Operating Segments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 330,411	$ 293,125	$ 291,277	$ 276,043	$ 293,607	$ 282,633	$ 256,914	$ 243,842	$ 1,190,856	$ 1,076,996	$ 876,828
Cost of services									962,897	894,893	726,866
Gross profit	68,160	$ 58,320	$ 51,264	$ 50,215	50,870	$ 46,397	$ 43,235	$ 41,601	227,959	182,103	149,962
Selling, general and administrative expenses									170,911	140,575	123,920
Goodwill impairment expense	6,976								6,976	0	0
Contingent consideration									(11)	(374)	(103)
Loss (gain) on sale of assets									0	52	(15)
Operating income (loss)									50,083	41,850	25,954
Depreciation and amortization									12,508	9,557	8,860
Capital expenditures									4,745	6,300	4,563
Total assets	560,528				445,258				560,528	445,258	421,994
Communications [Member]
Segment Reporting Information [Line Items]
Revenues									395,141	321,246	219,655
Cost of services									317,013	264,746	179,518
Gross profit									78,128	56,500	40,137
Selling, general and administrative expenses									37,674	31,850	26,003
Goodwill impairment expense									0
Contingent consideration									0	(97)	85
Loss (gain) on sale of assets									8	(6)	(4)
Operating income (loss)									40,446	24,753	14,223
Depreciation and amortization									1,351	1,513	1,247
Capital expenditures									830	973	647
Total assets	154,808				109,263				154,808	109,263	80,528
Residential [Member]
Segment Reporting Information [Line Items]
Revenues									411,790	313,336	285,711
Cost of services									318,034	248,562	227,355
Gross profit									93,756	64,774	58,356
Selling, general and administrative expenses									63,668	46,864	41,401
Goodwill impairment expense									0
Contingent consideration									0	0	0
Loss (gain) on sale of assets									2	(17)	8
Operating income (loss)									30,086	17,927	16,947
Depreciation and amortization									2,276	852	637
Capital expenditures									1,459	1,412	932
Total assets	110,998				63,903				110,998	63,903	55,176
Infrastructure Solutions [Member]
Segment Reporting Information [Line Items]
Revenues									128,379	136,790	97,163
Cost of services									93,358	105,863	75,337
Gross profit									35,021	30,927	21,826
Selling, general and administrative expenses									20,418	18,664	18,293
Goodwill impairment expense									0
Contingent consideration									0	(277)	(288)
Loss (gain) on sale of assets									35	105	18
Operating income (loss)									14,568	12,435	3,227
Depreciation and amortization									6,020	4,528	4,672
Capital expenditures									795	1,377	735
Total assets	124,640				116,867				124,640	116,867	109,506
Commercial and Industrial [Member]
Segment Reporting Information [Line Items]
Revenues									255,546	305,624	274,299
Cost of services									234,492	275,722	244,656
Gross profit									21,054	29,902	29,643
Selling, general and administrative expenses									32,128	27,815	27,031
Goodwill impairment expense									6,976
Contingent consideration									(11)	0	100
Loss (gain) on sale of assets									(45)	(30)	(37)
Operating income (loss)									(17,994)	2,117	2,749
Depreciation and amortization									2,768	2,563	2,197
Capital expenditures									1,362	2,402	2,216
Total assets	68,318				82,050				68,318	82,050	89,729
Corporate Segment [Member]
Segment Reporting Information [Line Items]
Revenues									0	0	0
Cost of services									0	0	0
Gross profit									0	0	0
Selling, general and administrative expenses									17,023	15,382	11,192
Goodwill impairment expense									0
Contingent consideration									0	0	0
Loss (gain) on sale of assets									0	0	0
Operating income (loss)									(17,023)	(15,382)	(11,192)
Depreciation and amortization									93	101	107
Capital expenditures									299	136	33
Total assets	$ 101,764				$ 73,175				$ 101,764	$ 73,175	$ 87,055