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, 2018 USD ($) claims	Dec. 31, 2017 USD ($)	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 ($)	Dec. 31, 2009 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 4,498
IBNR	824
2009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	310	$ 302	$ 309	$ 336	$ 346	$ 366	$ 392	$ 392	$ 390	$ 389
IBNR	$ 0
Cumulative Number of Claims | claims	14,716
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 356	360	365	395	415	426	423	412	$ 402
IBNR	$ 0
Cumulative Number of Claims | claims	14,615
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 437	437	434	439	468	443	437	$ 429
IBNR	$ 8
Cumulative Number of Claims | claims	16,505
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 493	484	493	498	508	469	$ 464
IBNR	$ 10
Cumulative Number of Claims | claims	17,691
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 535	525	513	500	479	$ 462
IBNR	$ 28
Cumulative Number of Claims | claims	19,442
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 535	530	537	489	$ 450
IBNR	$ 30
Cumulative Number of Claims | claims	19,602
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 494	510	499	$ 433
IBNR	$ 78
Cumulative Number of Claims | claims	17,835
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 485	487	$ 427
IBNR	$ 120
Cumulative Number of Claims | claims	15,427
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 449	$ 412
IBNR	$ 217
Cumulative Number of Claims | claims	13,777
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 404
IBNR	$ 333
Cumulative Number of Claims | claims	11,205