Fair value measurements (Details) - Recurring - USD ($) $ in Millions	May 31, 2017	Aug. 31, 2016
Money market funds
Assets [Abstract]
Fair value of assets	$ 10,491	$ 9,133
Available-for-sale investments
Assets [Abstract]
Fair value of assets	1	32
Interest rate swaps
Assets [Abstract]
Fair value of assets		3
Liabilities [Abstract]
Fair value of liabilities	0
Foreign currency forwards
Assets [Abstract]
Fair value of assets	0	16
Liabilities [Abstract]
Fair value of liabilities	5
Basis swaps
Liabilities [Abstract]
Fair value of liabilities	0	1
Level 1 | Money market funds
Assets [Abstract]
Fair value of assets	10,491	9,133
Level 1 | Available-for-sale investments
Assets [Abstract]
Fair value of assets	1	32
Level 1 | Interest rate swaps
Assets [Abstract]
Fair value of assets		0
Liabilities [Abstract]
Fair value of liabilities	0
Level 1 | Foreign currency forwards
Assets [Abstract]
Fair value of assets	0	0
Liabilities [Abstract]
Fair value of liabilities	0
Level 1 | Basis swaps
Liabilities [Abstract]
Fair value of liabilities	0	0
Level 2 | Money market funds
Assets [Abstract]
Fair value of assets	0	0
Level 2 | Available-for-sale investments
Assets [Abstract]
Fair value of assets	0	0
Level 2 | Interest rate swaps
Assets [Abstract]
Fair value of assets		3
Liabilities [Abstract]
Fair value of liabilities	0
Level 2 | Foreign currency forwards
Assets [Abstract]
Fair value of assets	0	16
Liabilities [Abstract]
Fair value of liabilities	5
Level 2 | Basis swaps
Liabilities [Abstract]
Fair value of liabilities	0	1
Level 3 | Money market funds
Assets [Abstract]
Fair value of assets	0	0
Level 3 | Available-for-sale investments
Assets [Abstract]
Fair value of assets	0	0
Level 3 | Interest rate swaps
Assets [Abstract]
Fair value of assets		0
Liabilities [Abstract]
Fair value of liabilities	0
Level 3 | Foreign currency forwards
Assets [Abstract]
Fair value of assets	0	0
Liabilities [Abstract]
Fair value of liabilities	0
Level 3 | Basis swaps
Liabilities [Abstract]
Fair value of liabilities	$ 0	$ 0