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Insurance service result - Summary of Insurance Service Result (Detail) - EUR (€)
€ in Millions
 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Disclosure Of Insurance Service Result [Line Items]    
Insurance revenue € 4,247 € 4,558
Total insurance service expenses 4,565 5,033
Net income / (expenses) on reinsurance held 372 588
Insurance service result 54 113
Reinsurance Contracts Held and Insurance Contracts Issued [Member]    
Disclosure Of Insurance Service Result [Line Items]    
Insurance revenue 4,247 4,558 [1]
Total insurance service expenses (4,565) (5,033) [1]
Net income / (expenses) on reinsurance held 372 588 [1]
Insurance service result 54 113 [1]
Reinsurance Contracts Held and Insurance Contracts Issued [Member] | Contracts not measured under the PAA [member]    
Disclosure Of Insurance Service Result [Line Items]    
Expected insurance claims and expenses 3,396 3,698 [1]
Earnings released from CSM 391 387 [1]
Release of risk adjustment 118 127 [1]
Recovery of acquisition cash flows 275 273 [1]
Other 6 18 [1]
Insurance revenue 4,186 4,503 [1]
Incurred claims and expenses (3,344) (4,074) [1]
Experience adjustments on incurred claims 265 259 [1]
Onerous contract losses (and reversals) (880) (624) [1]
Amortization Of Acquisition Cash Flows (275) (273) [1]
Total insurance service expenses (4,499) (4,972) [1]
Assumption changes that adjust underlying onerous contracts 185 14 [1]
Experience adjustments that adjust underlying onerous contracts (26) 355 [1]
CSM recognized for service received (18) (3) [1]
Release of risk adjustment (35) (38) [1]
Gain on retrospective reinsurance for incurred claims 1 1 [1]
Reversals of loss recovery component from onerous underlying contracts 1 0 [1]
Net income / (expenses) on reinsurance held 372 588 [1]
Reinsurance Contracts Held and Insurance Contracts Issued [Member] | Contracts measured under the PAA [member]    
Disclosure Of Insurance Service Result [Line Items]    
Insurance revenue 61 56 [1]
Total insurance service expenses (66) (61) [1]
Net income / (expenses) on reinsurance held € 0 € 0 [1]
[1] 1H 2025 figures are restated as Aegon UK is classified as discontinued operations.