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Fair value measurements Fair Value Measurements (narrative) (Details) - USD ($) $ in Millions | 9 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Realized Gain Losses As A Result of Annuitization Changes |  | $ 94
Minimum [Member] |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Notice Period For Redemption For Alternative Investments Investment Funds | 5 days |
Maximum [Member] |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Notice Period For Redemption For Alternative Investments Investment Funds | 120 days |

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