Fair value measurements (Details) - Recurring - USD ($) $ in Millions	Aug. 31, 2017	Aug. 31, 2016
Money Market Funds
Assets [Abstract]
Fair value of assets	$ 2,096	$ 9,133
Available-for-sale investments
Assets [Abstract]
Fair value of assets	1	32
Foreign currency forwards
Assets [Abstract]
Fair value of assets		16
Liabilities [Abstract]
Fair value of liabilities	19
Interest rate swaps
Assets [Abstract]
Fair value of assets		3
Basis swap
Liabilities [Abstract]
Fair value of liabilities		1
Level 1 | Money Market Funds
Assets [Abstract]
Fair value of assets	2,096	9,133
Level 1 | Available-for-sale investments
Assets [Abstract]
Fair value of assets	1	32
Level 1 | Foreign currency forwards
Assets [Abstract]
Fair value of assets		0
Liabilities [Abstract]
Fair value of liabilities	0
Level 1 | Interest rate swaps
Assets [Abstract]
Fair value of assets		0
Level 1 | Basis swap
Liabilities [Abstract]
Fair value of liabilities		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 | Foreign currency forwards
Assets [Abstract]
Fair value of assets		16
Liabilities [Abstract]
Fair value of liabilities	19
Level 2 | Interest rate swaps
Assets [Abstract]
Fair value of assets		3
Level 2 | Basis swap
Liabilities [Abstract]
Fair value of liabilities		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 | Foreign currency forwards
Assets [Abstract]
Fair value of assets		0
Liabilities [Abstract]
Fair value of liabilities	$ 0
Level 3 | Interest rate swaps
Assets [Abstract]
Fair value of assets		0
Level 3 | Basis swap
Liabilities [Abstract]
Fair value of liabilities		$ 0