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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