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Shareholders' equity Share Repurchases (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended
--- | --- | --- | ---
Oct. 29, 2018 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Nov 2016 Stock Repurchase Plan [Member] |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |
Number of shares repurchased |  |  | 1,615,383 |  | 5,033,013
Cost of shares repurchased |  |  | $ 232 |  | $ 707
Repurchase authorization remaining at end of period |  |  | $ 293 |  | $ 293
Dec 2017 Stock Repurchase Plan [Member] |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |
Number of shares repurchased |  | 2,781,307 |  | 5,225,162 |
Cost of shares repurchased |  | $ 379 |  | $ 703 |
Repurchase authorization remaining at end of period |  | $ 297 |  | $ 297 |
Subsequent Event [Member] | Dec 2017 Stock Repurchase Plan [Member] |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |
Number of shares repurchased | 750,000 |  |  |  |
Cost of shares repurchased | $ 94 |  |  |  |
Repurchase authorization remaining at end of period | $ 203 |  |  |  |

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