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, 2018 USD ($) claims	Dec. 31, 2017 USD ($)	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 ($)	Dec. 31, 2009 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 2,411
IBNR	239
2009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	277	$ 276	$ 276	$ 278	$ 281	$ 285	$ 282	$ 280	$ 276	$ 291
IBNR	$ 0
Cumulative Number of Claims | claims	48,499
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 288	288	289	293	298	291	287	283	$ 267
IBNR	$ 1
Cumulative Number of Claims | claims	48,030
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 294	294	294	300	302	288	281	$ 268
IBNR	$ 4
Cumulative Number of Claims | claims	47,905
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 299	299	307	303	299	289	$ 275
IBNR	$ 6
Cumulative Number of Claims | claims	46,288
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 245	245	249	265	265	$ 246
IBNR	$ 7
Cumulative Number of Claims | claims	39,429
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 205	205	212	223	$ 234
IBNR	$ 9
Cumulative Number of Claims | claims	33,609
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 190	190	199	$ 201
IBNR	$ 22
Cumulative Number of Claims | claims	30,388
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 186	186	$ 198
IBNR	$ 27
Cumulative Number of Claims | claims	30,342
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 198	$ 199
IBNR	$ 45
Cumulative Number of Claims | claims	30,580
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 229
IBNR	$ 118
Cumulative Number of Claims | claims	28,602