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Discontinued Operations, Goodwill and Intangible Assets - Narrative (Details) $ in Thousands |  |  | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Jan. 31, 2020 | Dec. 06, 2019 USD ($) | Jun. 30, 2025 USD ($) | Jun. 30, 2024 USD ($) | Mar. 31, 2024 USD ($) | Sep. 30, 2025 USD ($) acquisition | Sep. 30, 2024 USD ($) | Sep. 30, 2023 USD ($) | Dec. 31, 2014 USD ($) | Feb. 28, 2011 USD ($)
Discontinued Operations, Goodwill and Intangible Assets |  |  |  |  |  |  |  |  |  |
Losses related to contingent consideration receivable recognized |  |  |  |  |  | $ 0 | $ 90,412 | $ 43,222 |  |
Amortization expense of acquired intangible assets |  |  |  |  |  | $ 1,500 | 18,800 |  |  |
2025 Acquisitions |  |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets |  |  |  |  |  |  |  |  |  |
Number of business acquisitions | acquisition |  |  |  |  |  | 2 |  |  |  |
Total consideration |  |  |  |  |  | $ 375,900 |  |  |  |
Stock consideration |  |  |  |  |  | 146,400 |  |  |  |
Task Order, Deactivation Demolition And Removal Service |  |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets |  |  |  |  |  |  |  |  |  |
Percentage of claim recoveries due to purchaser | 10.00% |  |  |  |  |  |  |  |  |
Percentage of claim recoveries due to seller | 90.00% |  |  |  |  |  |  |  |  |
Refinery Turnaround Project |  |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets |  |  |  |  |  |  |  |  |  |
Loss from reduction in expected future net cash proceeds |  |  | $ 53,000 |  |  |  |  |  |  |
Department of Energy | Task Order, Deactivation Demolition And Removal Service |  |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets |  |  |  |  |  |  |  |  |  |
Maximum project cost to pay |  |  |  |  |  |  |  |  |  | $ 106,000
Additional fees on changed work scope |  | $ 60,400 |  |  |  |  |  |  | $ 103,000 |
Department of Energy | Task Order, Deactivation Demolition And Removal Service | Minimum | Former Affiliate |  |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets |  |  |  |  |  |  |  |  |  |
Damages sought |  | 148,500 |  |  |  |  |  |  |  |
Department of Energy | Task Order, Deactivation Demolition And Removal Service | Maximum | Former Affiliate |  |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets |  |  |  |  |  |  |  |  |  |
Damages sought |  | $ 329,400 |  |  |  |  |  |  |  |
Former Affiliate and DOE | Task Order, Deactivation Demolition And Removal Service |  |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets |  |  |  |  |  |  |  |  |  |
Maximum project cost to pay |  |  |  |  |  |  |  |  |  | 146,000
Minimum project cost to pay |  |  |  |  |  |  |  |  |  | 106,000
Former Affiliate | Task Order, Deactivation Demolition And Removal Service |  |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets |  |  |  |  |  |  |  |  |  |
Minimum project cost to pay |  |  |  |  |  |  |  |  |  | $ 146,000
Held for Sale |  |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets |  |  |  |  |  |  |  |  |  |
Losses related to contingent consideration receivable recognized |  |  |  |  |  | 83,800 | 97,100 | $ 50,600 |  |
Non-cash gain in discontinued operations |  |  |  |  |  |  | $ 12,700 |  |  |
Held for Sale | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets |  |  |  |  |  |  |  |  |  |
Investment commitment to fund |  |  |  | $ 30,000 |  |  |  |  |  |
Fund outflow in other investing activities and outstanding |  |  |  |  |  | $ 15,400 |  |  |  |
Held for Sale | Civil Infrastructure Construction Business |  |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets |  |  |  |  |  |  |  |  |  |
Losses related to contingent consideration receivable recognized |  |  |  |  | $ 103,100 |  |  |  |  |
Gain based on fair value of equity received |  |  |  | $ 12,700 |  |  |  |  |  |

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