SHORT-TERM INVESTMENTS - Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2016 USD ($)
Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position
Estimated Fair Value	$ 163,186
Gross Unrealized Losses	(140)
Corporate notes and bonds
Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position
Estimated Fair Value	145,463
Gross Unrealized Losses	(135)
U.S. agency obligations
Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position
Estimated Fair Value	12,056
Gross Unrealized Losses	(1)
U.S. treasury securities
Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position
Estimated Fair Value	708
Gross Unrealized Losses	(1)
Commercial paper
Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position
Estimated Fair Value	4,959
Gross Unrealized Losses	$ (3)