Derivative instruments (Cross-currency Swaps) (Details) - Cash flow hedge [Member] - Cross-currency swap contracts [Member] € in Millions, £ in Millions, SFr in Millions, $ in Millions	Sep. 30, 2018 USD ($)	Sep. 30, 2018 EUR (€)	Sep. 30, 2018 CHF (SFr)	Sep. 30, 2018 GBP (£)
2.125% 2019 euro Notes [Member]
Derivative [Line Items]
Notional amounts	$ 864	€ 675
1.25% 2022 euro Notes [Member]
Derivative [Line Items]
Notional amounts	1,388	1,250
0.41% 2023 Swiss franc Bonds [Member]
Derivative [Line Items]
Notional amounts	704		SFr 700
2.00% 2026 euro Notes [Member]
Derivative [Line Items]
Notional amounts	833	€ 750
5.50% 2026 pound sterling Notes [Member]
Derivative [Line Items]
Notional amounts	747			£ 475
4.00% 2029 pound sterling Notes [Member]
Derivative [Line Items]
Notional amounts	$ 1,111			£ 700
Euro Member Countries, Euro | 2.125% 2019 euro Notes [Member]
Derivative [Line Items]
Interest rates	2.125%	2.125%	2.125%	2.125%
Euro Member Countries, Euro | 1.25% 2022 euro Notes [Member]
Derivative [Line Items]
Interest rates	1.25%	1.25%	1.25%	1.25%
Euro Member Countries, Euro | 2.00% 2026 euro Notes [Member]
Derivative [Line Items]
Interest rates	2.00%	2.00%	2.00%	2.00%
Switzerland, Francs | 0.41% 2023 Swiss franc Bonds [Member]
Derivative [Line Items]
Interest rates	0.41%	0.41%	0.41%	0.41%
United Kingdom, Pounds | 5.50% 2026 pound sterling Notes [Member]
Derivative [Line Items]
Interest rates	5.50%	5.50%	5.50%	5.50%
United Kingdom, Pounds | 4.00% 2029 pound sterling Notes [Member]
Derivative [Line Items]
Interest rates	4.00%	4.00%	4.00%	4.00%
United States of America, Dollars | 2.125% 2019 euro Notes [Member]
Derivative [Line Items]
Interest rates	2.60%	2.60%	2.60%	2.60%
United States of America, Dollars | 1.25% 2022 euro Notes [Member]
Derivative [Line Items]
Interest rates	3.20%	3.20%	3.20%	3.20%
United States of America, Dollars | 0.41% 2023 Swiss franc Bonds [Member]
Derivative [Line Items]
Interest rates	3.40%	3.40%	3.40%	3.40%
United States of America, Dollars | 2.00% 2026 euro Notes [Member]
Derivative [Line Items]
Interest rates	3.90%	3.90%	3.90%	3.90%
United States of America, Dollars | 5.50% 2026 pound sterling Notes [Member]
Derivative [Line Items]
Interest rates	6.00%	6.00%	6.00%	6.00%
United States of America, Dollars | 4.00% 2029 pound sterling Notes [Member]
Derivative [Line Items]
Interest rates	4.50%	4.50%	4.50%	4.50%