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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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