Summary of significant accounting policies - Computation of Depreciation of Property and Equipment (Details) - USD ($) $ in Millions |
3 Months Ended | |
|---|---|---|
Dec. 31, 2023 |
Dec. 31, 2025 |
|
| Property Plant And Equipment [Line Items] | ||
| Asset impairment charges | $ 3.8 | |
| Buildings and improvements | Minimum | ||
| Property Plant And Equipment [Line Items] | ||
| Property and equipment useful life | 10 years | |
| Buildings and improvements | Maximum | ||
| Property Plant And Equipment [Line Items] | ||
| Property and equipment useful life | 40 years | |
| Machinery and equipment | Minimum | ||
| Property Plant And Equipment [Line Items] | ||
| Property and equipment useful life | 3 years | |
| Machinery and equipment | Maximum | ||
| Property Plant And Equipment [Line Items] | ||
| Property and equipment useful life | 20 years | |
| Mine development costs | ||
| Property Plant And Equipment [Line Items] | ||
| Property and equipment useful life | us-gaap:UnitsOfProductionDepreciationMethodMember |