SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Marketable Securities (Details)	9 Months Ended
	Sep. 30, 2017 USD ($) item	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Available-for-sale securities
Number of objectives of the entity's investment strategy | item	2
Realized gains	$ 17,000	$ 21,000
Other than temporary declines in value	$ 0
Minimum contractual term of certain current investments which can be liquidated	1 year
Amortized Cost	$ 411,891,000		$ 262,393,000
Gains in Accumulated Other Comprehensive Income	33,000		31,000
Losses in Accumulated Other Comprehensive Income	(240,000)		(245,000)
Estimated Fair Value	411,684,000		262,179,000
Realized losses	0	$ 0
Corporate bonds
Available-for-sale securities
Amortized Cost	213,058,000		137,013,000
Gains in Accumulated Other Comprehensive Income	27,000		17,000
Losses in Accumulated Other Comprehensive Income	(80,000)		(93,000)
Estimated Fair Value	213,005,000		136,937,000
Asset backed securities
Available-for-sale securities
Amortized Cost	101,167,000		55,667,000
Gains in Accumulated Other Comprehensive Income	2,000		3,000
Losses in Accumulated Other Comprehensive Income	(56,000)		(30,000)
Estimated Fair Value	101,113,000		55,640,000
U.S. government agency securities
Available-for-sale securities
Amortized Cost	64,972,000		49,591,000
Gains in Accumulated Other Comprehensive Income			3,000
Losses in Accumulated Other Comprehensive Income	(103,000)		(120,000)
Estimated Fair Value	64,869,000		49,474,000
Commercial paper
Available-for-sale securities
Amortized Cost	20,894,000		19,069,000
Gains in Accumulated Other Comprehensive Income	2,000		8,000
Losses in Accumulated Other Comprehensive Income	(1,000)		(1,000)
Estimated Fair Value	20,895,000		19,076,000
Certificates of deposit
Available-for-sale securities
Amortized Cost	11,800,000		1,053,000
Gains in Accumulated Other Comprehensive Income	2,000
Losses in Accumulated Other Comprehensive Income			(1,000)
Estimated Fair Value	$ 11,802,000		$ 1,052,000