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Income taxes - Provision for income taxes (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Income Tax Disclosure [Abstract]        
Taxes calculated using the estimated annual effective tax rate $ 542 $ 418 $ 1,382 $ 1,024
Discrete tax items (38) (55) (190) (139)
Provision for income taxes $ 504 $ 363 $ 1,192 $ 885
Estimated annual effective tax rate 31.00% 30.00% 31.00% 30.00%
Actual effective tax rate 28.00% 26.00% 26.00% 26.00%