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Discontinued Operations, Goodwill and Intangible Assets - Narrative (Details) - USD ($) $ in Millions |  |  | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Jan. 31, 2020 | Dec. 06, 2019 | Jun. 30, 2025 | Sep. 30, 2024 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024 | Dec. 31, 2014 | Feb. 28, 2011
Discontinued Operations, Goodwill and Intangible Assets [Line Items] |  |  |  |  |  |  |  |  |
Amortization expense of acquired intangible assets |  |  |  |  |  | $ 1.8 | $ 14.1 |  |
Refinery Turnaround Project |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets [Line Items] |  |  |  |  |  |  |  |  |
Loss from litigation settlement |  |  | $ 53.0 |  |  |  |  |  |
Task Order, Deactivation Demolition And Removal Service |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets [Line Items] |  |  |  |  |  |  |  |  |
Percentage of claim recoveries due to purchaser | 10.00% |  |  |  |  |  |  |  |
Percentage of claim recoveries due to seller | 90.00% |  |  |  |  |  |  |  |
Task Order, Deactivation Demolition And Removal Service | Department of Energy |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets [Line Items] |  |  |  |  |  |  |  |  |
Maximum project cost to pay |  |  |  |  |  |  |  |  | $ 106.0
Additional fees on changed work scope |  | $ 60.4 |  |  |  |  |  | $ 103.0 |
Task Order, Deactivation Demolition And Removal Service | Department of Energy | Minimum | Former Affiliate |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets [Line Items] |  |  |  |  |  |  |  |  |
Damages sought |  | 148.5 |  |  |  |  |  |  |
Task Order, Deactivation Demolition And Removal Service | Department of Energy | Maximum | Former Affiliate |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets [Line Items] |  |  |  |  |  |  |  |  |
Damages sought |  | $ 329.4 |  |  |  |  |  |  |
Task Order, Deactivation Demolition And Removal Service | Former Affiliate and DOE |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets [Line Items] |  |  |  |  |  |  |  |  |
Maximum project cost to pay |  |  | 146.0 |  |  | 146.0 |  |  |
Minimum project cost to pay |  |  | 106.0 |  |  | 106.0 |  |  |
Task Order, Deactivation Demolition And Removal Service | Former Affiliate |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets [Line Items] |  |  |  |  |  |  |  |  |
Minimum project cost to pay |  |  | 146.0 |  |  | 146.0 |  |  |
Held for Sale |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets [Line Items] |  |  |  |  |  |  |  |  |
Losses related to contingent consideration receivable recognized |  |  | 58.3 |  | $ (12.7) | 75.0 | $ 100.4 |  |
Held for Sale | Revolving Credit Facility |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets [Line Items] |  |  |  |  |  |  |  |  |
Investment commitment to fund |  |  |  | $ 30.0 |  |  |  |  |
Fund outflow in other investing activities and outstanding |  |  | $ 4.4 |  |  | $ 4.4 |  |  |
Held for Sale | Civil Infrastructure Construction Business |  |  |  |  |  |  |  |  |
Discontinued Operations, Goodwill and Intangible Assets [Line Items] |  |  |  |  |  |  |  |  |
Losses related to contingent consideration receivable recognized |  |  |  |  | $ 103.1 |  |  |  |
Gain based on fair value of equity received |  |  |  | $ 12.7 |  |  |  |  |

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