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Claim and Claim Adjustment Expense Reserves (Commercial - Workers' Compensation - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Workers' Compensation - 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 | $ 5,606 |  |  |  |  |  |  |  |  |
IBNR | 958 |  |  |  |  |  |  |  |  |
2008 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | 652 | $ 638 | $ 630 | $ 622 | $ 612 | $ 608 | $ 606 | $ 593 | $ 575 | $ 558
IBNR | $ 34 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 59,911 |  |  |  |  |  |  |  |  |
2009 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 632 | 625 | 617 | 611 | 600 | 596 | 594 | 587 | $ 583 |
IBNR | $ 34 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 51,161 |  |  |  |  |  |  |  |  |
2010 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 721 | 717 | 697 | 683 | 663 | 641 | 619 | $ 576 |  |
IBNR | $ 33 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 48,144 |  |  |  |  |  |  |  |  |
2011 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 668 | 666 | 642 | 648 | 637 | 628 | $ 593 |  |  |
IBNR | $ 23 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 44,691 |  |  |  |  |  |  |  |  |
2012 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 667 | 671 | 661 | 648 | 616 | $ 589 |  |  |  |
IBNR | $ 63 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 41,756 |  |  |  |  |  |  |  |  |
2013 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 584 | 610 | 584 | 563 | $ 528 |  |  |  |  |
IBNR | $ 57 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 38,153 |  |  |  |  |  |  |  |  |
2014 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 448 | 474 | 474 | $ 459 |  |  |  |  |  |
IBNR | $ 106 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 33,072 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 401 | 426 | $ 416 |  |  |  |  |  |  |
IBNR | $ 154 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 31,470 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 399 | $ 421 |  |  |  |  |  |  |  |
IBNR | $ 200 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 31,310 |  |  |  |  |  |  |  |  |
2017 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 434 |  |  |  |  |  |  |  |  |
IBNR | $ 254 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 27,929 |  |  |  |  |  |  |  |  |

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