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, 2023 USD ($) claim	Dec. 31, 2022 USD ($)	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 ($)	Dec. 31, 2014 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 4,443
IBNR	1,021
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	525	$ 527	$ 524	$ 527	$ 529	$ 535	$ 530	$ 537	$ 489	$ 450
IBNR	$ 6
Cumulative Number of Claims | claim	19,830
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 494	498	501	510	488	494	510	499	$ 433
IBNR	$ 15
Cumulative Number of Claims | claim	18,218
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 514	508	510	508	499	485	487	$ 427
IBNR	$ 17
Cumulative Number of Claims | claim	16,169
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 456	460	455	460	458	449	$ 412
IBNR	$ 20
Cumulative Number of Claims | claim	15,345
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 495	470	448	431	429	$ 404
IBNR	$ 41
Cumulative Number of Claims | claim	15,266
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 469	471	458	445	$ 430
IBNR	$ 62
Cumulative Number of Claims | claim	14,409
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 447	455	476	$ 477
IBNR	$ 180
Cumulative Number of Claims | claim	11,129
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 374	376	$ 377
IBNR	$ 193
Cumulative Number of Claims | claim	9,523
2022
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 329	$ 329
IBNR	$ 206
Cumulative Number of Claims | claim	9,237
2023
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 340
IBNR	$ 281
Cumulative Number of Claims | claim	8,240