Business Acquisitions - Summary of Purchase Price Attributable to Acquisitions (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Business Acquisition [Line Items]
Cash	$ 3,524	$ 1,527
Accounts receivable	8,629	17,059
Other current assets	382	1,265
Current assets	12,535	19,851
Property and equipment	1,523	7,117
Goodwill	29,570	147,609
Total assets	72,791	253,586
Current liabilities	3,935	9,721
Total liabilities	3,935	9,721
Purchase price	68,856	243,865
MSE Group
Business Acquisition [Line Items]
Cash	2,810
Accounts receivable	2,987
Other current assets	31
Current assets	5,828
Property and equipment	513
Goodwill	7,747
Total assets	24,251
Current liabilities	814
Total liabilities	814
Purchase price	23,437
EI
Business Acquisition [Line Items]
Cash	250
Accounts receivable	4,564
Other current assets	84
Current assets	4,898
Property and equipment	32
Goodwill	8,944
Total assets	25,646
Current liabilities	2,651
Total liabilities	2,651
Purchase price	22,995
Other Acquisitions
Business Acquisition [Line Items]
Cash	464
Accounts receivable	1,078
Other current assets	267
Current assets	1,809
Property and equipment	978
Goodwill	12,879
Total assets	22,894
Current liabilities	470
Total liabilities	470
Purchase price	22,424
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		146,983
Total assets		252,776
Current liabilities		9,721
Total liabilities		9,721
Purchase price		243,055
Other 2020 Acquisitions
Business Acquisition [Line Items]
Property and equipment		75
Goodwill		626
Total assets		810
Purchase price		810
Customer Relationships
Business Acquisition [Line Items]
Intangible assets	24,321	56,000
Customer Relationships | MSE Group
Business Acquisition [Line Items]
Intangible assets	8,720
Customer Relationships | EI
Business Acquisition [Line Items]
Intangible assets	10,265
Customer Relationships | Other Acquisitions
Business Acquisition [Line Items]
Intangible assets	5,336
Customer Relationships | CTEH
Business Acquisition [Line Items]
Intangible assets		56,000
Trade Names
Business Acquisition [Line Items]
Intangible assets	2,840	4,200
Trade Names | MSE Group
Business Acquisition [Line Items]
Intangible assets	521
Trade Names | EI
Business Acquisition [Line Items]
Intangible assets	996
Trade Names | Other Acquisitions
Business Acquisition [Line Items]
Intangible assets	1,323
Trade Names | CTEH
Business Acquisition [Line Items]
Intangible assets		4,200
Covenants Not to Compete
Business Acquisition [Line Items]
Intangible assets	1,681	4,109
Covenants Not to Compete | MSE Group
Business Acquisition [Line Items]
Intangible assets	922
Covenants Not to Compete | EI
Business Acquisition [Line Items]
Intangible assets	511
Covenants Not to Compete | Other Acquisitions
Business Acquisition [Line Items]
Intangible assets	248
Covenants Not to Compete | CTEH
Business Acquisition [Line Items]
Intangible assets		4,000
Covenants Not to Compete | Other 2020 Acquisitions
Business Acquisition [Line Items]
Intangible assets		109
Acquired Technology
Business Acquisition [Line Items]
Intangible assets	321
Acquired Technology | Other Acquisitions
Business Acquisition [Line Items]
Intangible assets	$ 321
Proprietary Software
Business Acquisition [Line Items]
Intangible assets		14,700
Proprietary Software | CTEH
Business Acquisition [Line Items]
Intangible assets		$ 14,700