Investments and Fair Value Measurements (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash, cash equivalents and investments [Abstract]
Cash	$ 39,189,000		$ 43,610,000
Cash equivalents [Abstract]
Certificates of deposit			742,000
Total cash and cash equivalents	39,189,000	$ 56,512,000	44,352,000	$ 36,152,000
Amortized cost	27,916,000		32,701,000
Cash, cash equivalents and investments, amortized costs	67,105,000		77,053,000
Gross Unrealized (Loss)	(210,000)		(93,000)
Fair Value	27,706,000		32,608,000
Cash, cash equivalents and investments, amortized costs	66,895,000		76,960,000
Contractual maturities on investments, amortized cost basis [Abstract]
Due within 1 year	15,288,000		20,056,000
Due after 1 through 5 years	12,628,000		12,645,000
Investments, amortized cost	27,916,000		32,701,000
Contractual maturities on investments, fair value basis [Abstract]
Due within 1 year	15,195,000		20,032,000
Due after 1 through 5 years	12,511,000		12,576,000
Investments, fair value	27,706,000		32,608,000
Proceeds from sales of available-for-sale investments	14,680,000	8,970,000
Investments held	5,453,000		5,578,000
Equity Securities, FV-NI, Realized Gain (Loss)		77,000
GHI
Contractual maturities on investments, fair value basis [Abstract]
Proceeds from sales of available-for-sale investments	0	125,000
Available-for-sale investments, expenses		48,000
Equity Securities, FV-NI, Realized Gain (Loss)		$ 77,000
Total Investments [Member]
Cash equivalents [Abstract]
Amortized cost	27,916,000		32,701,000
Gross Unrealized (Loss)	(210,000)		(93,000)
Fair Value	27,706,000		32,608,000
Contractual maturities on investments, amortized cost basis [Abstract]
Investments, amortized cost	27,916,000		32,701,000
Contractual maturities on investments, fair value basis [Abstract]
Investments, fair value	27,706,000		32,608,000
Certificates of Deposit [Member]
Cash equivalents [Abstract]
Amortized cost	7,340,000		7,099,000
Gross Unrealized (Loss)	(41,000)		(24,000)
Fair Value	7,299,000		7,075,000
Contractual maturities on investments, amortized cost basis [Abstract]
Investments, amortized cost	7,340,000		7,099,000
Contractual maturities on investments, fair value basis [Abstract]
Investments, fair value	7,299,000		7,075,000
Corporate Bonds [Member]
Cash equivalents [Abstract]
Amortized cost	20,576,000		25,602,000
Gross Unrealized (Loss)	(169,000)		(69,000)
Fair Value	20,407,000		25,533,000
Contractual maturities on investments, amortized cost basis [Abstract]
Investments, amortized cost	20,576,000		25,602,000
Contractual maturities on investments, fair value basis [Abstract]
Investments, fair value	$ 20,407,000		$ 25,533,000