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, 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	$ 2,371
IBNR	207
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	288	$ 288	$ 288	$ 289	$ 293	$ 298	$ 291	$ 287	$ 283	$ 267
IBNR	$ 1
Cumulative Number of Claims | claim	48,035
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 291	294	294	294	300	302	288	281	$ 268
IBNR	$ 0
Cumulative Number of Claims | claim	47,909
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 297	299	299	307	303	299	289	$ 275
IBNR	$ 3
Cumulative Number of Claims | claim	46,288
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 241	245	245	249	265	265	$ 246
IBNR	$ 2
Cumulative Number of Claims | claim	39,429
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 201	205	205	212	223	$ 234
IBNR	$ 3
Cumulative Number of Claims | claim	33,622
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 183	190	190	199	$ 201
IBNR	$ 7
Cumulative Number of Claims | claim	30,418
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 186	186	186	$ 198
IBNR	$ 7
Cumulative Number of Claims | claim	30,414
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 200	198	$ 199
IBNR	$ 9
Cumulative Number of Claims | claim	30,850
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 227	$ 229
IBNR	$ 47
Cumulative Number of Claims | claim	33,959
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 257
IBNR	$ 128
Cumulative Number of Claims | claim	31,455