Investments - Investments - Other Than Temporary Impairment (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Credit impairments	$ 0	$ 1	$ 0	$ 2
Intent-to-sell impairments	0	0	0	0
Impairments on equity securities and other impairments	0	2	0	3
Cumulative Credit Impairments
Beginning balance	(21)	(99)	(25)	(110)
Additions for credit impairments recognized on:
Securities not previously impaired	0	(1)	0	(1)
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, Additional Credit Losses	0	0	0	1
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period	1	1	5	9
Securities due to an increase in expected cash flows	0	5	0	9
Ending balance	(20)	(94)	(20)	(94)
Other than Temporary Impairment Losses, Investments	0	2	0	3
Equity Securities [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Impairments on equity securities and other impairments	$ 0	$ 1	$ 0	$ 1

Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions [Abstract]

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Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions [Abstract]

+ References

No definition available.

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The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=82895926&loc=d3e25351-111560

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- Definition

This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which a previous OTTI was recognized and the investor does not intend to sell the debt security and it is not more likely than not that the investor will be required to sell the debt security before recovery of its amortized cost basis.

+ References

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- Definition

This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which no such other than temporary impairment (OTTI) was previously recognized.

+ References

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- Definition

The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563

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- 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.

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This item represents reductions in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held due to increases in cash flows expected to be collected and recognized over the remaining life of the security.

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This item represents reductions in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security because of the sale of impaired securities during the period.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563

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The amount of other than temporary impairment losses on investments in debt and equity securities categorized as Available-for-sale which have been recognized in earnings during the period. Such impairment losses consist of other than temporary impairments (OTTI) on equity securities, credit losses on investments in debt securities, and when the entity intends to sell an impaired debt security or it is more likely than not that the entity will be required to sell the impaired debt security before recovery of its amortized cost basis.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=82896770&loc=SL6284422-111562

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The amount of other than temporary impairment losses on investments in debt securities categorized as Held-to-maturity which have been recognized in earnings during the period. Such impairment losses consist of other than temporary impairments (OTTI) of investments in debt securities due to credit losses and other than temporary impairments due to other than credit losses when the entity intends to sell an impaired debt security or it is more likely than not that the entity will be required to sell the impaired debt security before recovery of its amortized cost basis.

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