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Pension benefits - Expected Annual Pension Plan Contributions Under Defined Benefit Plans (Details)
$ in Millions
Dec. 31, 2019
USD ($)
Defined Benefit Plan, Expected Future Employer Contributions [Abstract]  
2020 $ 4
2021 2
2022 2
2023 3
2024 2
2025-2029 13
Total payments expected during the next 10 years $ 26