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, 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	$ 6,541
IBNR	1,789
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	667	$ 667	$ 669	$ 670	$ 681	$ 676	$ 677	$ 631	$ 589	$ 591
IBNR	$ 21
Cumulative Number of Claims | claim	39,405
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 632	630	635	635	619	636	639	611	$ 587
IBNR	$ 24
Cumulative Number of Claims | claim	35,276
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 623	620	623	613	655	650	655	$ 650
IBNR	$ 27
Cumulative Number of Claims | claim	33,649
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 659	658	635	631	654	658	$ 653
IBNR	$ 44
Cumulative Number of Claims | claim	27,972
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 602	600	589	574	576	$ 581
IBNR	$ 38
Cumulative Number of Claims | claim	24,005
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 673	671	667	659	$ 623
IBNR	$ 104
Cumulative Number of Claims | claim	24,215
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 634	632	632	$ 632
IBNR	$ 136
Cumulative Number of Claims | claim	21,781
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 646	644	$ 653
IBNR	$ 302
Cumulative Number of Claims | claim	19,234
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 682	$ 680
IBNR	$ 453
Cumulative Number of Claims | claim	17,294
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 723
IBNR	$ 640
Cumulative Number of Claims | claim	9,593