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Marketable Securities Marketable Securities Table (Details) - USD ($)
9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Gross Unrealized Loss $ 350,000 $ 640,000  
Available-for-sale Securities, Gross Unrealized Gain 939,000    
Available-for-sale Securities, Current 30,301,000 22,834,000 $ 32,424,000
Available-for-sale Securities, Noncurrent 90,436,000 89,705,000 88,465,000
Bonds [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Current 11,628,000   11,240,000
Available-for-sale Securities, Noncurrent 90,436,000   88,465,000
Certificates of Deposit [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Current 18,673,000   21,184,000
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 $ 35,000 962,000  
Available-for-sale Securities, Gross Unrealized Gain   $ 151,000