Commitments and Contingencies - Narrative (Details)	12 Months Ended
Sep. 30, 2020 USD ($)
Guarantor Obligations [Line Items]
Additional bonding capacity	$ 587,600,000
Cash collateral requirement for guarantees	2 years
Loss contingency, estimate of actual or projected loss	$ 2,400,000
Probable liquidated damages	200,000
Failure to meet contractual commitments, reduction to revenue	200,000
Facility Agreement \| Powell (UK) Limited
Guarantor Obligations [Line Items]
Guarantee liability	7,200,000
Revolving credit facility	9,000,000.0
Amount of credit facility remaining borrowing capacity	1,800,000
Surety Bond
Guarantor Obligations [Line Items]
Guarantee liability	162,400,000
U.S. Revolver \| Financial Standby Letter of Credit \| Facility Agreement
Guarantor Obligations [Line Items]
Guarantee liability	$ 38,700,000

Commitments and contingencies additional bonding capacity.

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- Definition

Credit Agreement, Cash Collateral Requirement for Guarantees, Period, Greater Than

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- Definition

Loss contingency probable loss in period.

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Amount of increase (decrease) in revenue recognized for cumulative catch-up adjustment from change in measure of progress which (increases) decreases obligation to transfer good or service to customer for which consideration from customer has been received or is due.

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- Definition

The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

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Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

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