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Legal Proceedings, Contingencies and Guarantees (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items] |  |
Adverse premium development due to the small business premium rate adjustment contingency, recognized | $ 36 |
Reduction to pretax operating income due to interest due to policyholders on the premium rate adjustments | 7 | $ 16
Guarantee Obligations |  |
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items] |  |
Guarantor obligations, maximum exposure, undiscounted | 1,800 |
Indemnification Agreement |  |
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items] |  |
Guarantor obligations, maximum exposure, undiscounted | 252 |
Guarantee agreement |  |
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items] |  |
Guarantor obligations, maximum exposure, undiscounted | 375 |
Guarantee and Indemnification |  |
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items] |  |
Guarantor obligations, current carrying calue | 5 | $ 5
Insurance Premium Rate Adjustment | Multi-Peril Package Product |  |
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items] |  |
Loss contingency accrual | $ 60 |

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- Definition Adverse Premium Development Due To The Small Business Premium Rate Adjustment Contingency, Recognized + References No definition available. + Details
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- Definition [Line Items] for Legal Proceedings, Commitments and Contingencies, and Guarantees [Table] + References No definition available. + Details
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- Definition The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82913245&loc=d3e12069-110248 + Details
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- Definition Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=82913245&loc=d3e12069-110248 + Details
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- Definition Amount of loss contingency liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349 + Details
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