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Shareholders' equity Repurchase of Common Shares (Details) - USD ($) $ in Millions | 2 Months Ended | 12 Months Ended
--- | --- | ---
Feb. 27, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Equity, Class of Treasury Stock [Line Items] |  |  |  |
Number of shares repurchased |  |  |  | 0
Common Shares repurchased |  |  |  | $ 0
2018 Stock Repurchase Plan [Member] [Domain] |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |
Number of shares repurchased |  | 7,719,035 |  |
Common Shares repurchased |  | $ 1,021 |  |
Nov 2016 Stock Repurchase Plan [Member] |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |
Number of shares repurchased |  |  | 5,866,612 |
Common Shares repurchased |  |  | $ 830 |
Subsequent Event [Member] |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |
Number of shares repurchased | 1,328,754 |  |  |
Common Shares repurchased | $ 174 |  |  |

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