FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS - Fair value of Cash and Stock Earn-outs (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2025 USD ($) Y	Sep. 30, 2025 USD ($) Y	Dec. 31, 2024 USD ($) Y	Aug. 31, 2024 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchase commitment and contingent consideration liabilities, current	$ 27,100	$ 27,100	$ 35,100
Purchase commitment and contingent consideration liabilities, net of current portion	79,000	79,000	87,400
Cash Earn-outs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in fair value of contingent consideration liabilities	5,100	8,600
Stock Earn-outs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in fair value of contingent consideration liabilities	(9,100)	(25,000)
Ecosio
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchase commitment and contingent consideration liabilities, current	27,100	27,100	35,100
Purchase commitment and contingent consideration liabilities, net of current portion	79,000	79,000	87,400
Ecosio \| Cash Earn-outs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchase commitment and contingent consideration liabilities, current	19,100	19,100	17,900
Purchase commitment and contingent consideration liabilities, net of current portion	63,900	63,900	56,500
Contingent consideration	83,000	83,000	74,400	$ 71,000
Ecosio \| Cash Earn-outs \| Revenue volatility
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration	$ 83,000	$ 83,000	$ 74,400
Contingent consideration measurement input	22.5	22.5	21
Ecosio \| Cash Earn-outs \| Revenue discount rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration measurement input	7.1	7.1	7.7
Ecosio \| Cash Earn-outs \| Term (in years)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration measurement input \| Y	2.4	2.4	3.2
Ecosio \| Stock Earn-outs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Purchase commitment and contingent consideration liabilities, current	$ 8,000	$ 8,000	$ 17,200
Purchase commitment and contingent consideration liabilities, net of current portion	15,100	15,100	30,900
Contingent consideration	23,100	23,100	48,100	$ 34,000
Ecosio \| Stock Earn-outs \| Revenue volatility
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration	$ 23,100	$ 23,100	$ 48,100
Contingent consideration measurement input	22.5	22.5	21
Ecosio \| Stock Earn-outs \| Revenue discount rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration measurement input	7.1	7.1	7.7
Ecosio \| Stock Earn-outs \| Term (in years)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration measurement input \| Y	2.4	2.4	3.2

Vertex, Inc. 10-Q