FAIR VALUE MEASUREMENTS - Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Short-term investments:
Short-term investments	$ 0	$ 7,000
Recurring
Liabilities / Other Current Liabilities:
Total	(22,817)	(689)
Recurring | Corporate notes and bonds
Short-term investments:
Short-term investments	0	7,000
Recurring | Money market funds
Cash equivalents:
Cash equivalents	41,761	34,064
Recurring | Level 1
Liabilities / Other Current Liabilities:
Total	41,761	34,064
Recurring | Level 1 | Corporate notes and bonds
Short-term investments:
Short-term investments	0	0
Recurring | Level 1 | Money market funds
Cash equivalents:
Cash equivalents	41,761	34,064
Recurring | Level 2
Liabilities / Other Current Liabilities:
Total	(47,836)	(2,929)
Recurring | Level 2 | Corporate notes and bonds
Short-term investments:
Short-term investments	0	7,000
Recurring | Level 2 | Money market funds
Cash equivalents:
Cash equivalents	0	0
Recurring | Level 3
Liabilities / Other Current Liabilities:
Total	(16,742)	(31,824)
Recurring | Level 3 | Corporate notes and bonds
Short-term investments:
Short-term investments	0	0
Recurring | Level 3 | Money market funds
Cash equivalents:
Cash equivalents	0	0
Recurring | Other Current Liabilities | Contingent consideration
Liabilities / Other Current Liabilities:
Contingent consideration	(2,282)	(15,182)
Recurring | Other Current Liabilities | Interest rate swaps
Liabilities / Other Current Liabilities:
Interest rate swaps	(7,979)	(1,778)
Recurring | Other Current Liabilities | Level 1 | Contingent consideration
Liabilities / Other Current Liabilities:
Contingent consideration	0	0
Recurring | Other Current Liabilities | Level 1 | Interest rate swaps
Liabilities / Other Current Liabilities:
Interest rate swaps	0	0
Recurring | Other Current Liabilities | Level 2 | Contingent consideration
Liabilities / Other Current Liabilities:
Contingent consideration	0	0
Recurring | Other Current Liabilities | Level 2 | Interest rate swaps
Liabilities / Other Current Liabilities:
Interest rate swaps	(7,979)	(1,778)
Recurring | Other Current Liabilities | Level 3 | Contingent consideration
Liabilities / Other Current Liabilities:
Contingent consideration	(2,282)	(15,182)
Recurring | Other Current Liabilities | Level 3 | Interest rate swaps
Liabilities / Other Current Liabilities:
Interest rate swaps	0	0
Recurring | Other Liabilities | Contingent consideration
Liabilities / Other Current Liabilities:
Contingent consideration	(14,460)	(16,642)
Recurring | Other Liabilities | Interest rate swaps
Liabilities / Other Current Liabilities:
Interest rate swaps	(39,857)	(8,151)
Recurring | Other Liabilities | Level 1 | Contingent consideration
Liabilities / Other Current Liabilities:
Contingent consideration	0	0
Recurring | Other Liabilities | Level 1 | Interest rate swaps
Liabilities / Other Current Liabilities:
Interest rate swaps	0	0
Recurring | Other Liabilities | Level 2 | Contingent consideration
Liabilities / Other Current Liabilities:
Contingent consideration	0	0
Recurring | Other Liabilities | Level 2 | Interest rate swaps
Liabilities / Other Current Liabilities:
Interest rate swaps	(39,857)	(8,151)
Recurring | Other Liabilities | Level 3 | Contingent consideration
Liabilities / Other Current Liabilities:
Contingent consideration	(14,460)	(16,642)
Recurring | Other Liabilities | Level 3 | Interest rate swaps
Liabilities / Other Current Liabilities:
Interest rate swaps	$ 0	$ 0