Claim and Claim Adjustment Expense Reserves (Commercial - Commercial Auto - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Commercial Auto - 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	$ 2,433
IBNR	245
2008
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	297	$ 296	$ 298	$ 298	$ 305	$ 309	$ 306	$ 316	$ 323	$ 322
IBNR	$ 0
Cumulative Number of Claims | claims	56,424
2009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 272	272	275	277	281	278	274	272	$ 287
IBNR	$ 0
Cumulative Number of Claims | claims	47,343
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 280	281	286	291	283	279	274	$ 262
IBNR	$ 1
Cumulative Number of Claims | claims	46,335
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 285	285	290	293	279	273	$ 262
IBNR	$ 3
Cumulative Number of Claims | claims	46,691
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 292	300	296	292	282	$ 270
IBNR	$ 7
Cumulative Number of Claims | claims	45,288
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 237	241	257	259	$ 242
IBNR	$ 10
Cumulative Number of Claims | claims	38,539
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 204	210	221	$ 231
IBNR	$ 19
Cumulative Number of Claims | claims	33,029
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 187	197	$ 199
IBNR	$ 35
Cumulative Number of Claims | claims	29,924
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 183	$ 196
IBNR	$ 53
Cumulative Number of Claims | claims	29,745
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 196
IBNR	$ 117
Cumulative Number of Claims | claims	25,173