FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Included in cash and cash equivalents:
Amortized Cost	$ 12,808	$ 11,193
Estimated Fair Value	12,808	11,193
Restricted cash:
Amortized Cost	268	268
Estimated Fair Value	268	268
Marketable securities:
Amortized Cost	106,629	116,042
Gross Unrealized Gains	22	54
Gross Unrealized Losses	(93)	(107)
Estimated Fair Value	106,558	115,989
Money market funds
Included in cash and cash equivalents:
Amortized Cost	9,946	11,193
Estimated Fair Value	9,946	11,193
Corporate notes
Included in cash and cash equivalents:
Amortized Cost	2,862
Estimated Fair Value	2,862
Certificate of deposit
Restricted cash:
Amortized Cost	268	268
Estimated Fair Value	268	268
Government-sponsored enterprise securities (due in less than one year)
Marketable securities:
Amortized Cost	7,500	5,000
Gross Unrealized Losses	(9)	(3)
Estimated Fair Value	7,491	4,997
Government-sponsored enterprise securities (due in one to two years)
Marketable securities:
Amortized Cost	10,000	12,500
Gross Unrealized Losses	(29)	(42)
Estimated Fair Value	9,971	12,458
Commercial paper (due in less than one year)
Marketable securities:
Amortized Cost	20,577	31,024
Gross Unrealized Gains	21	50
Gross Unrealized Losses	(3)	(5)
Estimated Fair Value	20,595	31,069
Corporate notes (due in less than one year)
Marketable securities:
Amortized Cost	56,382	66,012
Gross Unrealized Gains	1	4
Gross Unrealized Losses	(37)	(47)
Estimated Fair Value	56,346	65,969
Corporate notes (due in one to two years)
Marketable securities:
Amortized Cost	12,170	1,506
Gross Unrealized Losses	(15)	(10)
Estimated Fair Value	$ 12,155	$ 1,496

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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