Finance income and costs - Schedule of Components of Finance Income and Costs (Paranthetical) (Detail) € in Millions	12 Months Ended
Dec. 31, 2025 EUR (€)
Finance income and costs [Abstract]
Reclassification of cumulative translation impact on liquidation of subsidiaries	€ 36

Reclassification of cumulative translation impact on liquidation of subsidiaries.

No definition available.

Name:	sgbaf_ReclassificationOfCumulativeTranslationImpactOnLiquidationOfSubsidiaries
Namespace Prefix:	sgbaf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration