Claim and Claim Adjustment Expense Reserves (Specialty - Medical Professional Liability - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Specialty - USD ($) $ in Millions	12 Months Ended									24 Months Ended	36 Months Ended	48 Months Ended	60 Months Ended	72 Months Ended	84 Months Ended	96 Months Ended	108 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2025	Dec. 31, 2025	Dec. 31, 2025	Dec. 31, 2025	Dec. 31, 2025	Dec. 31, 2025	Dec. 31, 2025	Dec. 31, 2025
Claims Development [Line Items]
Total	$ 0	$ (8)	$ 5
Medical Professional Liability
Claims Development [Line Items]
Cumulative Number of Claims	1	(17)	15
Total net development for accident years prior to 2016	1	9	(10)
Total unallocated claim adjustment expense development	(2)	0	0
Total	0	(8)	5
Medical Professional Liability | 2016
Claims Development [Line Items]
Cumulative Number of Claims	(4)	(1)	6	$ (2)	$ 2	$ 9	$ 14	$ (2)	$ 60								$ 82
Medical Professional Liability | 2017
Claims Development [Line Items]
Cumulative Number of Claims	(5)	7	(4)	5	(5)	2	9	$ 37								$ 46
Medical Professional Liability | 2018
Claims Development [Line Items]
Cumulative Number of Claims	8	4	25	22	17	2	$ 25								$ 103
Medical Professional Liability | 2019
Claims Development [Line Items]
Cumulative Number of Claims	(3)	12	(2)	13	13	$ 15								$ 48
Medical Professional Liability | 2020
Claims Development [Line Items]
Cumulative Number of Claims	(19)	(28)	(8)	(21)	$ (1)								$ (77)
Medical Professional Liability | 2021
Claims Development [Line Items]
Cumulative Number of Claims	(31)	(25)	(2)	$ (1)								$ (59)
Medical Professional Liability | 2022
Claims Development [Line Items]
Cumulative Number of Claims	(10)	4	$ 0								$ (6)
Medical Professional Liability | 2023
Claims Development [Line Items]
Cumulative Number of Claims	32	$ 10								$ 42
Medical Professional Liability | 2024
Claims Development [Line Items]
Cumulative Number of Claims	$ 33