Commitments and Contingencies - Narrative (Details)	12 Months Ended
	Sep. 30, 2021 USD ($)
Guarantor Obligations [Line Items]
Additional bonding capacity	$ 457,000,000
Cash collateral requirement for guarantees	2 years
Loss contingency, estimate of actual or projected loss	$ 1,800,000
Probable liquidated damages	1,100,000
Failure to meet contractual commitments, reduction to revenue	1,100,000
Facility Agreement | Powell (UK) Limited
Guarantor Obligations [Line Items]
Guarantee liability, letters of credit	7,400,000
Revolving credit facility	9,400,000
Amount of credit facility remaining borrowing capacity	2,000,000
Surety Bond
Guarantor Obligations [Line Items]
Guarantee liability, letters of credit	143,000,000
U.S. Revolver | Amended And Restated Credit Agreement
Guarantor Obligations [Line Items]
Revolving credit facility	75,000,000
Amount of credit facility remaining borrowing capacity	40,200,000
U.S. Revolver | Financial Standby Letter of Credit | Amended And Restated Credit Agreement
Guarantor Obligations [Line Items]
Guarantee liability, letters of credit	$ 34,800,000