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Shareholders' equity Repurchase of Common Shares (Details) - USD ($) $ in Millions | 2 Months Ended | 12 Months Ended
--- | --- | ---
Feb. 26, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Equity, Class of Treasury Stock [Line Items] |  |  |  |
Number of shares repurchased |  | 10,442,238 | 7,719,035 | 5,866,612
2019 Stock Repurchase Plan [Member] |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |
Number of shares repurchased |  | 10,442,238 |  |
Common Shares repurchased |  | $ 1,531 |  |
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] | Nov 2019 Stock Repurchase Plan [Member] |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |
Number of shares repurchased | 947,400 |  |  |
Common Shares repurchased | $ 151 |  |  |

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