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Investments and Fair Value Measurements (Details) - USD ($)
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Dec. 31, 2015
Cash, cash equivalents and investments [Abstract]        
Cash $ 55,818,000   $ 23,948,000  
Cash equivalents [Abstract]        
Certificates of deposit 694,000   12,204,000  
Total cash and cash equivalents 56,512,000 $ 25,067,000 36,152,000 $ 24,875,000
Amortized Cost 86,835,000   53,678,000  
Gross Unrealized Gain 1,000   112,000  
Gross Unrealized (Loss) (79,000)   (67,000)  
Fair Value 86,757,000   53,723,000  
Contractual maturities on investments, amortized cost basis [Abstract]        
Due within 1 year 20,522,000   11,325,000  
Due after 1 through 5 years 9,801,000   6,201,000  
Investments, amortized cost 30,323,000   17,526,000  
Contractual maturities on investments, fair value basis [Abstract]        
Due within 1 year 20,483,000   11,415,000  
Due after 1 through 5 years 9,762,000   6,156,000  
Investments, fair value 30,245,000   17,571,000  
Gross realized gains from sales of available-for-sale investments 77,000 181,000    
GHI        
Contractual maturities on investments, fair value basis [Abstract]        
Realized gain 125,000 246,000    
Available-for-sale investments, expenses 48,000 65,000    
Gross realized gains from sales of available-for-sale investments 77,000 $ 181,000    
Auction Rate Securities [Member]        
Contractual maturities on investments, fair value basis [Abstract]        
Investments, fair value 0      
Total Investments [Member]        
Cash equivalents [Abstract]        
Amortized Cost 30,323,000   17,526,000  
Gross Unrealized Gain 1,000   112,000  
Gross Unrealized (Loss) (79,000)   (67,000)  
Fair Value 30,245,000   17,571,000  
Certificates of Deposit [Member]        
Cash equivalents [Abstract]        
Amortized Cost 6,928,000   8,999,000  
Gross Unrealized Gain 1,000   1,000  
Gross Unrealized (Loss) (10,000)   (20,000)  
Fair Value 6,919,000   8,980,000  
Corporate Bonds [Member]        
Cash equivalents [Abstract]        
Amortized Cost 23,395,000   8,479,000  
Gross Unrealized (Loss) (69,000)   (47,000)  
Fair Value $ 23,326,000   8,432,000  
Corporate Equity Securities [Member]        
Cash equivalents [Abstract]        
Amortized Cost     48,000  
Gross Unrealized Gain     111,000  
Fair Value     $ 159,000