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, 2019 USD ($) claim	Dec. 31, 2018 USD ($)	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 ($)	Dec. 31, 2010 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 4,685
IBNR	853
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	369	$ 356	$ 360	$ 365	$ 395	$ 415	$ 426	$ 423	$ 412	$ 402
IBNR	$ 1
Cumulative Number of Claims | claim	14,624
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 439	437	437	434	439	468	443	437	$ 429
IBNR	$ 2
Cumulative Number of Claims | claim	16,526
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 499	493	484	493	498	508	469	$ 464
IBNR	$ 8
Cumulative Number of Claims | claim	17,724
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 545	535	525	513	500	479	$ 462
IBNR	$ 27
Cumulative Number of Claims | claim	19,510
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 529	535	530	537	489	$ 450
IBNR	$ 16
Cumulative Number of Claims | claim	19,723
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 488	494	510	499	$ 433
IBNR	$ 29
Cumulative Number of Claims | claim	18,029
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 499	485	487	$ 427
IBNR	$ 63
Cumulative Number of Claims | claim	15,823
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 458	449	$ 412
IBNR	$ 127
Cumulative Number of Claims | claim	14,636
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 429	$ 404
IBNR	$ 216
Cumulative Number of Claims | claim	13,760
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 430
IBNR	$ 364
Cumulative Number of Claims | claim	10,467