| **Claim and Claim Adjustment Expense Reserves (Reconciliation of Liability for Unpaid Claim and Claim Adjustment Expense) (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]**                                                                                           |          |          |          |          |
| Total net liability for unpaid claim and claim adjustment expenses                                                                                            | $ 18,070      | $ 18,249      | $ 18,576      | $ 18,927      |
| Reinsurance receivables                                                                                                                                       | 3,934         |          |          |          |
| Claim and claim adjustment expenses                                                                                                                           | 22,004        | 22,343        |          |          |
| Specialty                                                                                                                                                     |          |          |          |          |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                           |          |          |          |          |
| Total net liability for unpaid claim and claim adjustment expenses                                                                                            | 5,179         | 5,404         |          |          |
| Reinsurance receivables                                                                                                                                       | 669           |          |          |          |
| Commercial                                                                                                                                                    |          |          |          |          |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                           |          |          |          |          |
| Total net liability for unpaid claim and claim adjustment expenses                                                                                            | 7,964         | 8,305         |          |          |
| Reinsurance receivables                                                                                                                                       | 621           |          |          |          |
| International                                                                                                                                                 |          |          |          |          |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                           |          |          |          |          |
| Total net liability for unpaid claim and claim adjustment expenses                                                                                            | 1,432         | 1,201         |          |          |
| Reinsurance receivables                                                                                                                                       | 204           |          |          |          |
| Corporate & Other                                                                                                                                         |          |          |          |          |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                           |          |          |          |          |
| Total net liability for unpaid claim and claim adjustment expenses                                                                                            | 205           | $ 230         |          |          |
| Reinsurance receivables                                                                                                                                       | 2,231         |          |          |          |
| Life and Group                                                                                                                                                |          |          |          |          |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                           |          |          |          |          |
| Total net liability for unpaid claim and claim adjustment expenses                                                                                            | 3,290         |          |          |          |
| Reinsurance receivables                                                                                                                                       | $ 209         |          |          |          |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.
+ References
Reference 1: 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=d3e14764-158437Reference 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
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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=d3e572229-122910Reference 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=d3e14764-158437Reference 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
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before valuation allowance of reinsurance recoverables for losses reported to the ceding insurer but not yet paid, and for amounts expected based upon statistical projections and other measures of incurred losses and loss settlement expenses which have not yet been reported to the ceding insurer.
+ References
No definition available.
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