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, 2017 USD ($) claims	Dec. 31, 2016 USD ($)	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 ($)	Dec. 31, 2008 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 6,147
IBNR	1,412
2008
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	592	$ 591	$ 600	$ 613	$ 632	$ 633	$ 647	$ 630	$ 604	$ 611
IBNR	$ 13
Cumulative Number of Claims | claims	44,655
2009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 627	622	619	623	629	633	634	637	$ 591
IBNR	$ 18
Cumulative Number of Claims | claims	44,038
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 654	659	675	695	649	599	597	$ 566
IBNR	$ 19
Cumulative Number of Claims | claims	43,472
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 615	621	611	610	564	534	$ 537
IBNR	$ 29
Cumulative Number of Claims | claims	38,216
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 569	558	570	579	563	$ 539
IBNR	$ 34
Cumulative Number of Claims | claims	34,249
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 604	643	634	645	$ 615
IBNR	$ 62
Cumulative Number of Claims | claims	33,255
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 627	635	634	$ 627
IBNR	$ 131
Cumulative Number of Claims | claims	27,478
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 585	574	$ 573
IBNR	$ 208
Cumulative Number of Claims | claims	23,082
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 647	$ 622
IBNR	$ 351
Cumulative Number of Claims | claims	21,893
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 627
IBNR	$ 547
Cumulative Number of Claims | claims	15,375