Debt - long-term debt (Details)	Aug. 31, 2018 USD ($)	Aug. 31, 2018 GBP (£)	Aug. 31, 2018 EUR (€)	Mar. 31, 2018 USD ($)	Aug. 31, 2017 USD ($)
Long-Term Debt [Abstract]
Other	$ 23,000,000				$ 24,000,000
Total long-term debt, less current portion	$ 12,431,000,000				12,684,000,000
3.450% unsecured notes due 2026
Long-Term Debt [Abstract]
Stated interest rate (as a percent)	3.45%	3.45%	3.45%
3.450% unsecured notes due 2026 | Unsecured Debt
Long-Term Debt [Abstract]
Carrying value of notes, net of unamortized discount and interest rate swap FMV adjustment	$ 1,888,000,000				1,887,000,000
4.650% unsecured notes due 2046
Long-Term Debt [Abstract]
Stated interest rate (as a percent)	4.65%	4.65%	4.65%
4.650% unsecured notes due 2046 | Unsecured Debt
Long-Term Debt [Abstract]
Carrying value of notes, net of unamortized discount and interest rate swap FMV adjustment	$ 590,000,000				590,000,000
2.700% unsecured notes due 2019
Long-Term Debt [Abstract]
Stated interest rate (as a percent)	2.70%	2.70%	2.70%
2.700% unsecured notes due 2019 | Unsecured Debt
Long-Term Debt [Abstract]
Carrying value of notes, net of unamortized discount and interest rate swap FMV adjustment	$ 1,248,000,000				1,246,000,000
3.300% unsecured notes due 2021
Long-Term Debt [Abstract]
Stated interest rate (as a percent)	3.30%	3.30%	3.30%
3.300% unsecured notes due 2021 | Unsecured Debt
Long-Term Debt [Abstract]
Carrying value of notes, net of unamortized discount and interest rate swap FMV adjustment	$ 1,245,000,000				1,244,000,000
3.800% unsecured notes due 2024
Long-Term Debt [Abstract]
Stated interest rate (as a percent)	3.80%	3.80%	3.80%
3.800% unsecured notes due 2024 | Unsecured Debt
Long-Term Debt [Abstract]
Carrying value of notes, net of unamortized discount and interest rate swap FMV adjustment	$ 1,990,000,000				1,988,000,000
4.500% unsecured notes due 2034
Long-Term Debt [Abstract]
Stated interest rate (as a percent)	4.50%	4.50%	4.50%
4.500% unsecured notes due 2034 | Unsecured Debt
Long-Term Debt [Abstract]
Carrying value of notes, net of unamortized discount and interest rate swap FMV adjustment	$ 495,000,000				495,000,000
4.800% unsecured notes due 2044
Long-Term Debt [Abstract]
Stated interest rate (as a percent)	4.80%	4.80%	4.80%
4.800% unsecured notes due 2044 | Unsecured Debt
Long-Term Debt [Abstract]
Carrying value of notes, net of unamortized discount and interest rate swap FMV adjustment	$ 1,492,000,000				1,492,000,000
2.875% unsecured Pound sterling notes due 2020
Long-Term Debt [Abstract]
Stated interest rate (as a percent)	2.875%	2.875%	2.875%
2.875% unsecured Pound sterling notes due 2020 | Unsecured Debt
Long-Term Debt [Abstract]
Carrying value of notes, net of unamortized discount and interest rate swap FMV adjustment	$ 517,000,000				513,000,000
3.600% unsecured Pound sterling notes due 2025
Long-Term Debt [Abstract]
Stated interest rate (as a percent)	3.60%	3.60%	3.60%
3.600% unsecured Pound sterling notes due 2025 | Unsecured Debt
Long-Term Debt [Abstract]
Carrying value of notes, net of unamortized discount and interest rate swap FMV adjustment	$ 387,000,000				384,000,000
2.125% unsecured Euro notes due 2026
Long-Term Debt [Abstract]
Stated interest rate (as a percent)	2.125%	2.125%	2.125%
2.125% unsecured Euro notes due 2026 | Unsecured Debt
Long-Term Debt [Abstract]
Carrying value of notes, net of unamortized discount and interest rate swap FMV adjustment	$ 868,000,000				884,000,000
3.100% unsecured notes due 2022
Long-Term Debt [Abstract]
Stated interest rate (as a percent)	3.10%	3.10%	3.10%
3.100% unsecured notes due 2022 | Unsecured Debt
Long-Term Debt [Abstract]
Carrying value of notes, net of unamortized discount and interest rate swap FMV adjustment	$ 1,196,000,000				1,195,000,000
4.400% unsecured notes due 2042
Long-Term Debt [Abstract]
Stated interest rate (as a percent)	4.40%	4.40%	4.40%
4.400% unsecured notes due 2042 | Unsecured Debt
Long-Term Debt [Abstract]
Carrying value of notes, net of unamortized discount and interest rate swap FMV adjustment	$ 492,000,000				492,000,000
5.250% unsecured notes due 2019
Long-Term Debt [Abstract]
Stated interest rate (as a percent)	5.25%	5.25%	5.25%
5.250% unsecured notes due 2019 | Unsecured Debt
Long-Term Debt [Abstract]
Carrying value of notes, net of unamortized discount and interest rate swap FMV adjustment	$ 0				$ 250,000,000
Total $6.0 billion debt issuance
Long-Term Debt [Abstract]
Face amount	6,000,000,000
Fair value of the notes	2,400,000,000
Carrying value of notes	2,500,000,000
Total $8.0 billion debt Issuance
Long-Term Debt [Abstract]
Face amount	8,000,000,000
Fair value of the notes	6,300,000,000
Carrying value of notes	6,500,000,000
Total 700 million pounds debt issuance
Long-Term Debt [Abstract]
Face amount | £		£ 700,000,000.0
Fair value of the notes	900,000,000
Carrying value of notes	900,000,000
Total 750 million euros debt issuance
Long-Term Debt [Abstract]
Face amount | €			€ 750,000,000.00
Fair value of the notes	900,000,000
Carrying value of notes	900,000,000
Total $4.0 billion debt issuance
Long-Term Debt [Abstract]
Face amount	4,000,000,000
Fair value of the notes	1,700,000,000
Carrying value of notes	1,700,000,000
Total $1.0 billion debt issuance
Long-Term Debt [Abstract]
Face amount	1,000,000,000
Fair value of the notes	300,000,000
Carrying value of notes	$ 200,000,000
Revolving Credit Facility | March 2018 Revolving Credit Facility
Long-Term Debt [Abstract]
Maximum borrowing capacity				$ 350,000,000