SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Marketable Securities (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) item	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Available-for-sale securities
Number of objectives of the entity's investment strategy | item			2
Realized gains			$ 10,000	$ 18,000
Minimum contractual term of certain current investments which can be liquidated			1 year
Amortized Cost	$ 304,113,000		$ 304,113,000		$ 262,393,000
Gains in Accumulated Other Comprehensive Income	14,000		14,000		31,000
Losses in Accumulated Other Comprehensive Income	(274,000)		(274,000)		(245,000)
Estimated Fair Value	303,853,000		303,853,000		262,179,000
Realized losses	0	$ 0	0	$ 0
Corporate bonds
Available-for-sale securities
Amortized Cost	135,469,000		135,469,000		137,013,000
Gains in Accumulated Other Comprehensive Income	11,000		11,000		17,000
Losses in Accumulated Other Comprehensive Income	(71,000)		(71,000)		(93,000)
Estimated Fair Value	135,409,000		135,409,000		136,937,000
Asset backed securities
Available-for-sale securities
Amortized Cost	94,705,000		94,705,000		55,667,000
Gains in Accumulated Other Comprehensive Income	2,000		2,000		3,000
Losses in Accumulated Other Comprehensive Income	(58,000)		(58,000)		(30,000)
Estimated Fair Value	94,649,000		94,649,000		55,640,000
U.S. government agency securities
Available-for-sale securities
Amortized Cost	62,964,000		62,964,000		49,591,000
Gains in Accumulated Other Comprehensive Income					3,000
Losses in Accumulated Other Comprehensive Income	(144,000)		(144,000)		(120,000)
Estimated Fair Value	62,820,000		62,820,000		49,474,000
Commercial paper
Available-for-sale securities
Amortized Cost	7,175,000		7,175,000		19,069,000
Gains in Accumulated Other Comprehensive Income					8,000
Losses in Accumulated Other Comprehensive Income	(1,000)		(1,000)		(1,000)
Estimated Fair Value	7,174,000		7,174,000		19,076,000
Certificates of deposit
Available-for-sale securities
Amortized Cost	3,800,000		3,800,000		1,053,000
Gains in Accumulated Other Comprehensive Income	1,000		1,000
Losses in Accumulated Other Comprehensive Income					(1,000)
Estimated Fair Value	$ 3,801,000		$ 3,801,000		$ 1,052,000