sale Securities, Gross Unrealized Loss 118,000 $ 388,000  
Available-for-sale Securities, Gross Unrealized Gain 53,000    
Available-for-sale Securities, Current 57,324,000 32,546,000 $ 39,495,000
Available-for-sale Securities, Noncurrent 41,871,000 90,144,000 70,559,000
Bonds [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Current 14,371,000   11,527,000
Available-for-sale Securities, Noncurrent 41,871,000   70,559,000
Certificates of Deposit [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Current 42,953,000   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 $ 48,000 155,000  
Available-for-sale Securities, Gross Unrealized Gain   $ 550,000