FAIR VALUE MEASUREMENTS - Fair Value Hierarchy Table (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Short-term investments:
Short-term investments	$ 611,598	$ 614,130
Corporate notes and bonds
Short-term investments:
Short-term investments	407,553	387,876
U.S. agency obligations
Short-term investments:
Short-term investments	97,592	121,329
U.S. treasury securities
Short-term investments:
Short-term investments	70,340	60,185
Commercial paper
Short-term investments:
Short-term investments	29,858	36,821
Certificates of deposit
Short-term investments:
Short-term investments	6,255	7,919
Recurring
Total
Total	628,596	614,351
Recurring | Money market funds
Cash Equivalents:
Cash equivalents	19,298	2,521
Recurring | Level 1
Total
Total	19,298	2,521
Recurring | Level 1 | Money market funds
Cash Equivalents:
Cash equivalents	19,298	2,521
Recurring | Level 2
Total
Total	611,598	614,130
Recurring | Level 3
Total
Total	(2,300)	(2,300)
Recurring | Contingent consideration
Other Current Liabilities:
Contingent consideration	(2,300)	(2,300)
Recurring | Contingent consideration | Level 3
Other Current Liabilities:
Contingent consideration	(2,300)	(2,300)
Recurring | Corporate notes and bonds
Short-term investments:
Short-term investments	407,553	387,876
Recurring | Corporate notes and bonds | Level 2
Short-term investments:
Short-term investments	407,553	387,876
Recurring | U.S. agency obligations
Short-term investments:
Short-term investments	97,592	121,329
Recurring | U.S. agency obligations | Level 2
Short-term investments:
Short-term investments	97,592	121,329
Recurring | U.S. treasury securities
Short-term investments:
Short-term investments	70,340	60,185
Recurring | U.S. treasury securities | Level 2
Short-term investments:
Short-term investments	70,340	60,185
Recurring | Commercial paper
Short-term investments:
Short-term investments	29,858	36,821
Recurring | Commercial paper | Level 2
Short-term investments:
Short-term investments	29,858	36,821
Recurring | Certificates of deposit
Short-term investments:
Short-term investments	6,255	7,919
Recurring | Certificates of deposit | Level 2
Short-term investments:
Short-term investments	$ 6,255	$ 7,919