FAIR VALUE MEASUREMENTS - Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Short-term investments:
Short-term investments	$ 293,101	$ 445,315
Corporate notes and bonds
Short-term investments:
Short-term investments	257,216	348,689
U.S. agency obligations
Short-term investments:
Short-term investments	15,267	34,808
U.S. treasury securities
Short-term investments:
Short-term investments	20,618	53,587
Commercial paper
Short-term investments:
Short-term investments		6,309
Certificates of deposit
Short-term investments:
Short-term investments		1,922
Recurring
Total
Total	444,229	476,823
Recurring | Money market funds
Cash Equivalents:
Cash equivalents	160,828	44,108
Recurring | Level 1
Total
Total	160,828	44,108
Recurring | Level 1 | Money market funds
Cash Equivalents:
Cash equivalents	160,828	44,108
Recurring | Level 2
Total
Total	293,101	445,315
Recurring | Level 2 | Money market funds
Cash Equivalents:
Cash equivalents	0	0
Recurring | Level 3
Total
Total	(9,700)	(12,600)
Recurring | Level 3 | Money market funds
Cash Equivalents:
Cash equivalents	0	0
Recurring | Contingent consideration
Liabilities / Other Current Liabilities:
Contingent consideration	(9,700)	(12,600)
Recurring | Contingent consideration | Level 1
Liabilities / Other Current Liabilities:
Contingent consideration	0	0
Recurring | Contingent consideration | Level 2
Liabilities / Other Current Liabilities:
Contingent consideration	0	0
Recurring | Contingent consideration | Level 3
Liabilities / Other Current Liabilities:
Contingent consideration	(9,700)	(12,600)
Recurring | Corporate notes and bonds
Short-term investments:
Short-term investments	257,216	348,689
Recurring | Corporate notes and bonds | Level 1
Short-term investments:
Short-term investments	0	0
Recurring | Corporate notes and bonds | Level 2
Short-term investments:
Short-term investments	257,216	348,689
Recurring | Corporate notes and bonds | Level 3
Short-term investments:
Short-term investments	0	0
Recurring | U.S. agency obligations
Short-term investments:
Short-term investments	15,267	34,808
Recurring | U.S. agency obligations | Level 1
Short-term investments:
Short-term investments	0	0
Recurring | U.S. agency obligations | Level 2
Short-term investments:
Short-term investments	15,267	34,808
Recurring | U.S. agency obligations | Level 3
Short-term investments:
Short-term investments	0	0
Recurring | U.S. treasury securities
Short-term investments:
Short-term investments	20,618	53,587
Recurring | U.S. treasury securities | Level 1
Short-term investments:
Short-term investments	0	0
Recurring | U.S. treasury securities | Level 2
Short-term investments:
Short-term investments	20,618	53,587
Recurring | U.S. treasury securities | Level 3
Short-term investments:
Short-term investments	0	0
Recurring | Commercial paper
Short-term investments:
Short-term investments		6,309
Recurring | Commercial paper | Level 1
Short-term investments:
Short-term investments		0
Recurring | Commercial paper | Level 2
Short-term investments:
Short-term investments		6,309
Recurring | Commercial paper | Level 3
Short-term investments:
Short-term investments		0
Recurring | Certificates of deposit
Short-term investments:
Short-term investments	0	1,922
Recurring | Certificates of deposit | Level 1
Short-term investments:
Short-term investments	0	0
Recurring | Certificates of deposit | Level 2
Short-term investments:
Short-term investments	0	1,922
Recurring | Certificates of deposit | Level 3
Short-term investments:
Short-term investments	$ 0	$ 0