Claim and Claim Adjustment Expense Reserves - Other Professional Liability and Management Liability (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($) Claim	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2015	$ 4,096	$ 3,679	$ 3,481
Total net liability for unpaid claim and claim adjustment expenses	19,263	18,163	16,929	$ 16,300
Property and casualty
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total net liability for unpaid claim and claim adjustment expenses	18,334	17,213
Other professional liability and management liability
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	9,796
IBNR	2,688
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	5,983
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	3,813
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2015	98
Liability for unallocated claim adjustment expenses for accident years presented	56
Total net liability for unpaid claim and claim adjustment expenses	3,967
Net Strengthening (Releases) of Prior Accident Year Reserves
Total net development for the accident years presented above	41	47	44
Total net development for accident years prior to 2015	3	(10)	6
Total unallocated claim adjustment expense development	5	0	0
Total	49	37	50
Other professional liability and management liability | Short-duration Insurance Contracts, Accident Year 2015
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	865	858	855	836	$ 813	$ 807	$ 832	$ 877	$ 892	$ 888
IBNR	$ 21
Cumulative number of claims | Claim	17,457
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 813	808	794	725	677	612	542	404	234	$ 60
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	7	3	19	23	6	(25)	(45)	(15)	4
IBNR, Total	(23)
Other professional liability and management liability | Short-duration Insurance Contracts, Accident Year 2016
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	912	906	888	891	907	904	900	900	901
IBNR	$ 37
Cumulative number of claims | Claim	17,989
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 856	826	784	736	701	625	466	248	$ 64
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	6	18	(3)	(16)	3	4	0	(1)
IBNR, Total	11
Other professional liability and management liability | Short-duration Insurance Contracts, Accident Year 2017
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	752	746	758	775	791	813	845	847
IBNR	$ 51
Cumulative number of claims | Claim	18,215
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 672	630	596	557	498	394	222	$ 57
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	6	(12)	(17)	(16)	(22)	(32)	(2)
IBNR, Total	(95)
Other professional liability and management liability | Short-duration Insurance Contracts, Accident Year 2018
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	987	941	923	906	869	864	850
IBNR	$ 57
Cumulative number of claims | Claim	20,071
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 847	779	706	599	473	282	$ 54
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	46	18	17	37	5	14
IBNR, Total	137
Other professional liability and management liability | Short-Duration Insurance Contract, Accident Year 2019
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	970	939	876	856	845	837
IBNR	$ 96
Cumulative number of claims | Claim	19,548
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 801	699	567	422	263	$ 64
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	31	63	20	11	8
IBNR, Total	133
Other professional liability and management liability | Short-Duration Insurance Contract, Accident Year 2020
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	945	945	951	944	930
IBNR	$ 168
Cumulative number of claims | Claim	19,509
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 660	523	400	248	$ 67
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	0	(6)	7	14
IBNR, Total	15
Other professional liability and management liability | Short-Duration Insurance Contract, Accident Year 2021
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	965	1,009	1,038	1,037
IBNR	$ 311
Cumulative number of claims | Claim	18,377
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 502	356	217	$ 58
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(44)	(29)	1
IBNR, Total	(72)
Other professional liability and management liability | Short-Duration Insurance Contract, Accident Year 2022
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	1,084	1,112	1,120
IBNR	$ 465
Cumulative number of claims | Claim	18,376
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 453	225	$ 64
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(28)	(8)
IBNR, Total	(36)
Other professional liability and management liability | Short-Duration Insurance Contract, Accident Year 2023
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	1,166	1,149
IBNR	$ 564
Cumulative number of claims | Claim	19,587
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 302	$ 64
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	17
IBNR, Total	17
Other professional liability and management liability | Short-Duration Insurance Contract, Accident Year 2024
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	1,150
IBNR	$ 918
Cumulative number of claims | Claim	17,921
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 77