SHORT-TERM INVESTMENTS - Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position for Less than Twelve Months (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($)
Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position for Less than Twelve Months
Estimated Fair Value	$ 352,123
Gross Unrealized Losses	(574)
Corporate notes and bonds
Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position for Less than Twelve Months
Estimated Fair Value	257,125
Gross Unrealized Losses	(528)
U.S. agency obligations
Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position for Less than Twelve Months
Estimated Fair Value	30,585
Gross Unrealized Losses	(13)
Commercial paper
Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position for Less than Twelve Months
Estimated Fair Value	12,475
Gross Unrealized Losses	(6)
Certificates of deposit
Fair Values and Gross Unrealized Losses of Available-for-sale Securities in a Gross Loss Position for Less than Twelve Months
Estimated Fair Value	$ 961