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Investments (Details) - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Available-for-sale securities    
Amortized Cost $ 87,014,000 $ 97,456,000
Gross Unrealized Gains 1,000  
Gross Unrealized Losses (159,000) (180,000)
Fair Value 86,856,000 97,276,000
Short-term investments    
Available-for-sale securities    
Amortized Cost 73,817,000 61,016,000
Gross Unrealized Gains 1,000  
Gross Unrealized Losses (127,000) (104,000)
Fair Value 73,691,000 60,912,000
Accrued interest 400,000 300,000
Long-term investments    
Available-for-sale securities    
Amortized Cost 13,197,000 36,440,000
Gross Unrealized Losses (32,000) (76,000)
Fair Value 13,165,000 36,364,000
Accrued interest $ 36,000 200,000
Minimum | Long-term investments    
Available-for-sale securities    
Maturity range of long term investment 12 months  
Maximum | Long-term investments    
Available-for-sale securities    
Maturity range of long term investment 20 months  
Corporate debt    
Available-for-sale securities    
Amortized Cost $ 56,892,000 85,474,000
Gross Unrealized Losses (105,000) (163,000)
Fair Value 56,787,000 85,311,000
U.S. Treasury notes    
Available-for-sale securities    
Amortized Cost 30,122,000 11,982,000
Gross Unrealized Gains 1,000  
Gross Unrealized Losses (54,000) (17,000)
Fair Value $ 30,069,000 $ 11,965,000