THE COMPANY AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021 USD ($) professional segment category project	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Noncontrolling Interest [Line Items]
Number of service categories | category	400
Number of domestic service professionals (over) | professional	260,000
Number of projects | project	33,000,000
Number of operating segments | segment	2
Current deferred revenue	$ 60,025		$ 54,654	$ 58,200
Noncurrent deferred revenue	100		200	$ 200
Deferred revenue recognized during period	52,300	$ 55,700
Current capitalized sales commissions	40,300		49,200
Noncurrent capitalized sales commissions	$ 1,200		$ 400
Angi | Class B Common Stock | IAC
Noncontrolling Interest [Line Items]
Economic interest (as a percent)	84.60%
Voting interest (as a percent)	98.20%