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Basis of preparation - Additional Information (Details) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
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Jun. 30, 2020 | Jun. 30, 2019 | Dec. 31, 2019
Basis Of Presentation [Abstract] |  |  |
Group materiality | $ 425 | $ 350 | $ 350

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