Fair value measurements (Details) - Recurring - USD ($) $ in Millions	Aug. 31, 2018	Aug. 31, 2017
Money Market Funds
Assets [Abstract]
Fair value of assets	$ 227	$ 2,096
Available-for-sale investments
Assets [Abstract]
Fair value of assets	1	1
Foreign currency forwards
Assets [Abstract]
Fair value of assets	52
Liabilities [Abstract]
Fair value of liabilities	4	19
Interest rate swaps
Liabilities [Abstract]
Fair value of liabilities	1
Level 1 | Money Market Funds
Assets [Abstract]
Fair value of assets	227	2,096
Level 1 | Available-for-sale investments
Assets [Abstract]
Fair value of assets	1	1
Level 1 | Foreign currency forwards
Assets [Abstract]
Fair value of assets	0
Liabilities [Abstract]
Fair value of liabilities	0	0
Level 1 | Interest rate swaps
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	52
Liabilities [Abstract]
Fair value of liabilities	4	19
Level 2 | Interest rate swaps
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	$ 0
Level 3 | Interest rate swaps
Liabilities [Abstract]
Fair value of liabilities	$ 0