Derivative instruments (Cross-currency Swaps) (Details) - Cash flow hedge [Member] - Cross-currency swap contracts [Member] € in Millions, £ in Millions, SFr in Millions, $ in Millions	Mar. 31, 2018 USD ($)	Mar. 31, 2018 GBP (£)	Mar. 31, 2018 CHF (SFr)	Mar. 31, 2018 EUR (€)
2.125% 2019 euro Notes [Member]
Derivative [Line Items]
Notional amount	$ 864			€ 675
1.25% 2022 euro Notes [Member]
Derivative [Line Items]
Notional amount	1,388			1,250
0.41% 2023 Swiss franc Bonds [Member]
Derivative [Line Items]
Notional amount	704		SFr 700
2.00% 2026 euro Notes [Member]
Derivative [Line Items]
Notional amount	833			€ 750
5.50% 2026 pound sterling Notes [Member]
Derivative [Line Items]
Notional amount	747	£ 475
4.00% 2029 pound sterling Notes [Member]
Derivative [Line Items]
Notional amount	$ 1,111	£ 700
Euro Member Countries, Euro | 2.125% 2019 euro Notes [Member]
Derivative [Line Items]
Interest rate	2.125%	2.125%	2.125%	2.125%
Euro Member Countries, Euro | 1.25% 2022 euro Notes [Member]
Derivative [Line Items]
Interest rate	1.25%	1.25%	1.25%	1.25%
Euro Member Countries, Euro | 2.00% 2026 euro Notes [Member]
Derivative [Line Items]
Interest rate	2.00%	2.00%	2.00%	2.00%
Switzerland, Francs | 0.41% 2023 Swiss franc Bonds [Member]
Derivative [Line Items]
Interest rate	0.41%	0.41%	0.41%	0.41%
United Kingdom, Pounds | 5.50% 2026 pound sterling Notes [Member]
Derivative [Line Items]
Interest rate	5.50%	5.50%	5.50%	5.50%
United Kingdom, Pounds | 4.00% 2029 pound sterling Notes [Member]
Derivative [Line Items]
Interest rate	4.00%	4.00%	4.00%	4.00%
United States of America, Dollars | 2.125% 2019 euro Notes [Member]
Derivative [Line Items]
Interest rate	2.60%	2.60%	2.60%	2.60%
United States of America, Dollars | 1.25% 2022 euro Notes [Member]
Derivative [Line Items]
Interest rate	3.20%	3.20%	3.20%	3.20%
United States of America, Dollars | 0.41% 2023 Swiss franc Bonds [Member]
Derivative [Line Items]
Interest rate	3.40%	3.40%	3.40%	3.40%
United States of America, Dollars | 2.00% 2026 euro Notes [Member]
Derivative [Line Items]
Interest rate	3.90%	3.90%	3.90%	3.90%
United States of America, Dollars | 5.50% 2026 pound sterling Notes [Member]
Derivative [Line Items]
Interest rate	6.00%	6.00%	6.00%	6.00%
United States of America, Dollars | 4.00% 2029 pound sterling Notes [Member]
Derivative [Line Items]
Interest rate	4.50%	4.50%	4.50%	4.50%