INVESTMENTS - Available for Sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total	$ 249,448
Fair Value	249,467
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Total	186,776	$ 59,920
Gross Unrealized Gains	38	10
Gross Unrealized Losses	(58)	(29)
Fair Value	186,756	59,901
Long-term Investments
Schedule of Available-for-sale Securities [Line Items]
Total	62,672	27,424
Gross Unrealized Gains	62	0
Gross Unrealized Losses	(23)	(58)
Fair Value	62,711	27,366
U.S. agency securities | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Total	10,023	9,048
Gross Unrealized Gains	1	7
Gross Unrealized Losses	(9)	0
Fair Value	10,015	9,055
U.S. agency securities | Long-term Investments
Schedule of Available-for-sale Securities [Line Items]
Total	3,601	3,502
Gross Unrealized Gains	3	0
Gross Unrealized Losses	(3)	0
Fair Value	3,601	3,502
Corporate bonds | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Total	35,779	17,318
Gross Unrealized Gains	19	0
Gross Unrealized Losses	(35)	(20)
Fair Value	35,763	17,298
Corporate bonds | Long-term Investments
Schedule of Available-for-sale Securities [Line Items]
Total	25,024	12,939
Gross Unrealized Gains	23	0
Gross Unrealized Losses	(8)	(25)
Fair Value	25,039	12,914
Commercial paper | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Total	55,352	6,980
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	55,352	6,980
Municipal securities | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Total	10,248	8,037
Gross Unrealized Gains	9	0
Gross Unrealized Losses	(4)	(9)
Fair Value	10,253	8,028
Municipal securities | Long-term Investments
Schedule of Available-for-sale Securities [Line Items]
Total	11,142	2,505
Gross Unrealized Gains	8	0
Gross Unrealized Losses	(9)	(13)
Fair Value	11,141	2,492
U.S. government securities | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Total	75,374	18,537
Gross Unrealized Gains	9	3
Gross Unrealized Losses	(10)	0
Fair Value	75,373	18,540
U.S. government securities | Long-term Investments
Schedule of Available-for-sale Securities [Line Items]
Total	22,905	8,478
Gross Unrealized Gains	28	0
Gross Unrealized Losses	(3)	(20)
Fair Value	$ 22,930	$ 8,458