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DISCONTINUED OPERATIONS AND DISPOSALS - Narrative (Details) € in Millions, $ in Millions			3 Months Ended								6 Months Ended		12 Months Ended
	Jan. 09, 2024 USD ($)	Jan. 09, 2024 EUR (€)	Jun. 30, 2025 USD ($)	Mar. 31, 2025 USD ($)	Mar. 31, 2025 EUR (€)	Sep. 30, 2024 USD ($)	Sep. 30, 2024 EUR (€)	Jun. 30, 2024 USD ($)	Jun. 30, 2024 EUR (€)	Mar. 31, 2024 USD ($)	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Dec. 31, 2024 USD ($)	May 31, 2025 USD ($)	May 31, 2025 EUR (€)	Feb. 03, 2025 USD ($)	Feb. 03, 2025 EUR (€)	Jan. 09, 2024 EUR (€)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of businesses, net of cash disposed											$ 67.7	$ 312.6
Gain (loss) on disposal											4.9	9.1
Deconsolidation of subsidiaries			$ 5.2					$ (22.9)			5.2	(22.9)
GfK's Consumer Panel \| Discontinued Operations, Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration received from sale of business	$ 350.0																	€ 316.6
Proceeds from sale of businesses, net of cash disposed	$ 301.7	€ 278.4		$ 10.9	€ 10.5	$ 3.3	€ 3.0	$ 10.9	€ 10.0
Gain (loss) on disposal										$ (0.3)		$ 9.1	$ 12.4
Netquest \| Disposal Group, Held-for-Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration received from sale of business														$ 5.9	€ 5.0	$ 60.3	€ 58.1
Gain (loss) on disposal			$ (0.7)								$ 4.9