SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Available-for-sale Securities (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Available-for-sale securities
Restricted cash	$ 0	$ 0
Number of objectives of the entity's investment strategy | item	2
Number of investments with other-than-temporary declines in value	$ 0
Minimum contractual term of certain current investments which can be liquidated	1 year
Realized gains	$ 24,132	14,205	$ 11,000
Amortized Cost	262,393,000	266,188,000
Gains in Accumulated Other Comprehensive Income	31,000	2,000
Losses in Accumulated Other Comprehensive Income	(245,000)	(446,000)
Estimated Fair Value	262,179,000	265,744,000
Allowance for Doubtful Accounts
Bad debt expense written off	0	0	$ 0
Corporate bonds
Available-for-sale securities
Amortized Cost	137,013,000	179,471,000
Gains in Accumulated Other Comprehensive Income	17,000	2,000
Losses in Accumulated Other Comprehensive Income	(93,000)	(262,000)
Estimated Fair Value	136,937,000	179,211,000
Asset backed securities
Available-for-sale securities
Amortized Cost	55,667,000	7,057,000
Gains in Accumulated Other Comprehensive Income	3,000
Losses in Accumulated Other Comprehensive Income	(30,000)	(18,000)
Estimated Fair Value	55,640,000	7,039,000
U.S. government agency securities
Available-for-sale securities
Amortized Cost	49,591,000	77,661,000
Gains in Accumulated Other Comprehensive Income	3,000
Losses in Accumulated Other Comprehensive Income	(120,000)	(166,000)
Estimated Fair Value	49,474,000	77,495,000
Commercial paper
Available-for-sale securities
Amortized Cost	19,069,000	1,999,000
Gains in Accumulated Other Comprehensive Income	8,000
Losses in Accumulated Other Comprehensive Income	(1,000)
Estimated Fair Value	19,076,000	$ 1,999,000
Certificates of deposit
Available-for-sale securities
Amortized Cost	1,053,000
Losses in Accumulated Other Comprehensive Income	(1,000)
Estimated Fair Value	$ 1,052,000