Summary of significant accounting policies - Goodwill Sensitivity (Details)	Mar. 31, 2022 rate
Central Services Organization
Goodwill [Line Items]
Fair value of reporting unit exceeded its carrying amount by	315.70%
Post-tax discount rate applied to the expected cash flow projections	32.50%
Decrease in the cash flow projections of	75.90%
Central Services Organization | Measurement Input, Discount Rate
Goodwill [Line Items]
Post-tax discount rate used to determine fair value	0.146
Growth rate used to extrapolate cash flow beyond the budget period	0.146
Central Services Organization | Measurement Input, Growth Rate
Goodwill [Line Items]
Post-tax discount rate used to determine fair value	0.046
Growth rate used to extrapolate cash flow beyond the budget period	0.046
Africa
Goodwill [Line Items]
Fair value of reporting unit exceeded its carrying amount by	321.00%
Post-tax discount rate applied to the expected cash flow projections	39.20%
Decrease in the cash flow projections of	76.20%
Africa | Measurement Input, Discount Rate
Goodwill [Line Items]
Post-tax discount rate used to determine fair value	0.146
Growth rate used to extrapolate cash flow beyond the budget period	0.146
Africa | Measurement Input, Growth Rate
Goodwill [Line Items]
Post-tax discount rate used to determine fair value	0.046
Growth rate used to extrapolate cash flow beyond the budget period	0.046
Europe
Goodwill [Line Items]
Fair value of reporting unit exceeded its carrying amount by	33.70%
Post-tax discount rate applied to the expected cash flow projections	13.10%
Decrease in the cash flow projections of	25.20%
Europe | Measurement Input, Discount Rate
Goodwill [Line Items]
Post-tax discount rate used to determine fair value	0.108
Growth rate used to extrapolate cash flow beyond the budget period	0.108
Europe | Measurement Input, Growth Rate
Goodwill [Line Items]
Post-tax discount rate used to determine fair value	0.034
Growth rate used to extrapolate cash flow beyond the budget period	0.034
Middle East and Australasia
Goodwill [Line Items]
Fair value of reporting unit exceeded its carrying amount by	300.90%
Post-tax discount rate applied to the expected cash flow projections	27.30%
Decrease in the cash flow projections of	75.10%
Middle East and Australasia | Measurement Input, Discount Rate
Goodwill [Line Items]
Post-tax discount rate used to determine fair value	0.084
Growth rate used to extrapolate cash flow beyond the budget period	0.084
Middle East and Australasia | Measurement Input, Growth Rate
Goodwill [Line Items]
Post-tax discount rate used to determine fair value	0.025
Growth rate used to extrapolate cash flow beyond the budget period	0.025