Short Duration Contracts - Reconciliation of claims development to liability (Details) - USD ($) $ in Millions		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		$ 21,118
Unpaid losses and loss adjustment expenses recoverable		7,821	$ 6,690	$ 6,280	$ 5,599
Lines other than short duration		146
Discounting		(68)
Unallocated claims adjustment expenses		352
Total reconciling items		430
Reserve for losses and loss adjustment expenses		29,369	$ 22,752	$ 20,032	$ 17,757
Other short duration lines not included
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance	[1]	2,384
Unpaid losses and loss adjustment expenses recoverable	[2]	474
Other short duration lines not included | Loss portfolio transfer
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses recoverable	[2]	168
Total for short duration lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses recoverable		7,821
Insurance | Property, energy, marine and aviation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		1,114
Unpaid losses and loss adjustment expenses recoverable		420
Insurance | Third party occurrence business
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		3,966
Unpaid losses and loss adjustment expenses recoverable		2,411
Insurance | Third party claims-made business
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		2,533
Unpaid losses and loss adjustment expenses recoverable		923
Insurance | Multi-line and other specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		1,941
Unpaid losses and loss adjustment expenses recoverable		356
Reinsurance | Casualty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		3,026
Unpaid losses and loss adjustment expenses recoverable		715
Reinsurance | Property catastrophe
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		812
Unpaid losses and loss adjustment expenses recoverable		783
Reinsurance | Property excluding property catastrophe
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		1,777
Unpaid losses and loss adjustment expenses recoverable		278
Reinsurance | Marine and aviation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		495
Unpaid losses and loss adjustment expenses recoverable		462
Reinsurance | Other specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		2,749
Unpaid losses and loss adjustment expenses recoverable		998
Mortgage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		321
Unpaid losses and loss adjustment expenses recoverable		33
Consolidating adjustments and eliminations
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses recoverable		$ (32)

[1] Includes amounts associated with the loss portfolio reinsurance agreement related to the MCE Acquisition.

[2] Includes unpaid loss and loss adjustment expenses recoverable of $168 million related to the loss portfolio transfer reinsurance agreements.