Commitments and Contingencies - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Jun. 30, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Guarantor Obligations [Line Items]
Restructuring and separation expenses		$ 787,000	$ 1,322,000	$ 8,441,000
Rstructuring costs related to exited facility	$ 500,000
Additional bonding capacity		589,400,000
Loss contingency, estimate of actual or projected loss		2,400,000
Probable liquidated damages		700,000
Failure to meet contractual commitments, reduction to revenue		700,000
Facility Agreement | Powell (UK) Limited
Guarantor Obligations [Line Items]
Guarantee liability		3,300,000
Revolving credit facility		6,512,500
Amount of credit facility remaining borrowing capacity		3,200,000
Surety Bond
Guarantor Obligations [Line Items]
Guarantee liability		160,600,000
U.S. Revolver
Guarantor Obligations [Line Items]
Revolving credit facility		75,000,000
Amount of credit facility remaining borrowing capacity		51,900,000
U.S. Revolver | Letters of Credit and Bank Guarantees
Guarantor Obligations [Line Items]
Guarantee liability		$ 26,400,000