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Investments and Fair Value Measurements (Details) - USD ($)
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Cash, cash equivalents and investments [Abstract]          
Cash   $ 47,054,000   $ 23,948,000  
Cash equivalents [Abstract]          
Certificates of deposit   1,024,000   12,204,000  
Total cash and cash equivalents $ 29,661,000 48,078,000 $ 29,661,000 36,152,000 $ 24,875,000
Amortized Cost   78,340,000   53,678,000  
Gross Unrealized Gain   1,000   112,000  
Gross Unrealized (Loss)   (54,000)   (67,000)  
Fair Value   78,287,000   53,723,000  
Contractual maturities on investments, amortized cost basis [Abstract]          
Due within 1 year   20,549,000   11,325,000  
Due after 1 through 5 years   9,713,000   6,201,000  
Investments, amortized cost   30,262,000   17,526,000  
Contractual maturities on investments, fair value basis [Abstract]          
Due within 1 year   20,524,000   11,415,000  
Due after 1 through 5 years   9,685,000   6,156,000  
Investments, fair value   30,209,000   17,571,000  
Investments held   6,015,000   6,599,000  
Gross realized gains from sales of available-for-sale investments   77,000 429,000    
GHI          
Contractual maturities on investments, fair value basis [Abstract]          
Proceeds from sales of available-for-sale investments 113,000 125,000 581,000    
Available-for-sale investments, expenses 29,000 48,000 152,000    
Gross realized gains from sales of available-for-sale investments $ 84,000 77,000 $ 429,000    
Auction Rate Securities [Member]          
Contractual maturities on investments, fair value basis [Abstract]          
Investments, fair value   0      
Total Investments [Member]          
Cash equivalents [Abstract]          
Amortized Cost   30,262,000   17,526,000  
Gross Unrealized Gain   1,000   112,000  
Gross Unrealized (Loss)   (54,000)   (67,000)  
Fair Value   30,209,000   17,571,000  
Certificates of Deposit [Member]          
Cash equivalents [Abstract]          
Amortized Cost   6,618,000   8,999,000  
Gross Unrealized Gain   1,000   1,000  
Gross Unrealized (Loss)   (12,000)   (20,000)  
Fair Value   6,607,000   8,980,000  
Corporate Bonds [Member]          
Cash equivalents [Abstract]          
Amortized Cost   23,644,000   8,479,000  
Gross Unrealized (Loss)   (42,000)   (47,000)  
Fair Value   $ 23,602,000   8,432,000  
Corporate Equity Securities [Member]          
Cash equivalents [Abstract]          
Amortized Cost       48,000  
Gross Unrealized Gain       111,000  
Fair Value       $ 159,000