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Employee benefit plans - Components of net periodic benefit (credits) cost (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Pension      
Employee benefit plans      
Interest cost $ 21 $ 19 $ 21
Expected return on plan assets (24) (27) (24)
Actual return on plan assets 70 (19)  
Expenses paid 2 2 2
Net periodic benefit (credit) cost (1) (6) (1)
Other Postretirement      
Employee benefit plans      
Interest cost 1 1 2
Amortization of prior service credit     (2)
Amortization of net loss (gain) $ (1) $ (1) (1)
Curtailment gain     (26)
Net periodic benefit (credit) cost     $ (27)