Claim and Claim Adjustment Expense Reserves (Commercial - General Liability - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - General Liability - Commercial $ 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	$ 6,492
IBNR	1,629
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	690	$ 691	$ 691	$ 697	$ 726	$ 750	$ 709	$ 658	$ 664	$ 646
IBNR	$ 19
Cumulative Number of Claims | claim	44,229
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 667	669	670	681	676	677	631	589	$ 591
IBNR	$ 20
Cumulative Number of Claims | claim	39,361
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 630	635	635	619	636	639	611	$ 587
IBNR	$ 31
Cumulative Number of Claims | claim	35,219
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 620	623	613	655	650	655	$ 650
IBNR	$ 27
Cumulative Number of Claims | claim	33,570
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 658	635	631	654	658	$ 653
IBNR	$ 57
Cumulative Number of Claims | claim	27,877
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 600	589	574	576	$ 581
IBNR	$ 73
Cumulative Number of Claims | claim	23,834
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 671	667	659	$ 623
IBNR	$ 166
Cumulative Number of Claims | claim	23,817
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 632	632	$ 632
IBNR	$ 226
Cumulative Number of Claims | claim	21,114
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 644	$ 653
IBNR	$ 408
Cumulative Number of Claims | claim	17,889
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 680
IBNR	$ 602
Cumulative Number of Claims | claim	12,916