Business Combination (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Apr. 15, 2016 USD ($)
Business Acquisition [Line Items]
Cash Paid for Acquisition	$ 20,213	$ 59,544	$ 3,113
Amount From Credit Facility Used For Acquisitions	$ 20,000
Interest Rate During Period	2.50%
Goodwill Written Off Related To Sale Of Business Unit	$ 51	577
Infrastructure Solutions [Member]
Business Acquisition [Line Items]
Goodwill Written Off Related To Sale Of Business Unit		577
Loss on Sale of Assets From Divestiture		$ 821
Assets held for sale				$ 2,200
Commercial & Industrial [Member]
Business Acquisition [Line Items]
Minority Interest Ownership Percentage By Parent	80.00%
Goodwill Written Off Related To Sale Of Business Unit	$ 0
AcquisitionRelatedCostsMember
Business Acquisition Pro Forma Information Nonrecurring Adjustment [Line Items]
Business Combination Acquisition Related Costs	458
Tax Provision Adjustment [Member]
Business Acquisition Pro Forma Information Nonrecurring Adjustment [Line Items]
Tax Provision Adjustment	665
All Business Acquisitions
Business Acquisition [Line Items]
Contingent Consideration Fair Value	$ 786
2017 Business Acquisitions
Business Acquisition [Line Items]
Number of Businesses Acquired	3
Total Consideration Transferred	$ 20,979
Cash Purchase Consideration	20,613
Cash Paid for Acquisition	20,213
Additional Consideration To Be Paid	400
Contingent Consideration Fair Value	366
Goodwill Tax Basis	3,411
Business Combination Acquired Receivables	11,223
Business Combination Acquired Receivables Estimated Uncollectible	0
Revenues From Acquisitions	18,413
Operating Income From Acquisitions	$ 769
Business Acquisition Freeman [Member] | Infrastructure Solutions [Member]
Business Acquisition [Line Items]
Date of Acquisition Agreement	Mar. 16, 2017
Businessacquisition Technical Services [Member] | Infrastructure Solutions [Member]
Business Acquisition [Line Items]
Minority Interest Description	Our agreements governing the operations of STR and NEXT Electric contain a provision where, at any time after five years from the acquisition date, we may purchase all or a portion of the 20% noncontrolling interest. Pursuant to this provision, we may purchase the noncontrolling interest, or, with notice, the noncontrolling interest holders may cause us to purchase their interests, for a contractually determined price based on the trailing 2 year earnings before interest, taxes, depreciation, and amortization of STR and NEXT Electric, calculated at the time of the purchase.
Businessacquisition Technical Services [Member] | Commercial & Industrial [Member]
Business Acquisition [Line Items]
Name of Acquired Business	Technical Services II, LLC
Description of Acquired Business	a Chesapeake, Virginia-based provider of mechanical maintenance services, including commercial heating, ventilation and air conditioning, food service equipment, electrical and plumbing services
Date of Acquisition Agreement	Jun. 15, 2017
Business Acquisition NEXT [Member] | Infrastructure Solutions [Member]
Business Acquisition [Line Items]
Minority Interest Description	Our agreements governing the operations of STR and NEXT Electric contain a provision where, at any time after five years from the acquisition date, we may purchase all or a portion of the 20% noncontrolling interest. Pursuant to this provision, we may purchase the noncontrolling interest, or, with notice, the noncontrolling interest holders may cause us to purchase their interests, for a contractually determined price based on the trailing 2 year earnings before interest, taxes, depreciation, and amortization of STR and NEXT Electric, calculated at the time of the purchase.
Business Acquisition NEXT [Member] | Commercial & Industrial [Member]
Business Acquisition [Line Items]
Name of Acquired Business	NEXT Electric, LLC
Description of Acquired Business	a Milwaukee, Wisconsin-based electrical contractor specializing in the design, installation and maintenance of electrical systems for commercial, industrial, healthcare, water treatment and education end markets
Date of Acquisition Agreement	Jul. 14, 2017