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Marketable Securities Marketable Securities Table (Details) - USD ($)
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Gross Unrealized Gain (Loss) $ 2,000    
Available-for-sale Securities, Gross Unrealized Loss 208,000 $ 443,000  
Available-for-sale Securities, Gross Unrealized Gain 109,000    
Available-for-sale Securities, Current 45,682,000 34,419,000 $ 39,495,000
Available-for-sale Securities, Noncurrent 53,298,000 87,575,000 70,559,000
Bonds [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Current 17,438,000   11,527,000
Available-for-sale Securities, Noncurrent 53,298,000   70,559,000
Certificates of Deposit [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Current 28,244,000   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 $ 45,000 350,000  
Available-for-sale Securities, Gross Unrealized Gain   $ 296,000