Claim, Claim Adjustment Expense 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, 2021 USD ($) claim	Dec. 31, 2020 USD ($)	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 ($)	Dec. 31, 2012 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 4,776
IBNR	1,016
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	497	$ 497	$ 499	$ 493	$ 484	$ 493	$ 498	$ 508	$ 469	$ 464
IBNR	$ 4
Cumulative Number of Claims | claim	17,755
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 530	531	545	535	525	513	500	479	$ 462
IBNR	$ 11
Cumulative Number of Claims | claim	19,565
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 524	527	529	535	530	537	489	$ 450
IBNR	$ 8
Cumulative Number of Claims | claim	19,800
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 501	510	488	494	510	499	$ 433
IBNR	$ 28
Cumulative Number of Claims | claim	18,170
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 510	508	499	485	487	$ 427
IBNR	$ 24
Cumulative Number of Claims | claim	16,085
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 455	460	458	449	$ 412
IBNR	$ 41
Cumulative Number of Claims | claim	15,197
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 448	431	429	$ 404
IBNR	$ 60
Cumulative Number of Claims | claim	14,997
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 458	445	$ 430
IBNR	$ 156
Cumulative Number of Claims | claim	13,804
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 476	$ 477
IBNR	$ 347
Cumulative Number of Claims | claim	9,935
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 377
IBNR	$ 337
Cumulative Number of Claims | claim	6,761