FAIR VALUE MEASUREMENTS - Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Short-term investments:
Corporate notes and bonds	$ 0	$ 7,000
Recurring
Total
Total	(14,168)	(689)
Recurring | Corporate notes and bonds
Short-term investments:
Corporate notes and bonds		7,000
Recurring | Money market funds
Cash equivalents:
Money market funds	41,933	34,064
Recurring | Level 1
Total
Total	41,933	34,064
Recurring | Level 1 | Corporate notes and bonds
Short-term investments:
Corporate notes and bonds		0
Recurring | Level 1 | Money market funds
Cash equivalents:
Money market funds	41,933	34,064
Recurring | Level 2
Total
Total	(39,311)	(2,929)
Recurring | Level 2 | Corporate notes and bonds
Short-term investments:
Corporate notes and bonds		7,000
Recurring | Level 2 | Money market funds
Cash equivalents:
Money market funds	0	0
Recurring | Level 3
Total
Total	(16,790)	(31,824)
Recurring | Level 3 | Corporate notes and bonds
Short-term investments:
Corporate notes and bonds		0
Recurring | Level 3 | Money market funds
Cash equivalents:
Money market funds	0	0
Recurring | Other Current Liabilities | Contingent consideration
Other liabilities:
Contingent consideration	(3,719)	(15,182)
Recurring | Other Current Liabilities | Interest Rate Swap
Other liabilities:
Interest rate swaps	(8,354)	(1,778)
Recurring | Other Current Liabilities | Level 1 | Contingent consideration
Other liabilities:
Contingent consideration	0	0
Recurring | Other Current Liabilities | Level 1 | Interest Rate Swap
Other liabilities:
Interest rate swaps	0	0
Recurring | Other Current Liabilities | Level 2 | Contingent consideration
Other liabilities:
Contingent consideration	0	0
Recurring | Other Current Liabilities | Level 2 | Interest Rate Swap
Other liabilities:
Interest rate swaps	(8,354)	(1,778)
Recurring | Other Current Liabilities | Level 3 | Contingent consideration
Other liabilities:
Contingent consideration	(3,719)	(15,182)
Recurring | Other Current Liabilities | Level 3 | Interest Rate Swap
Other liabilities:
Interest rate swaps	0	0
Recurring | Other Liabilities | Contingent consideration
Other liabilities:
Contingent consideration	(13,071)	(16,642)
Recurring | Other Liabilities | Interest Rate Swap
Other liabilities:
Interest rate swaps	(30,957)	(8,151)
Recurring | Other Liabilities | Level 1 | Contingent consideration
Other liabilities:
Contingent consideration	0	0
Recurring | Other Liabilities | Level 1 | Interest Rate Swap
Other liabilities:
Interest rate swaps	0	0
Recurring | Other Liabilities | Level 2 | Contingent consideration
Other liabilities:
Contingent consideration	0	0
Recurring | Other Liabilities | Level 2 | Interest Rate Swap
Other liabilities:
Interest rate swaps	(30,957)	(8,151)
Recurring | Other Liabilities | Level 3 | Contingent consideration
Other liabilities:
Contingent consideration	(13,071)	(16,642)
Recurring | Other Liabilities | Level 3 | Interest Rate Swap
Other liabilities:
Interest rate swaps	$ 0	$ 0