Claim and Claim Adjustment Expense Reserves (Specialty - Medical Professional Liability - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions	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, 2016	Dec. 31, 2015
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 19,263	$ 18,163	$ 16,929	$ 16,300
Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	5,979	5,916
Medical Professional Liability | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	2,884
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,360
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2015	33
Liability for unallocated claim adjustment expenses for accident years presented	32
Total net liability for unpaid claim and claim adjustment expenses	1,425
Medical Professional Liability | 2015 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	471	463	458	444	$ 420	$ 384	$ 313	$ 230	$ 101	$ 22
Medical Professional Liability | 2016 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	489	483	460	436	401	339	246	121	$ 18
Medical Professional Liability | 2017 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	427	417	388	355	308	235	107	$ 19
Medical Professional Liability | 2018 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	453	418	349	290	211	115	$ 21
Medical Professional Liability | 2019 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	395	349	280	183	91	$ 17
Medical Professional Liability | 2020 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	258	201	139	61	$ 11
Medical Professional Liability | 2021 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	170	118	49	$ 11
Medical Professional Liability | 2022 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	122	57	$ 10
Medical Professional Liability | 2023 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	86	$ 14
Medical Professional Liability | 2024 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 13