Fair value measurements (Details) - Recurring - USD ($) $ in Millions	Feb. 28, 2019	Aug. 31, 2018
Money market funds
Assets [Abstract]
Fair value of assets	$ 142	$ 227
Available-for-sale investments
Assets [Abstract]
Fair value of assets	5	1
Foreign currency forwards
Assets [Abstract]
Fair value of assets	3	52
Liabilities [Abstract]
Fair value of liabilities	92	4
Cross currency interest rate swap
Liabilities [Abstract]
Fair value of liabilities	16
Interest rate swaps
Liabilities [Abstract]
Fair value of liabilities		1
Level 1 | Money market funds
Assets [Abstract]
Fair value of assets	142	227
Level 1 | Available-for-sale investments
Assets [Abstract]
Fair value of assets	5	1
Level 1 | Foreign currency forwards
Assets [Abstract]
Fair value of assets	0	0
Liabilities [Abstract]
Fair value of liabilities	0	0
Level 1 | Cross currency interest rate swap
Liabilities [Abstract]
Fair value of liabilities	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	3	52
Liabilities [Abstract]
Fair value of liabilities	92	4
Level 2 | Cross currency interest rate swap
Liabilities [Abstract]
Fair value of liabilities	16
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	0
Liabilities [Abstract]
Fair value of liabilities	0	0
Level 3 | Cross currency interest rate swap
Liabilities [Abstract]
Fair value of liabilities	$ 0
Level 3 | Interest rate swaps
Liabilities [Abstract]
Fair value of liabilities		$ 0