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Investments - Rollforward of the cumulative credit losses recognized in earnings for fixed income securities held (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Credit Losses on Fixed Income Securities |  |  |
Beginning balance | $ (318) | $ (392) | $ (380)
Additional credit loss for securities previously other-than-temporarily impaired | (18) | (21) | (30)
Additional credit loss for securities not previously other-than-temporarily impaired | (8) | (23) | (45)
Reduction in credit loss for securities disposed or collected | 116 | 117 | 60
Change in credit loss due to accretion of increase in cash flows | 2 | 1 | 3
Ending balance | $ (226) | $ (318) | $ (392)

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- Definition This item represents a change in the cumulative amount of credit losses recognized in earnings for a debt security due to accretion of increase in cash flows and time value of cash flows. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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 + Details
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- Definition 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. + 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 (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563 + Details
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Balance Type: | credit
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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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