Business Acquisitions - Summary of Purchase Price Attributable to Acquisitions (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Business Acquisition [Line Items]
Cash	$ 1,527	$ 5,286
Restricted cash		629
Accounts receivable	17,059	5,458
Other current assets	1,265	559
Current assets	19,851	11,932
Property and equipment	7,117	4,064
Goodwill	147,251	36,622
Total assets	253,228	106,355
Current liabilities	9,721	6,001
Total liabilities	9,721	8,172
Purchase price	243,507	98,183
Non- current liabilities		2,171
Customer Relationships
Business Acquisition [Line Items]
Intangible assets	56,000	26,588
Trade Names
Business Acquisition [Line Items]
Intangible assets	4,200	1,667
Covenants Not to Compete
Business Acquisition [Line Items]
Intangible assets	4,109	5,443
Proprietary Software
Business Acquisition [Line Items]
Intangible assets	14,700	2,560
Patent
Business Acquisition [Line Items]
Intangible assets		17,479
CTEH
Business Acquisition [Line Items]
Cash	1,527
Accounts receivable	17,059
Other current assets	1,265
Current assets	19,851
Property and equipment	7,042
Goodwill	146,625
Total assets	252,418
Current liabilities	9,721
Total liabilities	9,721
Purchase price	242,697
CTEH | Customer Relationships
Business Acquisition [Line Items]
Intangible assets	56,000
CTEH | Trade Names
Business Acquisition [Line Items]
Intangible assets	4,200
CTEH | Covenants Not to Compete
Business Acquisition [Line Items]
Intangible assets	4,000
CTEH | Proprietary Software
Business Acquisition [Line Items]
Intangible assets	14,700
All Other Acquisitions
Business Acquisition [Line Items]
Property and equipment	75
Goodwill	626
Total assets	810
Purchase price	810
All Other Acquisitions | Covenants Not to Compete
Business Acquisition [Line Items]
Intangible assets	$ 109
ECT2
Business Acquisition [Line Items]
Cash		3,149
Restricted cash		629
Accounts receivable		1,707
Other current assets		498
Current assets		5,983
Property and equipment		776
Goodwill		16,395
Total assets		58,841
Current liabilities		5,024
Total liabilities		5,024
Purchase price		53,817
ECT2 | Customer Relationships
Business Acquisition [Line Items]
Intangible assets		13,840
ECT2 | Trade Names
Business Acquisition [Line Items]
Intangible assets		1,008
ECT2 | Covenants Not to Compete
Business Acquisition [Line Items]
Intangible assets		3,360
ECT2 | Patent
Business Acquisition [Line Items]
Intangible assets		17,479
All Other Acquisitions
Business Acquisition [Line Items]
Cash		2,137
Accounts receivable		3,751
Other current assets		61
Current assets		5,949
Property and equipment		3,288
Goodwill		20,227
Total assets		47,514
Current liabilities		977
Total liabilities		3,148
Purchase price		44,366
Non- current liabilities		2,171
All Other Acquisitions | Customer Relationships
Business Acquisition [Line Items]
Intangible assets		12,748
All Other Acquisitions | Trade Names
Business Acquisition [Line Items]
Intangible assets		659
All Other Acquisitions | Covenants Not to Compete
Business Acquisition [Line Items]
Intangible assets		2,083
All Other Acquisitions | Proprietary Software
Business Acquisition [Line Items]
Intangible assets		$ 2,560