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Claim and Claim Adjustment Expense Reserves (Specialty - Medical Professional Liability - Cumulative Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Medical Professional Liability - Specialty $ 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 | $ 4,971 |  |  |  |  |  |  |  |  |
IBNR | 995 |  |  |  |  |  |  |  |  |
2008 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | 438 | $ 442 | $ 455 | $ 467 | $ 468 | $ 468 | $ 480 | $ 496 | $ 451 | $ 426
IBNR | $ 4 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 14,102 |  |  |  |  |  |  |  |  |
2009 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 422 | 432 | 463 | 471 | 480 | 506 | 494 | 469 | $ 462 |
IBNR | $ 3 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 15,594 |  |  |  |  |  |  |  |  |
2010 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 398 | 403 | 446 | 470 | 486 | 478 | 478 | $ 483 |  |
IBNR | $ 9 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 15,239 |  |  |  |  |  |  |  |  |
2011 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 491 | 491 | 501 | 533 | 507 | 492 | $ 486 |  |  |
IBNR | $ 16 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 17,481 |  |  |  |  |  |  |  |  |
2012 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 563 | 559 | 567 | 575 | 529 | $ 526 |  |  |  |
IBNR | $ 39 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 18,503 |  |  |  |  |  |  |  |  |
2013 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 573 | 567 | 560 | 540 | $ 534 |  |  |  |  |
IBNR | $ 44 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 19,777 |  |  |  |  |  |  |  |  |
2014 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 564 | 585 | 548 | $ 511 |  |  |  |  |  |
IBNR | $ 78 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 19,764 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 543 | 539 | $ 480 |  |  |  |  |  |  |
IBNR | $ 164 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 17,690 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 527 | $ 469 |  |  |  |  |  |  |  |
IBNR | $ 268 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 14,743 |  |  |  |  |  |  |  |  |
2017 |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred claims and allocated claim adjustment expense, net | $ 452 |  |  |  |  |  |  |  |  |
IBNR | $ 370 |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | claims | 11,137 |  |  |  |  |  |  |  |  |

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