Exit and disposal activities - narrative (Details) - Store Optimization Program $ in Millions	Oct. 24, 2017 store	Aug. 31, 2018 USD ($)
Restructuring Cost and Reserve [Line Items]
Number of locations closed | store	600
Duration of restructuring plan	18 months
Expected restructuring costs		$ 450
Lease obligations and other real estate costs
Restructuring Cost and Reserve [Line Items]
Expected restructuring costs		270
Employee severance and other exit costs
Restructuring Cost and Reserve [Line Items]
Expected restructuring costs		$ 180