Claim and Claim Adjustment Expense Reserves - Other Professional Liability and Management Liability (Details) $ in Millions	12 Months Ended
	Dec. 31, 2025 USD ($) Claim	Dec. 31, 2024 USD ($)	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 ($)	Dec. 31, 2016 USD ($)
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2016	$ 4,685	$ 4,096	$ 3,679
Total net liability for unpaid claim and claim adjustment expenses	20,617	19,263	18,163	$ 16,929
Other professional liability and management liability
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	10,229
IBNR	2,689
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	6,324
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	3,905
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2016	101
Liability for unallocated claim adjustment expenses for accident years presented	67
Total net liability for unpaid claim and claim adjustment expenses	4,073
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	69	49	37
Total net development for the accident years presented above	90	34	44
Total net development for accident years prior to 2016	(18)	10	(7)
Total unallocated claim adjustment expense development	(3)	5	0
Other professional liability and management liability | Short-duration Insurance Contracts, Accident Year 2016
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	910	912	906	888	$ 891	$ 907	$ 904	$ 900	$ 900	$ 901
IBNR	$ 28
Cumulative number of claims | Claim	17,993
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 862	856	826	784	736	701	625	466	248	$ 64
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(2)	6	18	(3)	(16)	3	4	0	(1)
IBNR, Total	9
Other professional liability and management liability | Short-duration Insurance Contracts, Accident Year 2017
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	744	752	746	758	775	791	813	845	847
IBNR	$ 25
Cumulative number of claims | Claim	18,223
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 699	672	630	596	557	498	394	222	$ 57
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(8)	6	(12)	(17)	(16)	(22)	(32)	(2)
IBNR, Total	(103)
Other professional liability and management liability | Short-duration Insurance Contracts, Accident Year 2018
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	1,003	987	941	923	906	869	864	850
IBNR	$ 44
Cumulative number of claims | Claim	20,083
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 886	847	779	706	599	473	282	$ 54
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	16	46	18	17	37	5	14
IBNR, Total	153
Other professional liability and management liability | Short-Duration Insurance Contract, Accident Year 2019
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	984	970	939	876	856	845	837
IBNR	$ 79
Cumulative number of claims | Claim	19,577
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 864	801	699	567	422	263	$ 64
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	14	31	63	20	11	8
IBNR, Total	147
Other professional liability and management liability | Short-Duration Insurance Contract, Accident Year 2020
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	936	945	945	951	944	930
IBNR	$ 106
Cumulative number of claims | Claim	19,558
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 751	660	523	400	248	$ 67
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(9)	0	(6)	7	14
IBNR, Total	6
Other professional liability and management liability | Short-Duration Insurance Contract, Accident Year 2021
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	956	965	1,009	1,038	1,037
IBNR	$ 190
Cumulative number of claims | Claim	18,444
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 634	502	356	217	$ 58
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(9)	(44)	(29)	1
IBNR, Total	(81)
Other professional liability and management liability | Short-Duration Insurance Contract, Accident Year 2022
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	1,049	1,084	1,112	1,120
IBNR	$ 265
Cumulative number of claims | Claim	18,534
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 638	453	225	$ 64
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(35)	(28)	(8)
IBNR, Total	(71)
Other professional liability and management liability | Short-Duration Insurance Contract, Accident Year 2023
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	1,239	1,166	1,149
IBNR	$ 365
Cumulative number of claims | Claim	19,902
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 594	302	$ 64
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	73	17
IBNR, Total	90
Other professional liability and management liability | Short-Duration Insurance Contract, Accident Year 2024
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	1,200	1,150
IBNR	$ 609
Cumulative number of claims | Claim	20,579
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 315	$ 77
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	50
IBNR, Total	50
Other professional liability and management liability | Short-Duration Insurance Contract, Accident Year 2025
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	1,208
IBNR	$ 978
Cumulative number of claims | Claim	19,311
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 81