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, 2024 USD ($) claim	Dec. 31, 2023 USD ($)	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 ($)	Dec. 31, 2015 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 4,244
IBNR	916
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	494	$ 494	$ 498	$ 501	$ 510	$ 488	$ 494	$ 510	$ 499	$ 433
IBNR	$ 11
Cumulative Number of Claims | claim	18,228
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 513	514	508	510	508	499	485	487	$ 427
IBNR	$ 16
Cumulative Number of Claims | claim	16,195
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 463	456	460	455	460	458	449	$ 412
IBNR	$ 18
Cumulative Number of Claims | claim	15,384
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 499	495	470	448	431	429	$ 404
IBNR	$ 28
Cumulative Number of Claims | claim	15,331
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 481	469	471	458	445	$ 430
IBNR	$ 43
Cumulative Number of Claims | claim	14,515
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 419	447	455	476	$ 477
IBNR	$ 100
Cumulative Number of Claims | claim	11,289
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 349	374	376	$ 377
IBNR	$ 117
Cumulative Number of Claims | claim	9,935
2022
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 333	329	$ 329
IBNR	$ 143
Cumulative Number of Claims | claim	9,965
2023
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 350	$ 340
IBNR	$ 162
Cumulative Number of Claims | claim	10,424
2024
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 343
IBNR	$ 278
Cumulative Number of Claims | claim	8,561