Retroactive reinsurance - Narrative (Detail) - Retroactive Reinsurance [Member] - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2025 |
Dec. 31, 2024 |
Dec. 31, 2023 |
|
| Effects of Reinsurance [Line Items] | |||
| Insurance losses and loss adjustment expenses | $ 954 | $ 945 | $ 1,484 |
| Increase in incurred losses and loss adjustment expenses | 650 | ||
| Deferred charge assets on retroactive reinsurance contracts | 8,100 | 8,800 | |
| Liabilities for environmental, asbestos and latent injury claims | 11,100 | 11,900 | |
| Unpaid Losses and Loss Adjustment Expenses [Member] | |||
| Effects of Reinsurance [Line Items] | |||
| Insurance losses and loss adjustment expenses | $ 261 | $ 247 | $ 1,109 |