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Claim and Claim Adjustment Expense Reserves (Commercial - General Liability - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - General Liability - 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 | $ 6,147 |  |  |  |  |  |  |  |  |
IBNR | 1,412 |  |  |  |  |  |  |  |  |
2008 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | 592 | $ 591 | $ 600 | $ 613 | $ 632 | $ 633 | $ 647 | $ 630 | $ 604 | $ 611
IBNR | $ 13 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 44,655 |  |  |  |  |  |  |  |  |
2009 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 627 | 622 | 619 | 623 | 629 | 633 | 634 | 637 | $ 591 |
IBNR | $ 18 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 44,038 |  |  |  |  |  |  |  |  |
2010 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 654 | 659 | 675 | 695 | 649 | 599 | 597 | $ 566 |  |
IBNR | $ 19 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 43,472 |  |  |  |  |  |  |  |  |
2011 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 615 | 621 | 611 | 610 | 564 | 534 | $ 537 |  |  |
IBNR | $ 29 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 38,216 |  |  |  |  |  |  |  |  |
2012 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 569 | 558 | 570 | 579 | 563 | $ 539 |  |  |  |
IBNR | $ 34 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 34,249 |  |  |  |  |  |  |  |  |
2013 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 604 | 643 | 634 | 645 | $ 615 |  |  |  |  |
IBNR | $ 62 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 33,255 |  |  |  |  |  |  |  |  |
2014 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 627 | 635 | 634 | $ 627 |  |  |  |  |  |
IBNR | $ 131 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 27,478 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 585 | 574 | $ 573 |  |  |  |  |  |  |
IBNR | $ 208 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 23,082 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 647 | $ 622 |  |  |  |  |  |  |  |
IBNR | $ 351 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 21,893 |  |  |  |  |  |  |  |  |
2017 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 627 |  |  |  |  |  |  |  |  |
IBNR | $ 547 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 15,375 |  |  |  |  |  |  |  |  |

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