Claim and Claim Adjustment Expense Reserves (Specialty - Medical Professional Liability - Cumulative Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Medical Professional Liability - Specialty $ in Millions	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 ($)	Dec. 31, 2016 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 4,141
IBNR	920
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	509	$ 513	$ 514	$ 508	$ 510	$ 508	$ 499	$ 485	$ 487	$ 427
IBNR	$ 12
Cumulative Number of Claims | claim	16,205
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 458	463	456	460	455	460	458	449	$ 412
IBNR	$ 11
Cumulative Number of Claims | claim	15,400
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 507	499	495	470	448	431	429	$ 404
IBNR	$ 34
Cumulative Number of Claims | claim	15,358
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 478	481	469	471	458	445	$ 430
IBNR	$ 17
Cumulative Number of Claims | claim	14,572
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 400	419	447	455	476	$ 477
IBNR	$ 57
Cumulative Number of Claims | claim	11,381
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 318	349	374	376	$ 377
IBNR	$ 57
Cumulative Number of Claims | claim	10,085
2022
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 323	333	329	$ 329
IBNR	$ 87
Cumulative Number of Claims | claim	10,353
2023
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 382	350	$ 340
IBNR	$ 129
Cumulative Number of Claims | claim	11,129
2024
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 376	$ 343
IBNR	$ 199
Cumulative Number of Claims | claim	11,002
2025
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 390
IBNR	$ 317
Cumulative Number of Claims | claim	9,107