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Investments - Equity securities reported at fair value (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Debt and Equity Securities, FV-NI [Line Items] |  |
Equity securities | $ 1,827 | $ 1,662
Common stocks |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Equity securities, at cost | 1,040 |
Equity securities | 1,151 |
Equity securities, fair value in excess of cost | 111 |
Perpetual preferred stocks |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Equity securities, at cost | 683 |
Equity securities | 676 |
Equity securities, fair value in excess of cost | (7) |
Equity securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Equity securities, at cost | 1,723 |
Equity securities | 1,827 |
Equity securities, fair value in excess of cost | $ 104 |

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- Definition Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI) in excess of the cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value. + References No definition available. + Details
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- Definition Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740 + Details
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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. + References No definition available. + Details
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