SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Marketable Securities (Details)	3 Months Ended
	Mar. 31, 2017 USD ($) item	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Available-for-sale securities
Number of objectives of the entity's investment strategy | item	2
Realized gains	$ 4,000	$ 3,000
Minimum contractual term of certain current investments which can be liquidated	1 year
Amortized Cost	$ 235,683,000		$ 262,393,000
Gains in Accumulated Other Comprehensive Income	18,000		31,000
Losses in Accumulated Other Comprehensive Income	(237,000)		(245,000)
Estimated Fair Value	235,464,000		262,179,000
Realized losses	0	$ 0
Corporate bonds
Available-for-sale securities
Amortized Cost	110,463,000		137,013,000
Gains in Accumulated Other Comprehensive Income	13,000		17,000
Losses in Accumulated Other Comprehensive Income	(79,000)		(93,000)
Estimated Fair Value	110,397,000		136,937,000
Asset backed securities
Available-for-sale securities
Amortized Cost	54,274,000		55,667,000
Gains in Accumulated Other Comprehensive Income	2,000		3,000
Losses in Accumulated Other Comprehensive Income	(35,000)		(30,000)
Estimated Fair Value	54,241,000		55,640,000
U.S. government agency securities
Available-for-sale securities
Amortized Cost	49,586,000		49,591,000
Gains in Accumulated Other Comprehensive Income			3,000
Losses in Accumulated Other Comprehensive Income	(123,000)		(120,000)
Estimated Fair Value	49,463,000		49,474,000
Certificates of deposit
Available-for-sale securities
Amortized Cost	4,252,000		1,053,000
Gains in Accumulated Other Comprehensive Income	1,000
Losses in Accumulated Other Comprehensive Income			(1,000)
Estimated Fair Value	4,253,000		1,052,000
Commercial paper
Available-for-sale securities
Amortized Cost	17,108,000		19,069,000
Gains in Accumulated Other Comprehensive Income	2,000		8,000
Losses in Accumulated Other Comprehensive Income			(1,000)
Estimated Fair Value	$ 17,110,000		$ 19,076,000