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Claim and Claim Adjustment Expense Reserves (Gross and net carried claim and claim adjustment expense reserves) (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | --- | --- | ---
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Gross Case Reserves | $ 11,670 | $ 11,860 |  |
Gross IBNR Reserves | 10,334 | 10,483 |  |
Total Gross Carried Claim and Claim Adjustment Expense Reserves | 22,004 | 22,343 | $ 22,663 | $ 23,271
Net Case Reserves | 9,645 | 9,627 |  |
Net IBNR Reserves | 8,425 | 8,622 |  |
Total Net Carried Claim and Claim Adjustment Expense Reserves | 18,070 | 18,249 | $ 18,576 | $ 18,927
Specialty |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Gross Case Reserves | 1,805 | 1,871 |  |
Gross IBNR Reserves | 4,043 | 4,278 |  |
Total Gross Carried Claim and Claim Adjustment Expense Reserves | 5,848 | 6,149 |  |
Net Case Reserves | 1,656 | 1,681 |  |
Net IBNR Reserves | 3,523 | 3,723 |  |
Total Net Carried Claim and Claim Adjustment Expense Reserves | 5,179 | 5,404 |  |
Commercial |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Gross Case Reserves | 4,364 | 4,661 |  |
Gross IBNR Reserves | 4,221 | 4,233 |  |
Total Gross Carried Claim and Claim Adjustment Expense Reserves | 8,585 | 8,894 |  |
Net Case Reserves | 4,047 | 4,353 |  |
Net IBNR Reserves | 3,917 | 3,952 |  |
Total Net Carried Claim and Claim Adjustment Expense Reserves | 7,964 | 8,305 |  |
International |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Gross Case Reserves | 744 | 632 |  |
Gross IBNR Reserves | 892 | 696 |  |
Total Gross Carried Claim and Claim Adjustment Expense Reserves | 1,636 | 1,328 |  |
Net Case Reserves | 640 | 548 |  |
Net IBNR Reserves | 792 | 653 |  |
Total Net Carried Claim and Claim Adjustment Expense Reserves | 1,432 | 1,201 |  |
Life & Group |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Gross Case Reserves | 3,386 | 3,172 |  |
Gross IBNR Reserves | 113 | 186 |  |
Total Gross Carried Claim and Claim Adjustment Expense Reserves | 3,499 | 3,358 |  |
Net Case Reserves | 3,208 | 2,951 |  |
Net IBNR Reserves | 82 | 158 |  |
Total Net Carried Claim and Claim Adjustment Expense Reserves | 3,290 | 3,109 |  |
Corporate & Other |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Gross Case Reserves | 1,371 | 1,524 |  |
Gross IBNR Reserves | 1,065 | 1,090 |  |
Total Gross Carried Claim and Claim Adjustment Expense Reserves | 2,436 | 2,614 |  |
Net Case Reserves | 94 | 94 |  |
Net IBNR Reserves | 111 | 136 |  |
Total Net Carried Claim and Claim Adjustment Expense Reserves | $ 205 | $ 230 |  |

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- Definition Estimated costs as of the balance sheet date of settling insured claims and costs incurred in the claims settlement process for claims that have not yet been submitted to the insurance company for reimbursement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 5.W) -URI http://asc.fasb.org/extlink&oid=27015833&loc=d3e570705-122907 + Details
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- Definition Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(13)(a)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 + Details
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