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Investments - Unrealized loss and Limited partnerships (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Other details of unrealized loss |  |
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost | $ 269 |
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost | 26 |
Limited partnership interests | 6,740 | $ 5,814
Fixed income and equity securities |  |
Other details of unrealized loss |  |
Available-for-sale securities, continuous unrealized loss position, accumulated loss | 295 | 532
Investment grade fixed income securities |  |
Other details of unrealized loss |  |
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost | 219 |
Available-for-sale securities, continuous unrealized loss position, accumulated loss | 232 | 344
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost | 13 |
Equity securities |  |
Other details of unrealized loss |  |
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost | 11 |
Available-for-sale securities, continuous unrealized loss position, accumulated loss | 12 | 85
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost | 1 |
Below investment grade fixed income securities |  |
Other details of unrealized loss |  |
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost | 39 |
Available-for-sale securities, continuous unrealized loss position, accumulated loss | 51 | 103
Available for sale securities, unrealized losses having loss of less than twenty percent, less than 12 months | 22 |
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost | 12 |
Unrealized losses related to securities with unrealized loss position greater than 20% of cost or amortized cost, unrealized loss position of 12 or more consecutive months | 2 |
EMA limited partnerships |  |
Other details of unrealized loss |  |
Limited partnership interests | 5,410 | 4,530
Cost-method limited partnership interests |  |
Other details of unrealized loss |  |
Limited partnership interests | $ 1,330 | $ 1,280

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- Definition This item represents the excess of [amortized] cost over fair value of fixed income securities that have been in a loss position for twelve months or longer, and have unrealized loss position greater than 20% of cost or amortized cost for those securities which are categorized neither as held-to-maturity nor trading securities as of the balance sheet date. + References No definition available. + Details
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- Definition The amount of unrealized losses that are related to securities with an unrealized loss position less than 20% of cost or amortized cost. + References No definition available. + Details
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- Definition The amount of unrealized losses that are related to securities with an unrealized loss position greater than 20% of cost or amortized cost. + References No definition available. + Details
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- Definition Available for sale Securities, Unrealized Losses Having Loss of Less than Twenty Percent, Less than 12 Months + References No definition available. + Details
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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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- Definition Total investments in (A) an entity in which the entity has significant influence, but does not have control, (B) subsidiaries that are not required to be consolidated and are accounted for using the equity and or cost method, and (C) an entity in which the reporting entity shares control of the entity with another party or group. Includes long-term advances receivable from a party that is affiliated with the reporting entity by means of direct or indirect ownership. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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