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Taxes - Effective tax rate (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Taxes        
Effective tax rate (as a percent) 28.70% 29.60% 37.00% 28.20%
Devaluation of Mexican peso against U.S. dollar (as a percent)     22.00%  
Discrete tax expense (benefit) due to taxable translation gains on net U.S. dollar monetary assets held in Mexico     $ 22 $ 1