| Line Item | Jun 30, 2026 | Mar 31, 2026 | Sep 30, 2025 | Jun 30, 2025 | Mar 31, 2025 | Sep 30, 2024 | Jun 30, 2024 | Mar 31, 2024 | Sep 30, 2023 | Jun 30, 2023 | Mar 31, 2023 | Sep 30, 2022 | Jun 30, 2022 | Mar 31, 2022 | Sep 30, 2021 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Services revenue | — | ||||||||||||||
| Grant income | — | ||||||||||||||
| Total revenues and grant income | — | ||||||||||||||
| YoY growth | +162.5% | — | — | ||||||||||||
| Operating expenses | |||||||||||||||
| Direct costs | — | ||||||||||||||
| Selling, general and administrative | |||||||||||||||
| Research and development | |||||||||||||||
| Total operating expenses | |||||||||||||||
| % of revenue | 301.5% | 252.2% | — | ||||||||||||
| Operating loss | |||||||||||||||
| % of revenue | -201.5% | -152.2% | — | ||||||||||||
| Other income (expense) | |||||||||||||||
| Interest expense | |||||||||||||||
| Interest income | |||||||||||||||
| Other income (expense), net | — | ||||||||||||||
| Total other income (expense), net | — | ||||||||||||||
| Net loss | |||||||||||||||
| % of revenue | -192.9% | -382.8% | — | ||||||||||||
| Less: Net loss attributable to non-controlling interests | — | — | |||||||||||||
| Net loss attributable to X-Energy, Inc. | — | — | |||||||||||||
| % of revenue | -108.2% | — | — | ||||||||||||
| Net loss attributable to Class A common stock per share - basic | — | ||||||||||||||
| Net loss attributable to Class A common stock per share - diluted | — |