SHORT-TERM INVESTMENTS - Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2016 USD ($)
Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position
Estimated Fair Value	$ 239,133
Gross Unrealized Losses	(361)
Corporate notes and bonds
Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position
Estimated Fair Value	209,305
Gross Unrealized Losses	(347)
U.S. agency obligations
Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position
Estimated Fair Value	24,857
Gross Unrealized Losses	(10)
Commercial paper
Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position
Estimated Fair Value	4,971
Gross Unrealized Losses	$ (4)