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Investments - Equity securities reported at fair value (Details) - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017
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Debt Securities, Trading, and Equity Securities, FV-NI [Line Items] |  |
Equity securities, at fair value | $ 1,777 | $ 1,662
Common stocks |  |
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items] |  |
Equity securities, at cost | 1,066 |
Equity securities, at fair value | 1,180 |
Equity securities, fair value in excess of cost | 114 |
Perpetual preferred stocks |  |
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items] |  |
Equity securities, at cost | 603 |
Equity securities, at fair value | 597 |
Equity securities, fair value in excess of cost | (6) |
Equity securities |  |
Debt Securities, Trading, and Equity Securities, FV-NI [Line Items] |  |
Equity securities, at cost | 1,669 |
Equity securities, at fair value | 1,777 |
Equity securities, fair value in excess of cost | $ 108 |

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- Definition [Line Items] for Disclosure of information about investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security with change in fair value recognized in net income (FV-NI). + 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 No definition available. + Details
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- Definition 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) 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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