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Marketable Securities Marketable Securities Table (Details) - USD ($)
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Gross Unrealized Gain $ 296,000 $ 2,000  
Available-for-sale Securities, Current 34,419,000 27,337,000 $ 32,424,000
Available-for-sale Securities, Noncurrent 87,575,000 90,907,000 88,465,000
Bonds [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Current 13,303,000   11,240,000
Available-for-sale Securities, Noncurrent 87,575,000   88,465,000
Certificates of Deposit [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Current 21,116,000   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 Gain   $ 226,000