| Line Item | Jun 30, 2026 | Mar 31, 2026 | Dec 31, 2025 | Sep 30, 2025 | Jun 30, 2025 | Mar 31, 2025 | Dec 31, 2024 | Sep 30, 2024 | Jun 30, 2024 | Mar 31, 2024 | Dec 31, 2023 | Sep 30, 2023 | Jun 30, 2023 | Mar 31, 2023 | Dec 31, 2022 | Sep 30, 2022 |
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| Revenues | ||||||||||||||||
| Net premiums earned | ||||||||||||||||
| Net investment income | ||||||||||||||||
| Net realized gains (losses) | ||||||||||||||||
| Other underwriting income | ||||||||||||||||
| Equity in net income of investments accounted for using the equity method | ||||||||||||||||
| Net income (loss) available to Arch | ||||||||||||||||
| Preferred dividends | ||||||||||||||||
| Net income (loss) available to Arch common shareholders | ||||||||||||||||
| Basic (per share) | ||||||||||||||||
| Diluted (per share) |