SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Marketable Securities (Details)	9 Months Ended
	Sep. 30, 2016 USD ($) item	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Available-for-sale securities
Number of objectives of the entity's investment strategy | item	2
Realized gains	$ 21,000	$ 8,000
Minimum contractual term of certain current investments which can be liquidated	1 year
Amortized Cost	$ 274,419,000		$ 266,188,000
Gains in Accumulated Other Comprehensive Income	70,000		2,000
Losses in Accumulated Other Comprehensive Income	(104,000)		(446,000)
Estimated Fair Value	274,385,000		265,744,000
Realized losses	0	$ 0
Corporate bonds
Available-for-sale securities
Amortized Cost	143,221,000		179,471,000
Gains in Accumulated Other Comprehensive Income	27,000		2,000
Losses in Accumulated Other Comprehensive Income	(88,000)		(262,000)
Estimated Fair Value	143,160,000		179,211,000
Asset backed securities
Available-for-sale securities
Amortized Cost	56,542,000		77,661,000
Gains in Accumulated Other Comprehensive Income	18,000
Losses in Accumulated Other Comprehensive Income	(10,000)		(166,000)
Estimated Fair Value	56,550,000		77,495,000
U.S. government agency securities
Available-for-sale securities
Amortized Cost	49,596,000		7,057,000
Gains in Accumulated Other Comprehensive Income	18,000
Losses in Accumulated Other Comprehensive Income	(4,000)		(18,000)
Estimated Fair Value	49,610,000		7,039,000
Certificates of deposit
Available-for-sale securities
Amortized Cost	1,053,000		1,999,000
Losses in Accumulated Other Comprehensive Income	(1,000)
Estimated Fair Value	1,052,000		$ 1,999,000
Commercial paper
Available-for-sale securities
Amortized Cost	24,007,000
Gains in Accumulated Other Comprehensive Income	7,000
Losses in Accumulated Other Comprehensive Income	(1,000)
Estimated Fair Value	$ 24,013,000