Claim and Claim Adjustment Expense Reserves and Future Policy Benefit Reserves (Specialty - Medical Professional Liability - Cumulative Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Medical Professional Liability - Specialty $ in Millions	Dec. 31, 2022 USD ($) claim	Dec. 31, 2021 USD ($)	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 ($)	Dec. 31, 2013 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 4,624
IBNR	988
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	530	$ 530	$ 531	$ 545	$ 535	$ 525	$ 513	$ 500	$ 479	$ 462
IBNR	$ 5
Cumulative Number of Claims | claim	19,587
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 527	524	527	529	535	530	537	489	$ 450
IBNR	$ 12
Cumulative Number of Claims | claim	19,818
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 498	501	510	488	494	510	499	$ 433
IBNR	$ 22
Cumulative Number of Claims | claim	18,203
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 508	510	508	499	485	487	$ 427
IBNR	$ 12
Cumulative Number of Claims | claim	16,136
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 460	455	460	458	449	$ 412
IBNR	$ 30
Cumulative Number of Claims | claim	15,288
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 470	448	431	429	$ 404
IBNR	$ 31
Cumulative Number of Claims | claim	15,163
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 471	458	445	$ 430
IBNR	$ 101
Cumulative Number of Claims | claim	14,189
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 455	476	$ 477
IBNR	$ 226
Cumulative Number of Claims | claim	10,679
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 376	$ 377
IBNR	$ 259
Cumulative Number of Claims | claim	8,801
2022
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 329
IBNR	$ 290
Cumulative Number of Claims | claim	6,717