Legal Proceedings, Commitments and Contingencies, and Guarantees (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]
Reduction to Earned Premium due to the Small Business Premium Rate Adjustment Contingency	$ 16
Expected Premium Refund Related to the Small Business Premium Rate Adjustment Contingency	30
Property Lease Guarantee [Member]
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	11
Guarantee and Indemnification [Member]
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]
Guarantor Obligations, Current Carrying Value	5	$ 5
Guarantee agreement [Member]
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	375
Indemnification Agreement [Member]
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	258
Guarantee Obligations [Member]
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 1,900

Expected Premium Refund Related to the Small Business Premium Rate Adjustment Contingency

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- Definition

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

+ References

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Reduction to Earned Premium due to the Small Business Premium Rate Adjustment Contingency

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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=6851643&loc=d3e12069-110248

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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=6851643&loc=d3e12069-110248

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