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, 2020 USD ($) claim	Dec. 31, 2019 USD ($)	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 ($)	Dec. 31, 2011 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 4,825
IBNR	905
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	439	$ 439	$ 437	$ 437	$ 434	$ 439	$ 468	$ 443	$ 437	$ 429
IBNR	$ 4
Cumulative Number of Claims | claim	16,537
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 497	499	493	484	493	498	508	469	$ 464
IBNR	$ 5
Cumulative Number of Claims | claim	17,739
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 531	545	535	525	513	500	479	$ 462
IBNR	$ 11
Cumulative Number of Claims | claim	19,537
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 527	529	535	530	537	489	$ 450
IBNR	$ 11
Cumulative Number of Claims | claim	19,770
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 510	488	494	510	499	$ 433
IBNR	$ 29
Cumulative Number of Claims | claim	18,122
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 508	499	485	487	$ 427
IBNR	$ 27
Cumulative Number of Claims | claim	15,998
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 460	458	449	$ 412
IBNR	$ 62
Cumulative Number of Claims | claim	15,008
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 431	429	$ 404
IBNR	$ 98
Cumulative Number of Claims | claim	14,531
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 445	$ 430
IBNR	$ 232
Cumulative Number of Claims | claim	13,045
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 477
IBNR	$ 426
Cumulative Number of Claims | claim	7,787