Commitments and Contingencies - Narrative (Details)	12 Months Ended
	Sep. 30, 2017 USD ($)
Guarantor Obligations [Line Items]
Additional bonding capacity	$ 531,200,000
Liquidated damages	2,300,000
Probable liquidated damages	1,200,000
Loss contingency, estimate of actual or projected loss	1,200,000
Facility Agreement | Powell (UK) Limited
Guarantor Obligations [Line Items]
Guarantee liability	4,400,000
Revolving credit facility	6,700,000
Amount of credit facility remaining borrowing capacity	2,300,000
Surety Bond
Guarantor Obligations [Line Items]
Guarantee liability	218,800,000
U.S. Revolver
Guarantor Obligations [Line Items]
Revolving credit facility	75,000,000
Amount of credit facility remaining borrowing capacity	50,900,000
U.S. Revolver | Financial Standby Letter of Credit
Guarantor Obligations [Line Items]
Guarantee liability	$ 24,100,000