Payroll, staff and employee benefits obligations - Sensitivity analysis (Details) $ in Millions	Dec. 31, 2017 USD ($)
Discount rates
Sensitivity analysis
Increase in actuarial assumption (as a percent)	0.50%
Decrease in actuarial assumption (as a percent)	0.50%
Impact on benefit obligation from increase in actuarial assumption	$ (857)
Impact on benefit obligation from decrease in actuarial assumption	$ 974
Inflation rates
Sensitivity analysis
Increase in actuarial assumption (as a percent)	0.50%
Decrease in actuarial assumption (as a percent)	0.50%
Impact on benefit obligation from increase in actuarial assumption	$ 637
Impact on benefit obligation from decrease in actuarial assumption	$ (586)