Legal Proceedings, Contingencies and Guarantees (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
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

Adverse Premium Development Due To The Small Business Premium Rate Adjustment Contingency, Recognized

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- Definition

[Line Items] for Legal Proceedings, Commitments and Contingencies, and Guarantees [Table]

+ References

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Reduction To Pretax Operating Income Due To Interest Due To Policyholders On The Premium Rate Adjustments

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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

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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

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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

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