Investments and Fair Value Measurements (Details) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Cash, cash equivalents and investments [Abstract]
Cash		$ 55,775,000		$ 23,948,000
Cash equivalents [Abstract]
Certificates of deposit		708,000		12,204,000
Total cash and cash equivalents	$ 26,115,000	56,483,000	$ 26,115,000	36,152,000	$ 24,875,000
Amortized Cost		87,604,000		53,678,000
Gross Unrealized Gain		1,000		112,000
Gross Unrealized (Loss)		(81,000)		(67,000)
Fair Value		87,524,000		53,723,000
Contractual maturities on investments, amortized cost basis [Abstract]
Due within 1 year		22,504,000		11,325,000
Due after 1 through 5 years		8,617,000		6,201,000
Investments, amortized cost		31,121,000		17,526,000
Contractual maturities on investments, fair value basis [Abstract]
Due within 1 year		22,463,000		11,415,000
Due after 1 through 5 years		8,578,000		6,156,000
Investments, fair value		31,041,000		17,571,000
Gross realized gains from sales of available-for-sale investments		77,000	345,000
GHI
Contractual maturities on investments, fair value basis [Abstract]
Proceeds from sales of available-for-sale investments	222,000	125,000	468,000
Available-for-sale investments, expenses	58,000	48,000	123,000
Gross realized gains from sales of available-for-sale investments	$ 164,000	77,000	$ 345,000
Auction Rate Securities [Member]
Contractual maturities on investments, fair value basis [Abstract]
Investments, fair value		0
Total Investments [Member]
Cash equivalents [Abstract]
Amortized Cost		31,121,000		17,526,000
Gross Unrealized Gain		1,000		112,000
Gross Unrealized (Loss)		(81,000)		(67,000)
Fair Value		31,041,000		17,571,000
Certificates of Deposit [Member]
Cash equivalents [Abstract]
Amortized Cost		5,728,000		8,999,000
Gross Unrealized Gain		1,000		1,000
Gross Unrealized (Loss)		(12,000)		(20,000)
Fair Value		5,717,000		8,980,000
Corporate Bonds [Member]
Cash equivalents [Abstract]
Amortized Cost		25,393,000		8,479,000
Gross Unrealized (Loss)		(69,000)		(47,000)
Fair Value		$ 25,324,000		8,432,000
Corporate Equity Securities [Member]
Cash equivalents [Abstract]
Amortized Cost				48,000
Gross Unrealized Gain				111,000
Fair Value				$ 159,000