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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 ($) | Dec. 31, 2008 USD ($)
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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 |  |  |  |  |  |  |  |  |

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