FAIR VALUE MEASUREMENTS - Fair Value Hierarchy (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Other Current Liabilities / Other Liabilities:
Total	$ (24,202)	$ (44,440)
Money market funds
Cash equivalents:
Money market funds	42,100	42,096
Level 1
Other Current Liabilities / Other Liabilities:
Total	42,100	42,096
Level 1 | Money market funds
Cash equivalents:
Money market funds	42,100	42,096
Level 2
Other Current Liabilities / Other Liabilities:
Total	(50,119)	(70,353)
Level 2 | Money market funds
Cash equivalents:
Money market funds	0	0
Level 3
Other Current Liabilities / Other Liabilities:
Total	(16,183)	(16,183)
Level 3 | Money market funds
Cash equivalents:
Money market funds	0	0
Other current liabilities | Contingent consideration
Other Current Liabilities / Other Liabilities:
Contingent consideration	(14,810)	(14,810)
Other current liabilities | Interest rate swaps
Other Current Liabilities / Other Liabilities:
Interest rate swaps	(18,765)	(18,694)
Other current liabilities | Level 1 | Contingent consideration
Other Current Liabilities / Other Liabilities:
Contingent consideration	0	0
Other current liabilities | Level 1 | Interest rate swaps
Other Current Liabilities / Other Liabilities:
Interest rate swaps	0	0
Other current liabilities | Level 2 | Contingent consideration
Other Current Liabilities / Other Liabilities:
Contingent consideration	0	0
Other current liabilities | Level 2 | Interest rate swaps
Other Current Liabilities / Other Liabilities:
Interest rate swaps	(18,765)	(18,694)
Other current liabilities | Level 3 | Contingent consideration
Other Current Liabilities / Other Liabilities:
Contingent consideration	(14,810)	(14,810)
Other current liabilities | Level 3 | Interest rate swaps
Other Current Liabilities / Other Liabilities:
Interest rate swaps	0	0
Other liabilities | Contingent consideration
Other Current Liabilities / Other Liabilities:
Contingent consideration	(1,373)	(1,373)
Other liabilities | Interest rate swaps
Other Current Liabilities / Other Liabilities:
Interest rate swaps	(31,354)	(51,659)
Other liabilities | Level 1 | Contingent consideration
Other Current Liabilities / Other Liabilities:
Contingent consideration	0	0
Other liabilities | Level 1 | Interest rate swaps
Other Current Liabilities / Other Liabilities:
Interest rate swaps	0	0
Other liabilities | Level 2 | Contingent consideration
Other Current Liabilities / Other Liabilities:
Contingent consideration	0	0
Other liabilities | Level 2 | Interest rate swaps
Other Current Liabilities / Other Liabilities:
Interest rate swaps	(31,354)	(51,659)
Other liabilities | Level 3 | Contingent consideration
Other Current Liabilities / Other Liabilities:
Contingent consideration	(1,373)	(1,373)
Other liabilities | Level 3 | Interest rate swaps
Other Current Liabilities / Other Liabilities:
Interest rate swaps	$ 0	$ 0