Claim and Claim Adjustment Expense Reserves - Medical Professional 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
Medical professional liability
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	4,141
IBNR	920
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	2,771
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,370
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2016	47
Liability for unallocated claim adjustment expenses for accident years presented	51
Total net liability for unpaid claim and claim adjustment expenses	1,468
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	0	(8)	5
Total net development for the accident years presented above	1	(17)	15
Total net development for accident years prior to 2016	1	9	(10)
Total unallocated claim adjustment expense development	(2)	0	0
Medical professional liability | Short-duration Insurance Contracts, Accident Year 2016
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	509	513	514	508	$ 510	$ 508	$ 499	$ 485	$ 487	$ 427
IBNR	$ 12
Cumulative number of claims | Claim	16,205
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 491	489	483	460	436	401	339	246	121	$ 18
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(4)	(1)	6	(2)	2	9	14	(2)	60
IBNR, Total	82
Medical professional liability | Short-duration Insurance Contracts, Accident Year 2017
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	458	463	456	460	455	460	458	449	412
IBNR	$ 11
Cumulative number of claims | Claim	15,400
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 438	427	417	388	355	308	235	107	$ 19
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(5)	7	(4)	5	(5)	2	9	37
IBNR, Total	46
Medical professional liability | Short-duration Insurance Contracts, Accident Year 2018
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	507	499	495	470	448	431	429	404
IBNR	$ 34
Cumulative number of claims | Claim	15,358
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 463	453	418	349	290	211	115	$ 21
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	8	4	25	22	17	2	25
IBNR, Total	103
Medical professional liability | Short-Duration Insurance Contract, Accident Year 2019
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	478	481	469	471	458	445	430
IBNR	$ 17
Cumulative number of claims | Claim	14,572
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 425	395	349	280	183	91	$ 17
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(3)	12	(2)	13	13	15
IBNR, Total	48
Medical professional liability | Short-Duration Insurance Contract, Accident Year 2020
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	400	419	447	455	476	477
IBNR	$ 57
Cumulative number of claims | Claim	11,381
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 303	258	201	139	61	$ 11
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(19)	(28)	(8)	(21)	(1)
IBNR, Total	(77)
Medical professional liability | Short-Duration Insurance Contract, Accident Year 2021
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	318	349	374	376	377
IBNR	$ 57
Cumulative number of claims | Claim	10,085
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 223	170	118	49	$ 11
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(31)	(25)	(2)	(1)
IBNR, Total	(59)
Medical professional liability | Short-Duration Insurance Contract, Accident Year 2022
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	323	333	329	329
IBNR	$ 87
Cumulative number of claims | Claim	10,353
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 171	122	57	$ 10
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	(10)	4	0
IBNR, Total	(6)
Medical professional liability | Short-Duration Insurance Contract, Accident Year 2023
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	382	350	340
IBNR	$ 129
Cumulative number of claims | Claim	11,129
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 160	86	$ 14
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	32	10
IBNR, Total	42
Medical professional liability | Short-Duration Insurance Contract, Accident Year 2024
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	376	343
IBNR	$ 199
Cumulative number of claims | Claim	11,002
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 82	$ 13
Net Strengthening (Releases) of Prior Accident Year Reserves
Total	33
IBNR, Total	33
Medical professional liability | Short-Duration Insurance Contract, Accident Year 2025
Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses
Total	390
IBNR	$ 317
Cumulative number of claims | Claim	9,107
Cumulative Net Paid Claim and Allocated Claim Adjustment Expenses
Total	$ 15