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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 |  |

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