Investments and Fair Value Measurements (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) item	Jun. 30, 2017 USD ($) item	Jun. 30, 2018 USD ($) item	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Cash, cash equivalents and investments [Abstract]
Cash	$ 29,698,000		$ 29,698,000		$ 43,610,000
Cash equivalents [Abstract]
Certificates of deposit					742,000
Total cash and cash equivalents	29,698,000		29,698,000		44,352,000
Amortized cost	24,686,000		24,686,000		32,701,000
Cash, cash equivalents and investments, amortized costs	54,384,000		54,384,000		77,053,000
Gross Unrealized (Loss)	(170,000)		(170,000)		(93,000)
Fair Value	24,516,000		24,516,000		32,608,000
Cash, cash equivalents and investments, amortized costs			54,214,000		76,960,000
Contractual maturities on investments, amortized cost basis [Abstract]
Due within 1 year	18,912,000		18,912,000		20,056,000
Due after 1 through 5 years	5,774,000		5,774,000		12,645,000
Investments, amortized cost	24,686,000		24,686,000		32,701,000
Contractual maturities on investments, fair value basis [Abstract]
Due within 1 year	18,802,000		18,802,000		20,032,000
Due after 1 through 5 years	5,714,000		5,714,000		12,576,000
Investments, fair value	24,516,000		24,516,000		32,608,000
Realized gain on sale of available-for-sale securities				$ (77,000)
Transfers between fair value measurements	$ 0		$ 0
GHI
Contractual maturities on investments, fair value basis [Abstract]
Proceeds from sales of investments				125,000
Realized gain on sale of available-for-sale securities				(77,000)
Cost of investment		$ 48,000		$ 48,000
Number of transactions | item	0	0	0
Total Investments [Member]
Cash equivalents [Abstract]
Amortized cost	$ 24,686,000		$ 24,686,000		32,701,000
Gross Unrealized (Loss)	(170,000)		(170,000)		(93,000)
Fair Value	24,516,000		24,516,000		32,608,000
Contractual maturities on investments, amortized cost basis [Abstract]
Investments, amortized cost	24,686,000		24,686,000		32,701,000
Contractual maturities on investments, fair value basis [Abstract]
Investments, fair value	24,516,000		24,516,000		32,608,000
Certificates of Deposit [Member]
Cash equivalents [Abstract]
Amortized cost	6,189,000		6,189,000		7,099,000
Gross Unrealized (Loss)	(43,000)		(43,000)		(24,000)
Fair Value	6,146,000		6,146,000		7,075,000
Contractual maturities on investments, amortized cost basis [Abstract]
Investments, amortized cost	6,189,000		6,189,000		7,099,000
Contractual maturities on investments, fair value basis [Abstract]
Investments, fair value	6,146,000		6,146,000		7,075,000
Corporate Bonds [Member]
Cash equivalents [Abstract]
Amortized cost	18,497,000		18,497,000		25,602,000
Gross Unrealized (Loss)	(127,000)		(127,000)		(69,000)
Fair Value	18,370,000		18,370,000		25,533,000
Contractual maturities on investments, amortized cost basis [Abstract]
Investments, amortized cost	18,497,000		18,497,000		25,602,000
Contractual maturities on investments, fair value basis [Abstract]
Investments, fair value	$ 18,370,000		$ 18,370,000		$ 25,533,000