INVESTMENTS - Short-term and Long-term Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 374,857
Fair Value	373,243
Short-term Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	169,877	$ 59,920
Gross Unrealized Gains	31	10
Gross Unrealized Losses	(332)	(29)
Fair Value	169,576	59,901
Short-term Securities | U.S. agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,122	9,048
Gross Unrealized Gains	0	7
Gross Unrealized Losses	(39)	0
Fair Value	15,083	9,055
Short-term Securities | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	57,855	17,318
Gross Unrealized Gains	22	0
Gross Unrealized Losses	(79)	(20)
Fair Value	57,798	17,298
Short-term Securities | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17,428	6,980
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	17,428	6,980
Short-term Securities | Municipal securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	23,743	8,037
Gross Unrealized Gains	8	0
Gross Unrealized Losses	(51)	(9)
Fair Value	23,700	8,028
Short-term Securities | U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	55,729	18,537
Gross Unrealized Gains	1	3
Gross Unrealized Losses	(163)	0
Fair Value	55,567	18,540
Long-term Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	204,980	27,424
Gross Unrealized Gains	59	0
Gross Unrealized Losses	(1,372)	(58)
Fair Value	203,667	27,366
Long-term Securities | U.S. agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,288	3,502
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(121)	0
Fair Value	20,169	3,502
Long-term Securities | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	91,959	12,939
Gross Unrealized Gains	25	0
Gross Unrealized Losses	(571)	(25)
Fair Value	91,413	12,914
Long-term Securities | Municipal securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	26,371	2,505
Gross Unrealized Gains	13	0
Gross Unrealized Losses	(160)	(13)
Fair Value	26,224	2,492
Long-term Securities | U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	66,362	8,478
Gross Unrealized Gains	19	0
Gross Unrealized Losses	(520)	(20)
Fair Value	$ 65,861	$ 8,458