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, 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	$ 2,425
IBNR	262
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	292	$ 291	$ 294	$ 294	$ 294	$ 300	$ 302	$ 288	$ 281	$ 268
IBNR	$ 0
Cumulative Number of Claims | claim	47,907
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 296	297	299	299	307	303	299	289	$ 275
IBNR	$ 2
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	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	201	205	205	212	223	$ 234
IBNR	$ 1
Cumulative Number of Claims | claim	33,625
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 181	183	190	190	199	$ 201
IBNR	$ 3
Cumulative Number of Claims | claim	30,426
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 190	186	186	186	$ 198
IBNR	$ 2
Cumulative Number of Claims | claim	30,430
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 221	200	198	$ 199
IBNR	$ 7
Cumulative Number of Claims | claim	30,913
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 227	227	$ 229
IBNR	$ 10
Cumulative Number of Claims | claim	34,225
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 266	$ 257
IBNR	$ 48
Cumulative Number of Claims | claim	36,779
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 310
IBNR	$ 187
Cumulative Number of Claims | claim	24,427