Montrose Environmental (MEG) 10-Q 10 Nov 21 2021 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Nov. 08, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2021
Document Quarterly Report	true
Document Transition Report	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Trading Symbol	MEG
Entity Registrant Name	Montrose Environmental Group, Inc.
Entity Central Index Key	0001643615
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		29,491,400
Entity Interactive Data Current	Yes
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity File Number	001-39394
Entity Tax Identification Number	46-4195044
Entity Incorporation State Country Code	DE
Entity Address Address Line1	5120 Northshore Drive
Entity Address City Or Town	North Little Rock
Entity Address State Or Province	AR
Entity Address Postal Zip Code	72118
City Area Code	501
Local Phone Number	900-6400
Security12b Title	Common Stock, par value $0.000004 per share
Security Exchange Name	NYSE

UNAUDITED CONDENSED CONSOLIDATE

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL POSITION - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
CURRENT ASSETS:
Cash and restricted cash	$ 16,006	$ 34,881
Accounts receivable—net	66,471	54,102
Contract assets	46,270	38,576
Prepaid and other current assets	9,839	6,709
Total current assets	138,586	134,268
NON-CURRENT ASSETS:
Property and equipment—net	31,078	34,399
Operating lease right-of-use asset—net	23,111
Finance lease right-of-use asset—net	7,493
Goodwill	304,237	274,667
Other intangible assets—net	160,239	154,854
Other assets	2,874	4,538
TOTAL ASSETS	667,618	602,726
CURRENT LIABILITIES:
Accounts payable and other accrued liabilities	42,373	34,877
Accrued payroll and benefits	22,485	21,181
Business acquisitions contingent consideration, current	31,152	49,902
Current portion of operating lease liabilities	6,715
Current portion of finance lease liabilities	3,174
Current portion of long-term debt	8,750	5,583
Total current liabilities	114,649	111,543
NON-CURRENT LIABILITIES:
Business acquisitions contingent consideration, long-term	4,200	4,565
Other non-current liabilities	2,446	2,523
Deferred tax liabilities—net	3,059	2,815
Operating lease liability—net of current portion	16,584
Finance lease liability—net of current portion	4,641
Long-term debt—net of deferred financing fees	200,876	170,321
Total liabilities	368,992	312,653
COMMITMENTS AND CONTINGENCIES
STOCKHOLDERS’ EQUITY:
Additional paid-in-capital	291,850	259,427
Accumulated deficit	(146,206)	(122,353)
Accumulated other comprehensive income	54	71
Total stockholders’ equity	145,698	137,145
TOTAL LIABILITIES, CONVERTIBLE AND REDEEMABLE SERIES A-2 PREFERRED STOCK AND STOCKHOLDERS’ EQUITY	667,618	602,726
Conversion Option
NON-CURRENT LIABILITIES:
Embedded derivative	22,537	20,886
Convertible And Redeemable Series A-2 Preferred Stock
NON-CURRENT LIABILITIES:
REDEEMABLE SERIES PREFERRED STOCK	$ 152,928	$ 152,928

UNAUDITED CONDENSED CONSOLIDA_2

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL POSITION (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Common stock, par value	$ 0.000004	$ 0.000004
Common stock, shares authorized	190,000,000	190,000,000
Common stock, shares issued	26,525,844	24,932,527
Common stock, shares outstanding	26,525,844	24,932,527
Convertible And Redeemable Series A-2 Preferred Stock
Temporary equity, par value	$ 0.0001	$ 0.0001
Temporary equity, shares authorized	17,500	17,500
Temporary equity, shares issued	17,500	17,500
Temporary equity, shares outstanding	17,500	17,500
Temporary equity, aggregate liquidation preference	$ 182.2	$ 182.2

UNAUDITED CONDENSED CONSOLIDA_3

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
REVENUES	$ 132,578	$ 84,705	$ 402,619	$ 219,502
COST OF REVENUES (exclusive of depreciation and amortization shown below)	85,242	51,828	272,662	142,115
SELLING, GENERAL AND ADMINISTRATIVE EXPENSE	30,499	24,442	82,865	64,810
FAIR VALUE CHANGES IN BUSINESS ACQUISITIONS CONTINGENT CONSIDERATION		13,404	24,035	17,387
DEPRECIATION AND AMORTIZATION	11,471	9,740	33,145	27,084
INCOME (LOSS) FROM OPERATIONS	5,366	(14,709)	(10,088)	(31,894)
OTHER EXPENSE
Other expense	(516)	(9,637)	(1,909)	(17,534)
Interest expense—net	(1,722)	(3,043)	(11,208)	(10,896)
Total other expenses—net	(2,238)	(12,680)	(13,117)	(28,430)
INCOME (LOSS) BEFORE EXPENSE (BENEFIT) FROM INCOME TAXES	3,128	(27,389)	(23,205)	(60,324)
INCOME TAX EXPENSE (BENEFIT)	902	3,348	648	(1,563)
NET INCOME (LOSS)	2,226	(30,737)	(23,853)	(58,761)
EQUITY ADJUSTMENT FROM FOREIGN CURRENCY TRANSLATION	(74)	80	(17)	27
COMPREHENSIVE INCOME (LOSS)	2,152	(30,657)	(23,870)	(58,734)
ACCRETION OF REDEEMABLE SERIES A-1 PREFERRED STOCK		(6,542)		(17,601)
REDEEMABLE SERIES A-1 PREFERRED STOCK DEEMED DIVIDEND		(24,341)		(24,341)
CONVERTIBLE AND REDEEMABLE SERIES A-2 PREFERRED STOCK DIVIDEND	(4,100)	(2,870)	(12,300)	(2,870)
NET LOSS ATTRIBUTABLE TO COMMON STOCKHOLDERS	$ (1,874)	$ (64,490)	$ (36,153)	$ (103,573)
WEIGHTED AVERAGE COMMON SHARES OUTSTANDING— BASIC AND DILUTED	26,220	21,554	25,798	13,669
NET LOSS PER SHARE ATTRIBUTABLE TO COMMON STOCKHOLDERS— BASIC AND DILUTED	$ (0.07)	$ (2.99)	$ (1.40)	$ (7.58)

UNAUDITED CONDENSED CONSOLIDA_4

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF REDEEMABLE SERIES A-1 PREFERRED STOCK, CONVERTIBLE AND REDEEMABLE SERIES A-2 PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Redeemable Series A-1 Preferred Stock	Convertible And Redeemable Series A-2 Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive (Loss) Income
Beginning balance at Dec. 31, 2019	$ (26,291)				$ 38,153	$ (64,404)	$ (40)
Beginning balance, shares at Dec. 31, 2019				8,370,107
Beginning balance at Dec. 31, 2019		$ 128,822
Beginning balance, shares at Dec. 31, 2019		12,000
Net income (loss)	(41,248)					(41,248)
Accretion of the redeemable series A-1 preferred stock to redeemable value	(5,415)				(5,415)
Accretion of the redeemable series A-1 preferred stock to redeemable value		$ 5,415
Stock-based compensation	1,150				1,150
Accumulated other comprehensive income (loss)	37						37
Ending balance at Mar. 31, 2020	(71,767)				33,888	(105,652)	(3)
Ending balance, shares at Mar. 31, 2020		12,000
Ending balance at Mar. 31, 2020		$ 134,237
Ending balance, shares at Mar. 31, 2020				8,370,107
Beginning balance at Dec. 31, 2019	(26,291)				38,153	(64,404)	(40)
Beginning balance, shares at Dec. 31, 2019				8,370,107
Beginning balance at Dec. 31, 2019		$ 128,822
Beginning balance, shares at Dec. 31, 2019		12,000
Net income (loss)	(58,761)
Redemption of the series A-1	$ 26,801
Redemption of the series A-1, shares	1,786,739
Common stock issued	$ 224,515
Common stock issued, shares	16,523,015
Dividend payment to the Series A-2 preferred shareholders			$ (2,900)
Issuance of common stock in connection with the initial public offering, net of issuance costs of $15.6 million, shares	11,500,000
Cancellation of shares	(117,785)
Ending balance at Sep. 30, 2020	$ 138,767				261,945	(123,165)	(13)
Ending balance, shares at Sep. 30, 2020			17,500
Ending balance at Sep. 30, 2020			$ 152,928
Ending balance, shares at Sep. 30, 2020				24,893,122
Beginning balance at Mar. 31, 2020	(71,767)				33,888	(105,652)	(3)
Beginning balance, shares at Mar. 31, 2020				8,370,107
Beginning balance at Mar. 31, 2020		$ 134,237
Beginning balance, shares at Mar. 31, 2020		12,000
Net income (loss)	13,224					13,224
Accretion of the redeemable series A-1 preferred stock to redeemable value	(5,644)				(5,644)
Accretion of the redeemable series A-1 preferred stock to redeemable value		$ 5,644
Issuance of the convertible and redeemable series A-2 preferred stock			$ 152,928
Issuance of the convertible and redeemable series A-2 preferred stock, shares			17,500
Stock-based compensation	1,140				1,140
Common stock issued	25,021				25,021
Common stock issued, shares				794,639
Accumulated other comprehensive income (loss)	(90)						(90)
Ending balance at Jun. 30, 2020	(38,116)				54,405	(92,428)	(93)
Ending balance, shares at Jun. 30, 2020		12,000	17,500
Ending balance at Jun. 30, 2020		$ 139,881	$ 152,928
Ending balance, shares at Jun. 30, 2020				9,164,746
Net income (loss)	(30,737)					(30,737)
Accretion of the redeemable series A-1 preferred stock to redeemable value	(6,542)				(6,542)
Accretion of the redeemable series A-1 preferred stock to redeemable value		6,542
Series A-1 preferred stock deemed dividend	(24,341)				(24,341)
Series A-1 preferred stock deemed dividend		$ 24,341
Redemption of the series A-1	$ 26,801				26,801
Temporary equity redemption of the series A-1, shares		(12,000)
Temporary equity redemption of the series A-1, value		$ (170,764)
Redemption of the series A-1, shares	1,786,739			1,786,739
Stock-based compensation	$ 1,149				1,149
Common stock issued	$ 150				150
Common stock issued, shares	15,728,376			26,850
Dividend payment to the Series A-2 preferred shareholders	$ (2,870)		$ (2,900)		(2,870)
Exercise of the series A-1 preferred stock warrant	11,871				11,871
Exercise of the series A-1 preferred stock warrant, shares				534,240
Exercise of the series A-2 preferred stock warrant	44,441				44,441
Exercise of the series A-2 preferred stock warrant, shares				1,999,999
Issuance of common stock in connection with the initial public offering, net of issuance costs of $15.6 million	$ 156,881				156,881
Issuance of common stock in connection with the initial public offering, net of issuance costs of $15.6 million, shares	11,500,000			11,500,000
Cancellation of shares	(117,785)			(119,452)
Accumulated other comprehensive income (loss)	$ 80						80
Ending balance at Sep. 30, 2020	138,767				261,945	(123,165)	(13)
Ending balance, shares at Sep. 30, 2020			17,500
Ending balance at Sep. 30, 2020			$ 152,928
Ending balance, shares at Sep. 30, 2020				24,893,122
Beginning balance at Dec. 31, 2020	137,145				259,427	(122,353)	71
Beginning balance, shares at Dec. 31, 2020				24,932,527
Beginning balance at Dec. 31, 2020			$ 152,928
Beginning balance, shares at Dec. 31, 2020			17,500
Net income (loss)	(11,596)					(11,596)
Stock-based compensation	1,805				1,805
Common stock issued	4,456				4,456
Common stock issued, shares				506,330
Dividend payment to the Series A-2 preferred shareholders	(4,100)				(4,100)
Accumulated other comprehensive income (loss)	29						29
Ending balance at Mar. 31, 2021	127,739				261,588	(133,949)	100
Ending balance, shares at Mar. 31, 2021			17,500
Ending balance at Mar. 31, 2021			$ 152,928
Ending balance, shares at Mar. 31, 2021				25,438,857
Beginning balance at Dec. 31, 2020	137,145				259,427	(122,353)	71
Beginning balance, shares at Dec. 31, 2020				24,932,527
Beginning balance at Dec. 31, 2020			$ 152,928
Beginning balance, shares at Dec. 31, 2020			17,500
Net income (loss)	(23,853)
Common stock issued	$ 38,136
Common stock issued, shares	1,593,317
Dividend payment to the Series A-2 preferred shareholders			$ (12,300)
Ending balance at Sep. 30, 2021	$ 145,698				291,850	(146,206)	54
Ending balance, shares at Sep. 30, 2021			17,500
Ending balance at Sep. 30, 2021			$ 152,928
Ending balance, shares at Sep. 30, 2021				26,525,844
Beginning balance at Mar. 31, 2021	127,739				261,588	(133,949)	100
Beginning balance, shares at Mar. 31, 2021				25,438,857
Beginning balance at Mar. 31, 2021			$ 152,928
Beginning balance, shares at Mar. 31, 2021			17,500
Net income (loss)	(14,483)					(14,483)
Stock-based compensation	2,417				2,417
Common stock issued	27,460				27,460
Common stock issued, shares				669,331
Dividend payment to the Series A-2 preferred shareholders	(4,100)				(4,100)
Accumulated other comprehensive income (loss)	28						28
Ending balance at Jun. 30, 2021	139,061				287,365	(148,432)	128
Ending balance, shares at Jun. 30, 2021			17,500
Ending balance at Jun. 30, 2021			$ 152,928
Ending balance, shares at Jun. 30, 2021				26,108,188
Net income (loss)	2,226					2,226
Stock-based compensation	2,365				2,365
Common stock issued	$ 6,220				6,220
Common stock issued, shares	417,656			417,656
Dividend payment to the Series A-2 preferred shareholders	$ (4,100)		$ (4,100)		(4,100)
Accumulated other comprehensive income (loss)	(74)						(74)
Ending balance at Sep. 30, 2021	$ 145,698				$ 291,850	$ (146,206)	$ 54
Ending balance, shares at Sep. 30, 2021			17,500
Ending balance at Sep. 30, 2021			$ 152,928
Ending balance, shares at Sep. 30, 2021				26,525,844

UNAUDITED CONDENSED CONSOLIDA_5

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF REDEEMABLE SERIES A-1 PREFERRED STOCK, CONVERTIBLE AND REDEEMABLE SERIES A-2 PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Parenthetical) $ in Millions	3 Months Ended
	Sep. 30, 2020USD ($)
Statement Of Stockholders Equity [Abstract]
Stock issuance costs	$ 15.6

UNAUDITED CONDENSED CONSOLIDA_6

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
OPERATING ACTIVITIES:
Net loss	$ 2,226	$ (30,737)		$ (23,853)	$ (58,761)
Adjustments to reconcile net loss to net cash used in operating activities:
Provision for bad debt			$ 6,300	803	6,445	$ 4,532
DEPRECIATION AND AMORTIZATION	11,471	9,740		33,145	27,084
Amortization of right-of-use asset				5,947
Stock-based compensation expense	2,365	1,149		6,587	3,439
Fair value changes in embedded derivatives				1,651	17,492
FAIR VALUE CHANGES IN BUSINESS ACQUISITIONS CONTINGENT CONSIDERATION		13,404		24,035	17,387
Deferred income taxes				232	(1,563)
Other				68	(1,180)
Debt extinguishment costs				4,052
Changes in operating assets and liabilities—net of acquisitions:
Accounts receivable and contract assets				(12,503)	(7,736)
Prepaid expenses and other current assets				(1,781)	(1,349)
Accounts payable and other accrued liabilities				(3,422)	(4,829)
Accrued payroll and benefits				61	6,084
Payment of contingent consideration and other assumed purchase price obligations				(15,549)	(6,390)
Change in operating leases				(5,765)
Net cash provided by (used in) operating activities				13,708	(3,877)
INVESTING ACTIVITIES:
Purchases of property and equipment				(5,405)	(5,366)
Proprietary software development and other software costs				(241)	(370)
Purchase price true ups				(8,562)
Proceeds from net working capital adjustment related to acquisitions					2,819
Cash paid for acquisitions—net of cash acquired				(36,480)	(173,923)
Net cash used in investing activities				(50,688)	(176,840)
FINANCING ACTIVITIES:
Proceeds from line of credit				109,000	104,390
Payments on line of credit				(72,000)	(201,980)
Proceeds from term loans				175,000	175,000
Repayment of term loan				(173,905)	(49,297)
Payment of contingent consideration and other purchase price obligations				(9,605)	(6,004)
Repayment of finance leases				(1,884)	(2,257)
Proceeds from issuance of common stock in connection with initial public offering, net of issuance costs					161,288
Payments of deferred offering costs					(2,925)
Prepayment premium on credit facility					(351)
Debt issuance costs				(2,590)	(4,866)
Proceeds from issuance of common stock for exercised stock options				6,032	171
Issuance of convertible and redeemable Series A-2 preferred stock and warrant					173,664
Redemption of the Series A-1 preferred stock					(131,821)
Dividend payment to the Series A-2 shareholders				(12,300)	(2,870)
Exercise of warrant options					25
Net cash provided by financing activities				17,748	212,167
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH				(19,232)	31,450
Foreign exchange impact on cash balance				357	43
Beginning of year			$ 6,884	34,881	6,884	6,884
End of period	$ 16,006	$ 38,377		16,006	38,377	$ 34,881
SUPPLEMENTAL DISCLOSURES OF CASH FLOWS INFORMATION:
Cash paid for interest				4,649	9,368
Cash paid for income tax				958	171
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING ACTIVITIES:
Series A-1 preferred stock deemed dividends—net of return from holders					24,341
Series A-1 preferred stock dividend paid in common shares					26,801
Accrued purchases of property and equipment				1,171	486
Property and equipment purchased under finance leases				1,766	1,753
Accretion of the redeemable series A-1 preferred stock to redeemable value					17,601
Common stock issued to acquire new businesses				6,020	25,000
Acquisitions unpaid contingent consideration				35,352	58,912
Offering costs included in accounts payable and other accrued liabilities				389	$ 1,237
Acquisitions contingent consideration and purchase price true ups paid in shares				$ 26,084

Description of the Business and

Description of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Description of the Business and Basis of Presentation	1. DESCRIPTION OF THE BUSINESS AND BASIS OF PRESENTATION Description of the Business —Montrose Environmental Group, Inc. (“Montrose” or the “Company”) is a corporation formed on November 2013, under the laws of the State of Delaware. The Company has approximately 80 offices across the United States, Canada and Australia and over 2,300 employees as of September 30, 2021. Montrose is an environmental services company serving the recurring environmental needs of a diverse client base, including Fortune 500 companies and federal, state and local governments through the following three segments: Assessment, Permitting and Response — Through its Assessment, Permitting and Response segment, Montrose provides scientific advisory and consulting services to support environmental assessments, environmental emergency response, and environmental audits and permits for current operations, facility upgrades, new projects, decommissioning projects and development projects. The Company’s technical advisory and consulting offerings include regulatory compliance support and planning, environmental, ecosystem and toxicological assessments and support during responses to environmental disruption. Montrose helps clients navigate regulations at the local, state, provincial and federal levels. Measurement and Analysis — Through its Measurement and Analysis segment, Montrose’s teams test and analyze air, water and soil to determine concentrations of contaminants, including emerging contaminants such as PFAS, as well as determine the toxicological impact of contaminants on flora, fauna and human health. Montrose’s offerings include source and ambient air testing and monitoring, leak detection and repair (“LDAR”) and advanced analytical laboratory services such as air, storm water, wastewater and drinking water analysis. Remediation and Reuse — Through its Remediation and Reuse segment, Montrose provides clients with engineering, design, implementation and operations and maintenance services, primarily to treat contaminated water, remove contaminants from soil or create biogas from waste. The Company does not own the properties or facilities at which it implements these projects or the underlying liabilities, nor does it own material amounts of the equipment used in projects; instead, the Company assists clients in designing solutions, managing projects and mitigating their environmental risks and liabilities at their locations. Initial Public Offering —On July 27, 2020, the Company completed its initial public offering (“IPO”) of common stock, in which it sold 11,500,000 shares, including 1,500,000 shares issued pursuant to the underwriters full exercise on July 24, 2020 of the underwriters’ option to purchase additional shares, at a price to the public of $15.00 per share, resulting in net proceeds to the Company of approximately $161.3 million after deducting underwriting discounts of $11.2 million. Additionally, the Company offset $4.4 million of deferred IPO costs against IPO proceeds recorded to additional paid in capital. These deferred IPO costs were directly attributable to the IPO offering in accordance with Staff Accounting Bulletin Topic 5: Miscellaneous Accounting. The Company’s common stock began trading on the New York Stock Exchange on July 23, 2020. Basis of Presentation —The unaudited condensed consolidated financial statements include the operations of the Company and its wholly-owned subsidiaries. These unaudited condensed consolidated financial statements are presented in conformity with accounting principles generally accepted in the United States (“U.S. GAAP”) and have been prepared pursuant to the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”) that permit reduced disclosure for interim periods. The unaudited condensed consolidated financial statements include all accounts of the Company and, in the opinion of management, include all recurring adjustments and normal accruals necessary for a fair statement of the Company’s financial position, results of operations and cash flows for the dates and periods presented. These unaudited condensed consolidated financial statements should be read in conjunction with the Company’s audited financial statements as of and for the year ended December 31, 2020. Results for interim periods are not necessarily indicative of the results to be expected during the remainder of the current year or for any future period. All intercompany transactions, accounts and profits, have been eliminated in the unaudited condensed consolidated financial statements.

Summary of New Accounting Prono

Summary of New Accounting Pronouncements	9 Months Ended
	Sep. 30, 2021
New Accounting Pronouncements And Changes In Accounting Principles [Abstract]
Summary of New Accounting Pronouncements	2. SUMMARY OF NEW ACCOUNTING PRONOUNCEMENTS Recently Adopted Accounting Pronouncements —The Company qualifies as an emerging growth company as defined in the Jumpstart Our Business Startups Act of 2012 (“JOBS Act”) and therefore intends to take advantage of certain exemptions from various public company reporting requirements, including delaying adoption of new or revised accounting standards until those standards apply to private companies. The Company has elected to use this extended transition period under the JOBS Act. The effective dates shown below reflect the election to use the extended transition period. However, as June 30, 2021, the end of the Company’s second fiscal quarter, the market value of the Company’s common stock held by non-affiliates exceeded $700.0 million and, as a result, the Company will no longer qualify as an emerging growth company at the end of the fiscal year ended December 31, 2021. The adoption dates discussed below are based on the Company no longer qualifying as an emerging growth company. In December 2019, the Financial Accounting Standards Board (“FASB”) issued Accounting Standard Update (“ASU”) 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. ASU 2019-12 removes certain exceptions to the general principles in Accounting Standard Codification (“ASC”) 740 and clarifies and amends certain guidance to promote consistent application. The standard was adopted as of January 1, 2021 and did not have a material impact on the Company’s unaudited condensed consolidated financial statements. In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation: Improvements to Nonemployee Share-Based Payment Accounting. Under the revised guidance, the accounting for awards issued to non-employees will be similar to the accounting for employee awards. The new guidance is effective for fiscal years beginning after December 15, 2019. The standard was adopted as of January 1, 2020 and did not have a material impact on the Company’s unaudited condensed consolidated financial statements. In January 2017, the FASB issued ASU 2017-04, Intangibles—Goodwill and Other (Topic 350): Simplifying the Accounting for Goodwill Impairment. The revised guidance eliminates Step 2 of the current goodwill impairment analysis test, which requires hypothetical purchase price allocation to measure goodwill impairment. A goodwill impairment loss will instead be measured at the amount by which a reporting unit’s carrying amount exceeds its fair value, not to exceed the carrying amount of goodwill. The revised guidance was adopted as of January 1, 2020 and did not have a material impact on the Company’s unaudited condensed consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), to improve financial reporting regarding leasing transactions. In June 2021, with an effective adoption date of January 1, 2021, the Company adopted ASU No. 2016-02, using the modified retrospective transition approach with a cumulative effect adjustment to the income statement as of the date of adoption (Note 6). As a result of electing the modified retrospective approach, the Company is not required to restate comparative periods for the effects of ASC 842. The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification determines whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability, regardless of lease classification. The Company elected the package of practical expedients, which allows it not to have to reassess previous conclusions about lease identification, lease classification and initial direct costs. In addition, the Company elected to not record operating lease right-of-use assets or operating lease liabilities for leases with an initial term of 12 months or less and elected the practical expedient to not separate lease and non-lease components for all of its leases. As of adoption date, the Company recognized operating lease right-of-use assets, current operating lease liabilities and operating lease liabilities, net of current portion of $24.6 million, $7.3 million and $17.3 million, respectively. The Company recognized finance lease right-of-use assets, current finance lease liabilities and finance lease liabilities, net of current portion of $7.2 million, $2.9 million and $4.6 million, respectively. Recently Issued Accounting Pronouncements Not Yet Adopted —In August 2020, the FASB issued ASU 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40)—Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity. The ASU simplifies accounting for convertible instruments by removing major separation models required under current U.S. GAAP. Consequently, more convertible debt instruments will be reported as a single liability instrument with no separate accounting for embedded conversion features. The ASU removes certain settlement conditions that are required for equity contracts to qualify for the derivative scope exception, which will permit more equity contracts to qualify for the exception. The ASU also simplifies the diluted net income per share calculation in certain areas. The new guidance is effective for fiscal years beginning after December 15, 2021, including interim periods within those fiscal years, and early adoption is permitted. The Company is currently evaluating the impact of the adoption of the standard on the unaudited condensed consolidated financial statements. In March 2020, the FASB issued ASU 2020-04, Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting. ASU 2020-04 provides optional expedients and exceptions for applying U.S. GAAP to contracts, hedging relationships and other transactions affected by the expected transition away from reference rates that are expected to be discontinued, such as LIBOR. ASU 2020-04 was effective upon issuance. The Company may elect to apply the guidance prospectively through December 31, 2022. The Company is currently evaluating the impact of the adoption of the standard on the unaudited condensed consolidated financial statements. In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326). The standard introduces a new model for recognizing credit losses on financial instruments based on an estimate of current expected credit losses and will apply to trade receivables. The new guidance will be effective for the Company’s annual and interim periods beginning after December 15, 2021. The Company does not anticipate that the adoption of this standard will have a material impact on the unaudited condensed consolidated financial statements.

Revenues and Accounts Receivabl

Revenues and Accounts Receivable	9 Months Ended
	Sep. 30, 2021
Revenues And Accounts Receivable [Abstract]
Revenues and Accounts Receivable	3. REVENUES AND ACCOUNTS RECEIVABLE The Company’s main revenue sources derive from the following revenue streams: Assessment, Permitting and Response Revenues —Assessment, Permitting and Response revenues are generated from multidisciplinary environmental consulting services. The majority of the contracts are fixed-price or time and material based. Measurement and Analysis Revenues —Measurement and Analysis revenues are generated from emissions sampling, testing and reporting services, leak detection services, ambient air monitoring services and laboratory testing services. The majority of the contracts are fixed-price or time-and-materials based. Remediation and Reuse Revenues — Remediation and Reuse revenues are generated from operating and maintenance (“O&M”) services (on biogas and waste water treatment facilities), as well as remediation, monitoring and environmental compliance services. Services on the majority of O&M contracts are provided under long-term fixed-fee contracts. Remediation, monitoring and environmental compliance contracts are predominantly fixed-fee and time-and-materials based. Disaggregation of Revenue —The Company disaggregates revenue by its operating segments The Company believes disaggregating revenue into these categories achieves the disclosure objectives to depict how the nature, amount, and uncertainty of revenue and cash flows are affected by economic factors. Disaggregated revenue disclosures are provided in Note 20. Contract Balances —The Company presents contract balances for unbilled receivables (contract assets), as well as customer advances, deposits and deferred revenue (contract liabilities) within contract assets and accounts payable and accrued expenses, respectively, on the unaudited condensed consolidated statements of financial position. Amounts are generally billed at periodic intervals (e.g., weekly, bi-weekly or monthly) as work progresses in accordance with agreed-upon contractual terms. The Company utilizes the practical expedient to not adjust the promised amount of consideration for the effects of a significant financing component as the period between when the Company transfers services to a customer and when the customer pays for those services is one year or less. Amounts recorded as unbilled receivables are generally for services the Company is not entitled to bill based on the passage of time. Under certain contracts, billing occurs subsequent to revenue recognition, resulting in contract assets. The Company sometimes receives advances or deposits from customers before revenue is recognized, resulting in contract liabilities. The following table presents the Company’s contract balances: September 30, December 31, 2021 2020 Contract assets $ 46,270 $ 38,576 Contract liabilities 8,037 6,114 Contract assets acquired through business acquisitions amounted to $0.4 million and $6.5 million as of September 30, 2021 and December 31, 2020, respectively. Contract liabilities acquired through business acquisitions amounted to $0.5 million and zero as of September 30, 2021 and December 31, 2020, respectively. Revenue recognized during the three and nine months ended September 30, 2021, included in the contract liabilities balance at the beginning of the year was $1.3 million and $3.8 million, respectively. The revenue recognized from the contract liabilities consisted of the Company satisfying performance obligations during the normal course of business. The amount of revenue recognized from changes in the transaction price associated with performance obligations satisfied in prior periods during the three and nine months ended September 30, 2021 was not material. Remaining Unsatisfied Performance Obligations —Remaining unsatisfied performance obligations represent the total dollar value of work to be performed on contracts awarded and in progress. The amount of remaining unsatisfied performance obligations increases with new contracts or additions to existing contracts and decreases as revenue is recognized on existing contracts. Contracts are included in the amount of remaining unsatisfied performance obligations when an enforceable agreement has been reached. As of September 30, 2021 and December 31, 2020, the estimated revenue expected to be recognized in the future related to performance obligations that are unsatisfied was approximately $64.1 million and $24.4 million, respectively. As of September 30, 2021, the Company expected to recognize approximately $51.7 million of this amount as revenue within the next year and $12.4 million the year after Accounts Receivable, Net —Accounts receivable, net consisted of the following: September 30, December 31, 2021 2020 Accounts receivable, invoiced $ 68,974 $ 57,228 Accounts receivable, other 1,876 1,139 Allowance for doubtful accounts (4,379 ) (4,265 ) Accounts receivable—net $ 66,471 $ 54,102 The Company extends non-interest-bearing trade credit to its customers in the ordinary course of business. Accounts receivable are shown on the face of the unaudited condensed consolidated statements of financial position, net of an allowance for doubtful accounts. In determining the allowance for doubtful accounts, the Company analyzes the aging of accounts receivable, historical bad debts, customer creditworthiness and current economic trends. During the first quarter of 2020, there was a global outbreak of a new strain of coronavirus, COVID-19. The COVID-19 pandemic added uncertainty to the collectability of certain receivables, particularly in industries hard hit by the pandemic. As a result, the Company recorded a $6.3 million bad debt reserve during the first quarter of 2020. The bad debt adjustment included a $5.5 million reserve for one customer in the Company’s Remediation and Reuse segment in which management concluded to discontinue select service lines as of June 30, 2020 (Note 20). The Company did not have any customers that exceeded 10.0% of gross accounts receivable as of September 30, 2021. As of December 31, 2020, the Company had one customer that accounted for 10.2% of gross accounts receivable. During the three months ended September 30, 2021, the Company did not have any customers that exceeded 10.0% of revenue. During the nine months ended September 30, 2021, the Company had three customers who accounted for 12.1%, 11.4% and 10.7% of revenue. The Company did not have any customers that exceeded 10.0% of revenue during the three or nine months ended September 30, 2020. The Company performs ongoing credit evaluations, and accordingly, believes that the balances from these customers do not represent a significant credit risk. The allowance for doubtful accounts consisted of the following: Beginning Balance Bad Debt Expense Charged to Allowance Other ( 1) Ending Balance Nine months ended September 30, 2021 $ 4,265 $ 803 $ (1,229 ) $ 540 $ 4,379 Year ended December 31, 2020 1,327 4,532 (2,633 ) 1,039 4,265 ____________________ (1) This amount consists of additions to the allowance due to business acquisitions.

Prepaid and Other Current Asset

Prepaid and Other Current Assets	9 Months Ended
	Sep. 30, 2021
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid and Other Current Assets	4. PREPAID AND OTHER CURRENT ASSETS Prepaid and other current assets consisted of the following: September 30, December 31, 2021 2020 Deposits $ 795 $ 708 Prepaid expenses 6,574 3,510 Supplies 2,470 2,491 Prepaid and other current assets $ 9,839 $ 6,709

Property and Equipment, Net

Property and Equipment, Net	9 Months Ended
	Sep. 30, 2021
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. PROPERTY AND EQUIPMENT, NET Property and equipment are stated at cost or estimated fair value for assets acquired through business combinations. Depreciation and amortization are provided using the straight-line method over the estimated useful lives of the assets. Leasehold improvements are amortized using the straight-line method over the shorter of the remaining lease term, including options that are deemed to be reasonably assured, or the estimated useful life of the improvement. Property and equipment, net, consisted of the following: Estimated September 30, December 31, Useful Life 2021 2020 Lab and test equipment 7 years $ 18,183 $ 18,631 Vehicles 5 years 5,147 13,320 Equipment 3-7 years 35,099 32,177 Furniture and fixtures 7 years 2,696 2,938 Leasehold improvements 7 years 6,807 6,767 Aircraft 10 years 931 834 Building 39 years 2,975 2,975 71,838 77,642 Land 725 725 Construction in progress 1,660 219 Less accumulated depreciation (43,145 ) (44,187 ) Total property and equipment— net $ 31,078 $ 34,399 Total depreciation expense included in the unaudited condensed consolidated statements of operations was $2.5 million and $7.2 million for the three and nine months ended September 30, 2021, respectively, and $2.1 million and $6.1 million for the three and nine months ended September 30, 2020, respectively. Total property and equipment, net as of December 31, 2020 included $7.2 million related to financed assets, which, following the adoption of ASC 842, are presented as part of finance lease liabilities and excluded from property and equipment, net on the face of the condensed consolidated balance sheet as of September 30, 2021.

Leases

Leases	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Leases	6. LEASES Leases are classified as either finance leases or operating leases based on criteria in ASC 842. The Company has finance leases for its vehicle and equipment leases and operating leases for its real estate space and office equipment leases. The Company’s operating and finance leases generally have original lease terms between 1 year and 15 years, and in some instances include one or more options to renew. The Company includes options to extend the lease term if the options are reasonably certain of being exercised. The Company currently considers some of its renewal options to be reasonably certain to be exercised. Some leases also include early termination options, which can be exercised under specific conditions. The Company does not have material residual value guarantees or restrictive covenants associated with its leases. In June 2021, with an effective adoption date of January 1, 2021, the Company adopted ASU 2016-02 using the modified retrospective approach, which permits application of this new guidance at the beginning of the period of adoption, with comparative periods continuing to be reported under ASC 840. Finance and operating lease assets represent the right to use an underlying asset for the lease term, and finance and operating lease liabilities represent the obligation to make lease payments arising from the lease. The Company calculates the present value of its finance and operating leases using an estimated incremental borrowing rate (“IBR”), which requires judgment. For real estate operating leases, the Company estimates the IBR based on prevailing market rates for collateralized debt in a similar economic environment with similar payment terms and maturity dates commensurate with the terms of the lease. For all other leases, the Company estimates the IBR based on the stated interest rate on the contract. Since many of the inputs used to calculate the rate implicit in the leases are not readily determinable from the lessee’s perspective, the Company will not use the implicit interest rate. Certain leases contain variable payments, these payments are expensed as incurred and not included in the Company’s operating lease right-of-use assets and operating lease liabilities. These amounts primarily include payments for maintenance, utilities, taxes, and insurance and are excluded from the present value of the Company’s lease obligations. As of the adoption date, the Company recognized operating lease right-of-use assets, current operating lease liabilities and operating lease liabilities, net of current portion of $24.6 million, $7.3 million and $17.3 million, respectively. As of the adoption date, the Company recognized finance lease right-of-use assets, current finance lease liabilities and finance lease liabilities, net of current portion of $7.2 million, $2.9 million and $4.6 million, respectively. As part of this adoption, the Company elected to not record operating lease right-of-use assets or operating lease liabilities for leases with an initial term of 12 months or less. The Company also elected to combine lease and non-lease components on all new or modified operating leases into a single lease component for all classes of assets. Total rent expense under operating leases was $2.2 million and $6.4 million for the three and nine months ended September 30, 2020, respectively. The components of lease expense were as follows: Three Months Ended Nine Months Ended Statement of Operations Location September 30, 2021 September 30, 2021 Operating lease cost Lease cost Selling, general and administrative expense $ 2,203 $ 6,398 Variable lease cost Selling, general and administrative expense 120 329 Total operating lease cost 2,323 6,727 Finance lease cost Amortization of right of use assets Depreciation and amortization 825 2,391 Interest on lease liabilities Interest expense—net 97 300 Total finance lease cost 922 2,691 Total lease cost $ 3,245 $ 9,418 Supplemental cash flows information related to leases was as follows: Nine Months Ended September 30, 2021 Cash paid for amounts included in the measurement of lease liabilities Operating cash flows used in operating leases $ 6,273 Operating cash flows used in finance leases 300 Financing cash flows used in finance leases 2,367 Lease liabilities arising from new ROU assets Operating leases 5,078 Finance leases 2,719 Weighted average remaining lease terms and weighted average discount rates were: September 30, 2021 Operating Leases Finance Leases Weighted average remaining lease term (years) 5.22 2.80 Weighted average discount rate 2.62 % 4.95 % The following is a schedule by year of the maturities of lease liabilities with original terms in excess of one year: September 30, 2021 Operating Leases Finance Leases Remainder of 2021 $ 2,103 $ 923 2022 6,659 3,295 2023 4,557 2,316 2024 3,031 1,317 2025 and thereafter 8,637 484 Total undiscounted future minimum lease payments 24,987 8,335 Less imputed interest (1,688 ) (520 ) Total discounted future minimum lease payments $ 23,299 $ 7,815 A schedule of the future minimum rental commitments under the Company’s capital lease agreements and non-cancelable operating lease agreements with an initial or remaining term in excess of one year as of December 31, 2020, in accordance with ASC 840, the predecessor to ASC 842, were as follows: December 31, 2020 Operating Leases Finance Leases 2021 $ 5,946 $ 2,652 2022 4,865 2,172 2023 3,146 1,444 2024 1,812 496 2025 and thereafter 4,954 69 Total $ 20,723 $ 6,833

Business Acquisitions

Business Acquisitions	9 Months Ended
	Sep. 30, 2021
Business Combinations [Abstract]
Business Acquisitions	7. BUSINESS ACQUISITIONS In line with the Company’s strategic growth initiatives, the Company acquired four businesses during the nine months ended September 30, 2021 and several businesses during the year ended December 31, 2020. The results of each The identifiable intangible assets for acquisitions are valued using the excess earnings method discounted cash flow approach for customer relationships, the relief from royalty method for trade names, the patent and external proprietary software, the “with and without” method for covenants not to compete and the replacement cost method for the internal proprietary software by incorporating Level 3 inputs as described under the fair value hierarchy of ASC 820. These unobservable inputs reflect the Company’s own assumptions about which assumptions market participants would use in pricing an asset on a non-recurring basis. These assets will be amortized over their respective estimated useful lives. Other purchase price obligations (primarily deferred purchase price liabilities and target working capital liabilities or receivables) are included on the unaudited condensed consolidated statements of financial position in accounts payable and other accrued liabilities, other non-current liabilities or accounts receivable-net in the case of working capital deficits. Contingent consideration outstanding from acquisitions are included on the unaudited condensed consolidated statements of financial position in business acquisition contingent consideration, current or in business acquisitions contingent consideration, long-term. These obligations are scheduled to be settled if certain performance thresholds are met. The Company considers several factors when determining whether or not contingent consideration liabilities are part of the purchase price, including the following: (i) the valuation of its acquisitions is not supported solely by the initial consideration paid, (ii) the former stockholders of acquired companies that remain as key employees receive compensation other than contingent consideration payments at a reasonable level compared with the compensation of the Company’s other key employees and (iii) contingent consideration payments are not affected by employment termination. The Company reviews and assesses the estimated fair value of contingent consideration at each reporting period. Transaction costs related to business combinations totaled $0.9 million and $1.7 million zero and $3.8 million Acquisitions Completed During the Nine months Ended September 30, 2021 MSE Group (“MSE”) —In January 2021, the Company completed the acquisition of MSE Group by acquiring 100.0% of its membership interests. MSE is a provider of environmental assessment, compliance, engineering, and design services primarily to the U.S. federal government. MSE is based in Orlando, FL with additional offices in Tampa, Orlando, Jacksonville, San Antonio, TX, and Wilmington, NC, and satellite locations nationwide. Vista Analytical Laboratory, Inc. (“Vista”) —In June 2021, the Company completed the acquisition of Vista Analytical Laboratory, Inc. (“Vista”) by acquiring 100.0% of its common stock. Vista provides specialty analytical services related to Per- and polyfluoroalkyl substances (“PFAS”) and other semi-volatile organic compounds. Vista is based in Dorado Hills, CA. The upfront cash payment made to acquire Vista was funded through cash on hand and the common stock portion of the purchase price was funded through the issuance of 9,322 shares of common stock Environmental Intelligence, LLC (“EI”) —In July, 2021 the Company completed the acquisition of Environmental Intelligence (“EI”) by acquiring 100.0% of its membership interests. EI is an environmental consulting company recognized for its innovative work in wildlife mitigation, biological assessments, and other environmental services. EI is based in Laguna Beach, CA and enhances Montrose’s ecological planning and service capabilities in California and the US West Coast. The upfront cash payment made to acquire EI was funded through cash on hand and the common stock portion of the purchase price was funded through the issuance of 43,100 shares of common stock. SensibleIoT, LLC (“Sensible”) — In August, 2021, t he Company completed the business acquisition of SensibleIoT, LLC (“Sensible”) by acquiring 100.0% of its membership interests. Sensible is a technology platform that connects sensors and sources of environment data to a central, proprietary database that enables real-time client interaction. Sensible provides Montrose with an advanced ability to integrate environmental services and enhance environmental data analytics for clients. The upfront cash payment made to acquire Sensible was funded through cash on hand and the common stock portion of the purchase price was funded through the issuance of 19,638 shares of common stock. The following table summarizes the elements of the purchase price of the acquisitions completed during the nine months ended September 30, 2021: Cash Common Stock Other Purchase Price Components Current Contingent Consideration Current Contingent Consideration Long Term Total Purchase Price MSE $ 9,082 $ 2,271 $ 10,280 $ — $ 1,804 $ 23,437 Vista 9,025 475 172 — 1,000 10,672 EI 15,397 2,274 5,324 — — 22,995 Sensible 6,500 1,000 52 1,000 3,200 11,752 Total $ 40,004 $ 6,020 $ 15,828 $ 1,000 $ 6,004 $ 68,856 The other purchase price components of the MSE purchase price consist of a target working capital amount, a 338 election tax liability, 2020 and 2021 purchase price true ups and contingent consideration. The 2020 and 2021 purchase price true up elements are based on MSE’s actual 2020 and 2021 results. The other purchase price components of the Vista purchase price consist of a target working capital amount and other contingent consideration. The other purchase price component for EI consists primarily of cash in escrow, as well as, a working capital target payment, which is not finalized at this time. The other purchase price components of Sensible consist of a working capital target payment and other contingent consideration, which are not finalized at this time. The contingent consideration elements of the acquisitions are related to earn-outs which are based on the expected achievement of revenue or earnings thresholds as of the date of the acquisition and for which the maximum potential amount is limited. The Company may be required to make up to $12.7 million in aggregate true up and earn-out payments between the years 2022 and 2025 in connection with the acquisitions of MSE, Vista and Sensible. The Company paid the MSE target working capital amount and the 2020 purchase price true up in April 2021. The preliminary purchase price attributable to the acquisitions was allocated as follows: MSE EI All other acquisitions Total Cash $ 2,810 $ 250 $ 464 $ 3,524 Accounts receivable 2,987 4,564 1,078 8,629 Other current assets 31 84 267 382 Current assets 5,828 4,898 1,809 12,535 Property and equipment 513 32 978 1,523 Customer relationships 8,720 10,265 5,336 24,321 Trade names 521 996 1,323 2,840 Covenants not to compete 922 511 248 1,681 Acquired Technology — — 321 321 Goodwill 7,747 8,944 12,879 29,570 Total assets 24,251 25,646 22,894 72,791 Current liabilities (814 ) (2,651 ) (470 ) (3,935 ) Total liabilities (814 ) (2,651 ) (470 ) (3,935 ) Purchase price $ 23,437 $ 22,995 $ 22,424 $ 68,856 For the acquisitions completed during the nine months ended September 30, 2021, the results of operations since the acquisition dates have been combined with those of the Company. The Company’s unaudited condensed consolidated statement of operations for the three and nine months ended September 30, 2021 includes revenue of $11.9 million and $19.7 million, respectively, and pre-tax loss of $(0.2) million and zero , respectively. MSE is included in the Company’s Remediation and Reuse segment, Vista and Sensible are included in the Company’s Measurement and Analysis segment and EI is included the Company’s Assessment, Permit and Response segment. The weighted average useful lives for the acquired customer relationships and related backlog for MSE are 7 years and 2 years, respectively. The weighted average useful lives for the acquired tradenames and covenants not to compete for MSE are 2 years The weighted average useful life for the acquired customer relationships for Vista is 10 years. The weighted average useful lives for the acquired tradenames and covenants not to compete for Vista are 3 years The weighted average useful life for the acquired customer relationships for EI is 10 years. The weighted average useful life for the acquired tradenames and covenants not to compete for EI is 5 years. The weighted average useful life for the acquired customer relationships for Sensible is 10 years 5 years Goodwill associated with these four acquisitions is deductible for income tax purposes. Acquisitions Completed During the Year Ended December 31, 2020 The Center for Toxicology and Environmental Health, L.L.C. —In April 2020, the Company completed the acquisition of The Center for Toxicology and Environmental Health, L.L.C. (“CTEH”) by acquiring 100.0% of its membership interests. CTEH is an environmental consulting company headquartered in Arkansas that specializes in environmental response and toxicology. The cash payment made to acquire CTEH was funded through the issuance of the Convertible and Redeemable Series A-2 Preferred Stock (Note 17) and the common stock portion of the purchase price was funded through the issuance of 791,139 shares of common stock. Leed Environmental Inc .— In September 2020, the Company acquired certain testing assets, and operations from Leed Environmental Inc. (“LEED”). LEED provides environmental project management and coordination services. LEED expands the Company’s remediation capabilities in the Northeast region of the United States. The cash payment made to acquire LEED was funded via cash on hand. American Environmental Testing Co . — In September 2020, the Company acquired certain assets and operations of American Environmental Testing Co. (“AETC”), a stack testing company in Utah. AETC expands the Company’s air measurement and analysis capabilities in the West Coast region. The cash payment made to acquire AETC was funded via cash on hand. The following table summarizes the elements of purchase price of the acquisitions completed during the year ended December 31, 2020: Cash Common Stock Other Purchase Price Components Current Other Purchase Price Components Long Term Contingent Consideration Current Contingent Consideration Long Term Total Purchase Price CTEH $ 175,000 $ 25,000 $ (1,939 ) $ — $ 34,451 $ 10,543 $ 243,055 Other acquisitions 450 — 50 100 210 — 810 Total $ 175,450 $ 25,000 $ (1,889 ) $ 100 $ 34,661 $ 10,543 $ 243,865 The contingent consideration elements of the purchase price of the acquisitions are related to earn-outs which are based on the expected achievement of revenue or earnings thresholds as of the date of the acquisition and for which the maximum potential amount is limited. The CTEH first year earn-out was calculated at twelve times CTEH’s 2020 EBITDA (as defined in the purchase agreement) in excess of $18.3 million, with a maximum first year earn-out payment of $50.0 million, which was fully achieved. The second year earn-out is to be calculated at ten times CTEH’s 2021 EBITDA in excess of actual 2020 EBITDA (with actual 2020 EBITDA subject to a minimum of $18.3 million and a maximum of $22.5 million), with a maximum second year earn-out payment of $30.0 million. The 2020 earn-out was initially payable 100.0% in common stock, but as a result of the completion of the Company’s IPO (Note 1), 50.0% was payable in cash. In April 2021, the 2020 earn-out payment was made with 50.0% paid in cash and the remaining 50.0% paid in common stock of the Company (Notes 14 and 18). The 2021 earn-out, if any, is payable 100.0% in cash. The purchase price attributable to the acquisitions was allocated as follows: CTEH Other 2020 Acquisitions Total Cash $ 1,527 $ — $ 1,527 Accounts receivable 17,059 — 17,059 Other current assets 1,265 — 1,265 Current assets 19,851 — 19,851 Property and equipment 7,042 75 7,117 Customer relationships 56,000 — 56,000 Trade names 4,200 — 4,200 Covenants not to compete 4,000 109 4,109 Proprietary software 14,700 — 14,700 Goodwill 146,983 626 147,609 Total assets 252,776 810 253,586 Current liabilities 9,721 — 9,721 Total liabilities 9,721 — 9,721 Purchase price $ 243,055 $ 810 $ 243,865 CTEH, LEED and AETC are included in the Company’s Assessment, Permitting and Response, Remediation and Reuse and Measurement and Analysis segments, respectively. The weighted average useful lives for the acquired customer relationships and internal proprietary software for the CTEH acquisition are 15 years and 3 years, respectively. The weighted average useful lives for the acquired tradenames, covenants not to compete and external proprietary software for the CTEH acquisition is 5 years Goodwill associated with the CTEH, LEED and AETC acquisitions is deductible for income tax purposes. Supplemental Unaudited Pro-Forma — The unaudited condensed consolidated financial information summarized in the following table gives effect to the 2021 and the 2020 acquisitions discussed above assuming they occurred on January 1, 2020. These unaudited consolidated pro forma operating results do not assume any impact from revenue, cost or other operating synergies that are expected or may have been realized as a result of the acquisitions. These unaudited consolidated pro forma operating results are presented for illustrative purposes only and are not indicative of the operating results that would have been achieved had the acquisitions occurred on January 1, 2020, nor does the information purport to reflect results for any future period. For the Three Months Ended September 30, 2021 2020 As reported Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) As reported Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) Revenues $ 132,578 $ — $ 132,578 $ 84,705 $ 5,064 $ 89,769 Net (loss) income 2,226 — 2,226 (30,737 ) 628 (30,109 ) For the Nine Months Ended September 30, 2021 2020 As reported Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) As reported Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) Revenues $ 402,619 $ 12,454 $ 415,073 $ 219,502 $ 61,583 $ 281,085 Net (loss) income (23,853 ) 2,205 (21,648 ) (58,761 ) 15,847 (42,914 ) During the first quarter of 2020, the Company determined to reduce the footprint of its environmental lab in Berkeley, California, and to exit its non-specialized municipal water engineering service line and its food waste biogas engineering service line, (together, “the Discontinued Service Lines”). Revenues from Discontinued Service Lines included in revenues in the above table was zero for the three and nine months ended September 30, 2021 and zero and $3.8 million for the three and nine months ended September 30, 2020, respectively.

Goodwill and Intangible Assets

Goodwill and Intangible Assets	9 Months Ended
	Sep. 30, 2021
Goodwill And Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	8. GOODWILL AND INTANGIBLE ASSETS Amounts related to goodwill are as follows: Assessment, Permitting and Response Measurement and Analysis Remediation and Reuse Total Balance as of December 31, 2020 $ 162,156 $ 69,054 $ 43,457 $ 274,667 Goodwill acquired during the period 8,944 12,879 7,747 29,570 Balance as of September 30, 2021 $ 171,100 $ 81,933 $ 51,204 $ 304,237 Amounts related to finite-lived intangible assets are as follows: September 30, 2021 Estimated Useful Life Gross Balance Accumulated Amortization Total Intangible Assets—Net Finite lived intangible assets Customer relationships 2-15 years $ 189,103 $ 67,328 $ 121,775 Covenants not to compete 4-5 years 31,623 24,832 6,791 Trade names 1-5 years 19,778 15,107 4,671 Proprietary software 3-5 years 21,595 9,853 11,742 Patent 16 years 17,479 2,219 15,260 Total other intangible assets —net $ 279,578 $ 119,339 $ 160,239 December 31, 2020 Estimated Useful Life Gross Balance Accumulated Amortization Total Intangible Assets—Net Finite lived intangible assets Customer relationships 7-15 years $ 164,782 $ 53,446 $ 111,336 Covenants not to compete 4-5 years 29,942 21,469 8,473 Trade names 1-5 years 16,938 12,849 4,089 Proprietary software 3-5 years 21,007 6,132 14,875 Patent 16 years 17,479 1,398 16,081 Total other intangible assets —net $ 250,148 $ 95,294 $ 154,854 Intangible assets with finite lives are stated at cost, less accumulated amortization and impairment losses, if any. These intangible assets are amortized using the straight-line method over the estimated useful lives of the assets. Future amortization expense is estimated to be as follows for each of the five following years and thereafter: December 31, 2021 (remaining) $ 8,463 2022 30,298 2023 24,407 2024 20,623 2025 14,225 2026 and thereafter 62,223 Total $ 160,239

Accounts Payable and Other Accr

Accounts Payable and Other Accrued Liabilities	9 Months Ended
	Sep. 30, 2021
Payables And Accruals [Abstract]
Accounts Payable and Other Accrued Liabilities	9. ACCOUNTS PAYABLE AND OTHER ACCRUED LIABILITIES Accounts payable and other accrued liabilities consisted of the following: September 30, December 31, 2021 2020 Accounts payable $ 17,233 $ 15,481 Accrued expenses 15,703 11,469 Other business acquisitions purchase price obligations 274 50 Contract liabilities 8,037 6,114 Other current liabilities 1,126 1,507 Income tax payable — 256 Total accounts payable and other accrued liabilities $ 42,373 $ 34,877

Accrued Payroll and Benefits

Accrued Payroll and Benefits	9 Months Ended
	Sep. 30, 2021
Statement Of Financial Position [Abstract]
Accrued Payroll and Benefits	10. ACCRUED PAYROLL AND BENEFITS Accrued payroll and benefits consisted of the following: September 30, December 31, 2021 2020 Accrued bonuses $ 8,233 $ 5,416 Accrued paid time off 1,990 2,067 Accrued payroll 7,472 9,133 Accrued other 4,790 4,565 Total accrued payroll and benefits $ 22,485 $ 21,181

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Income Taxes	11. INCOME TAXES The Company calculates its interim income tax provision in accordance with ASC Topic 270, Interim Reporting (“ASC 270”), and ASC 740. The Company’s effective tax rate (“ETR”) from continuing operations was 28.8% and (2.8)% for the three and nine months ended September 30, 2021, respectively, and (12.2)% and 2.6% for the three and nine months ended September 30, 2020, respectively. Income tax expense recorded by the Company during the three and nine months ended September 30, 2021 was $0.9 million and $0.6 million, respectively. The Company recorded an income tax expense of $3.3 million and a benefit of $1.6 million during the three and nine months ended September 30, 2020, respectively. The difference between the ETR and federal statutory rate of 21.0% is primarily attributable to items recorded for U.S. GAAP but permanently disallowed for U.S. federal income tax purposes, recognition of a U.S. federal and state valuation allowance, state and foreign income tax provisions and Global Intangible Low Taxed Income (“GILTI”). A valuation allowance is recorded when it is more-likely-than-not some of the Company’s deferred tax assets may not be realized. Significant judgment is applied when assessing the need for a valuation allowance and the Company considers future taxable income, reversals of existing deferred tax assets and liabilities and ongoing prudent and feasible tax planning strategies, in making such assessment. As of September 30, 2021, the Company’s U.S. federal, state and various foreign net deferred tax assets are not more-likely-than-not to be realized and a full valuation allowance is maintained. The Company records uncertain tax positions in accordance with ASC 740, on the basis of a two-step process in which (i) the Company determines whether it is more likely than not a tax position will be sustained on the basis of the technical merits of such position and (ii) for those tax positions meeting the more-likely-than-not recognition threshold, the Company would recognize the largest amount of tax benefit that is more than 50.0% likely to be realized upon ultimate settlement with the related tax authority. The Company has determined it has no uncertain tax positions as of September 30, 2021. The Company classifies interest and penalties recognized on uncertain tax positions as a component of income tax expense.

Warrant Options

Warrant Options	9 Months Ended
	Sep. 30, 2021
Warrants And Rights Note Disclosure [Abstract]
Warrant Options	12. WARRANT OPTIONS In October 2018, in connection with the issuance of the Redeemable Series A-1 Preferred Stock, the Company issued a detachable warrant to acquire 534,240 shares of common stock at a price of $0.01 per share at any given time during a period of ten years beginning on the instrument’s issuance date. For the three and nine months ended September 30, 2020, fair value gains/(losses) recorded in other expense on the unaudited condensed consolidated statements of operations related to the Redeemable Series A-1 warrant were $5.0 million. In April 2020, in connection with the issuance of the Convertible and Redeemable Series A-2 Preferred Stock, the Company issued a detachable warrant to acquire 1,351,960 shares of common stock at a price of $0.01 per share at any time following the occurrence of a qualifying IPO, a sale of the Company, or a redemption in full of the Series A-2 preferred stock (each, an “Adjustment Event”), with an expiration date of ten years from the instrument’s issuance date. The number of shares underlying the warrant and issuable upon exercise was subject to adjustment based upon the price per share of common stock upon the occurrence of an Adjustment Event (Note 17) to reflect an aggregate value of $30.0 million. As a result of the $15.00 per share public offering price in the IPO, the warrant issued in connection with the issuance of the Convertible and Redeemable Series A-2 Preferred Stock was adjusted pursuant to its terms and, upon closing of the IPO, represented a warrant to purchase 1,999,999 shares of common stock (an increase of 648,039 shares). On July 30, 2020, the Redeemable Series A-1 Preferred Stock and the Convertible and Redeemable Series A-2 Preferred Stock warrants were

Debt

Debt	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Debt	13. DEBT Debt consisted of the following: September 30, December 31, 2021 2020 Term loan facility $ 175,000 $ 173,906 Revolving Line of Credit 37,000 — Capital leases — 3,088 Equipment line of credit — 3,018 Less deferred debt issuance costs (2,374 ) (4,108 ) Total debt 209,626 175,904 Less current portion of long-term debt (8,750 ) (5,583 ) Long-term debt, less current portion $ 200,876 $ 170,321 Deferred Financing Costs —Costs relating to debt issuance have been deferred and are presented as discounted against the underlying debt instrument. These costs are amortized to interest expense over the terms of the underlying debt instruments. 2021 Credit Facility — On April 27, 2021 , the Company entered into a new Senior Secured Credit Agreement providing for a new $300.0 million credit facility comprised of a $175.0 million term loan and a $125.0 million revolving line of credit (the “2021 Credit Facility”) , and used a portion of the proceeds from the 2021 Credit Facility to repay all amounts outstanding under the 2020 Credit Facility (as defined below) . The 2021 revolving credit facility includes a $20.0 million sublimit for the issuance of letters of credit. Subject to certain exceptions, all amounts under the 2021 Credit Facility will become due on April 27, 2026. The Company has the option to borrow incremental term loans or request an increase in the aggregate commitments under the revolving credit facility up to an aggregate amount of $150.0 million subject to the satisfaction of certain conditions. The 2021 Credit Facility term loan must be repaid in quarterly installments and Amortization Table Year 1 Year 2 Year 3 Year 4 Year 5 Term Loan 5.0 % 5.0 % 7.5 % 7.5 % 10.0 % The 2021 Credit Facility term loan and the revolver bear interest subject to the Company’s leverage ratio and LIBOR as follows: Pricing Tier Net Leverage Ratio 2021 Credit Facility Commitment Fee Letter of Credit Fee LIBOR Base Rate 1 ≥ 3.75x to 1.0 2.50 % 1.50 % 0.25 % 2.50 % 2 <3.75x to 1.0 but ≥ 3.25 to 1.0 2.25 1.25 0.23 2.25 3 <3.25 to 1.0 but ≥ 2.50 to 1.0 2.00 1.00 0.20 2.00 4 <2.50 to 1.0 but ≥ 1.75 to 1.0 1.75 0.75 0.15 1.75 5 <1.75 to 1.0 1.50 0.50 0.15 1.50 Additionally, the Company may receive an interest rate adjustment of up to 0.05% under the 2021 Credit Facility based on the Company’s performance against certain defined sustainability and environmental, social and governance related objectives. The 2021 Credit Facility includes a number of covenants imposing certain restrictions on the Company’s business, including, among other things, financial covenants requiring the Company to remain below a maximum total net leverage ratio of 4.25 times, which steps down to 4.00 times beginning with the fiscal quarter ending December 31, 2022 through and including the fiscal quarter ending September 30, 2023 and then to 3.75 times beginning with the fiscal quarter ending December 31, 2023, and a minimum fixed charge coverage ratio of 1.25 times. As of September 30, 2021, the Company’s consolidated total leverage ratio (as defined in the 2021 Credit Facility) was 2.8 times. The 2021 Credit Facility requires customary mandatory prepayments of the term loan and revolver and cash collateralization of letters of credit, subject to customary exceptions, including 100% of the proceeds of debt not permitted by the 2021 Credit Facility, 100% of the proceeds of certain dispositions, subject to customary reinvestment rights, where applicable, and 100% of insurance or condemnation proceeds, subject to customary reinvestment rights, where applicable. The 2021 Credit Facility also includes customary events of default and related acceleration and termination rights. The weighted average interest rate on the 2021 Credit Facility as of September 30, 2021 was 2.1%. The Company’s obligations under the 2021 Credit Facility are guaranteed by certain of the Company’s existing and future direct and indirect subsidiaries, and such obligations are secured by substantially all of the Company’s assets, including the capital stock or other equity interests in those subsidiaries. 2020 Credit Facility — On April 13, 2020, the Company entered into a Unitranche Credit Agreement (the “2020 Credit Facility”) , which was paid in full in April 2021 via proceeds from the issuance of the 2021 Credit Facility, which consisted of a $225.0 million credit facility comprised of a $175.0 million term loan and a $50.0 million revolving credit facility. T he 2020 Credit Facility maturity date was April 2025. Up until October 6, 2020, the term loan and the revolver bore interest at LIBOR plus 5.0% with a 1.0% LIBOR floor or the base rate plus 4.0% and LIBOR plus 3.5% or the base rate plus 2.5% , respectively. Effective October 6, 2020, the Company amended the 2020 Credit Facility to provide for a reduction on the applicable interest rate on the term loan from LIBOR plus 5.0% with a 1.0% LIBOR floor to LIBOR plus 4.5% with a 1.0% LIBOR floor. The revolver interest rate remained unchanged. The revolver was also subject to an unused commitment fee of 0.35%. The Term Loan had quarterly repayments that started on September 30, 2020 of $0.5 million, increasing to $1.1 million on September 30, 2021 and further increasing to $1.6 million on September 30, 2022, with the remaining outstanding principal amount due on the maturity date. The 2020 Credit Facility contained financial covenants requiring the Company to remain below a maximum consolidated total leverage ratio of 4.25 times, which would have stepped down to 4.00 times beginning December 31, 2021 and then to 3.75 times beginning December 31, , and a minimum consolidated fixed charge coverage ratio of 1.25 times. As of December 31, 2020, the Company’s consolidated total leverage ratio (as defined in the 2020 Credit Facility) was 2.7 times. The resulting loss on extinguishment upon repayment of the 2020 Credit Facility amounted to $4.1 million, of which $1.0 million was related to fees paid and $3.1 related to unamortized debt issuance costs. Total loss on extinguishment is recorded in interest expense-net within the condensed consolidated statement of operations for the three and nine months ended September 30, 2021. Prior Senior Secured Credit Facility —The Company’s Senior Secured Credit Facility prior to the 2020 Credit Facility (the “Prior Senior Credit Facility”), which was paid in full in April 2020 via proceeds from the issuance of the 2020 Credit Facility, consisted of a $50.0 million term loan and a $130.0 million revolving credit facility. Borrowings under the Prior Senior Credit Facility bore interest at either (i) LIBOR plus the applicable margin or (ii) a base rate (equal to the highest of (a) the federal funds rate plus 0.5%, (b) Lender A’s prime rate and (c) Eurodollar Rate, which is based on LIBOR, (using a one-month period plus 1.0%), plus the applicable margin, as the Company elects. The applicable margin means a percentage per annum determined in accordance with the following table: Pricing Tier Consolidated Leverage Ratio Commitment Fee Eurodollar Rate Loans and LIBOR Letter of Credit Fee Daily Floating Rate Loans Rate Loans 1 > 3.75 to 1.0 0.50 % 4.00 % 4.00 % 3.00 % 2 ≤ 3.75 to 1.0 but > 3.00 to 1.0 0.50 3.50 3.50 2.50 3 ≤ 3.00 to 1.0 but > 2.25 to 1.0 0.40 3.00 3.00 2.00 4 < 2.25 to 1.0 0.30 2.50 2.50 1.50 Equipment Line of Credit —On March 12, 2019, the Company increased its equipment line of credit facility for the purchase of equipment and related freight, installation costs and taxes paid for an additional amount not to exceed $2.0 million. On May 16, 2019, the Company entered into a Canadian equipment line of credit facility for an amount not to exceed $1.0 million Canadian dollars. Interest on the line of credit is determined based on a three-year swap rate at the time of funding. The following is a schedule of the aggregate annual maturities of long-term debt presented on the unaudited condensed consolidated statement of financial position, based on the terms of the 2021 Credit Facility and equipment line of credit: September 30, 2022 $ 8,750 2023 10,938 2024 13,125 2025 15,313 2026 163,874 Total $ 212,000

Fair Value of Financial Instrum

Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	14. FAIR VALUE OF FINANCIAL INSTRUMENTS The following financial liabilities are measured at fair value on a recurring basis using significant unobservable inputs (Level 3): September 30, December 31, 2021 2020 Business acquisitions contingent consideration, current $ 31,152 $ 49,902 Business acquisitions contingent consideration, long-term 4,200 4,565 Conversion option 22,537 20,886 Total $ 57,889 $ 75,353 The estimated fair value amounts shown above are not necessarily indicative of the amounts that the Company would realize upon disposition, nor do they indicate the Company’s intent or ability to dispose of the financial instrument. The following table sets forth the Company’s financial instruments that were measured at fair value on a recurring basis: Compound Embedded Option Business Acquisitions Contingent Consideration, Current Business Acquisitions Contingent Consideration, Long-term Conversion Option Contingent Put Option Warrant Options Total Liabilities Balance—at January 1, 2020 $ — $ 8,614 $ 379 $ — $ 7,100 $ 16,878 $ 32,971 Series A-2 compound embedded option (9,361 ) — — — — — $ (9,361 ) Issuance of warrant option — — — — — 30,097 $ 30,097 Acquisitions — 34,661 10,543 — — — $ 45,204 Changes in fair value included in earnings 27,420 18,387 (1,000 ) — (19,240 ) 9,312 $ 34,879 Payment of contingent consideration payable — (12,464 ) — — — — $ (12,464 ) Foreign currency translation of contingent consideration payment — (208 ) — — — — $ (208 ) Reclass of long term to short term contingent liabilities — 180 (180 ) — — — $ — Write off of the contingent put option — — — — 12,140 — $ 12,140 Exercise of warrant options — — — — — (56,287 ) $ (56,287 ) Balance—at September 30, 2020 $ 18,059 $ 49,170 $ 9,742 $ — $ — $ — $ 76,971 . Balance—at January 1, 2021 $ — $ 49,902 $ 4,565 $ 20,886 $ — $ — $ 75,353 Acquisitions — 1,000 6,004 — — — 7,004 Changes in fair value included in earnings — 13,774 10,261 1,651 — — 25,686 Payment of contingent consideration payable — (50,154 ) — — — — (50,154 ) Reclass of long term to short term contingent liabilities 16,630 (16,630 ) — Balance—at September 30, 2021 $ — $ 31,152 $ 4,200 $ 22,537 $ — $ — $ 57,889 Quantitative Information about Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3): Compound Embedded Option —Prior to the Company’s IPO, the fair value of the compound embedded option associated with the issuance of the Convertible and Redeemable Series A-2 Preferred Stock (Note 17) was estimated using a “with-and-without” method. The “with-and-without” methodology involves valuing the whole instrument on an as-is basis and then valuing the instrument without the embedded derivative feature. The difference between the entire instrument with the embedded derivative feature compared to the instrument without the embedded derivative feature is the fair value of the derivative. The unobservable inputs were based on a 100.0% probability of an IPO event and IPO date. The considerable quantifiable inputs in the compound embedded option were: (i) the future value of the compound embedded option, (ii) the fair value of the Convertible and Redeemable Series A-2 Preferred Stock, (iii) the present value of the total instrument, as well as the present value of the compound embedded feature plus the fair value of the instrument, and (iv) the risk free, discount rates, conversion date and maximum conversion amounts. Business Acquisitions Contingent Consideration —The fair value of the contingent consideration payable associated with the acquisition of CTEH, MSE and Sensible was determined using a Monte Carlo simulation of earnings in a risk-neutral Geometric Brownian Motion framework. The fair values of the contingent consideration payables for the other acquisitions were calculated based on expected target achievement amounts, which are measured quarterly and then subsequently adjusted to actuals at the target measurement date. The method used to price these liabilities is considered level 3 due to the subjective nature of the unobservable inputs used to determine the fair value. The input is the expected achievement of earn-out thresholds. Conversion Option —Upon the Company’s IPO, the fair value of the conversion option associated with the issuance of the Convertible and Redeemable Series A-2 Preferred Stock (Note 17) was estimated using a “with-and-without” method. The “with-and-without” methodology considers the value of the security on an as-is basis and then without the embedded conversion premium. The difference between the two scenarios is the implied fair value of the embedded derivative. The unobservable input is the required rate of return on the Series A-2. The considerable quantifiable inputs in the valuation relate to the timing of conversions or redemptions. Contingent Put Option —The fair value of the contingent put option associated with the issuance of the Redeemable Series A-1 Preferred Stock was estimated using a “with-and-without” method. The “with-and-without” methodology considers the value of the security on an as-is basis and then without the embedded contingent put option. The difference between the two scenarios is the implied fair value of the embedded derivative, recorded as the contingent put option liability. In this case the Series A-1 was redeemed on the date of value so the value of the “with” scenario is known. The unobservable input is the required rate of return on the Series A-1 through to maturity in the “without” scenario. The contingent put option was redeemed in July 2020 (Note 16). Warrant Options — The warrant options were exercised on July 30, 2020 (Note 12). The fair value of the warrant option associated with the issuance of the Redeemable Series A-1 Preferred Stock was calculated based on the Black-Sholes pricing model using the following assumptions: July 30, 2020 Common stock value (per share) $ 22.22 Expected volatility 44.35 % Risk-free interest rate 0.55 % Expected life (years) 10 As of July 30, 2020, the date the warrant options were exercised in full, the fair value of the warrant option associated with the issuance of the Convertible and Redeemable Series A-2 Preferred Stock (Note 17) was calculated based on the Black-Scholes pricing model with assumptions for (i) stock price, (ii) volatility based on the median historical volatility of publicly listed comparable companies’ stock price returns, (iii) risk-free rates based on U.S. treasury yields and (iv) dividend yield. The method used to price these liabilities is considered Level 3 due to the subjective nature of the unobservable inputs (common stock value and expected volatility) used to determine the fair value.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2021
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	15. COMMITMENTS AND CONTINGENCIES Leases —The Company leases office facilities over various terms expiring through 2030. Certain of these operating leases contain rent escalation clauses. The Company also has office equipment leases that expire through 2026 Other Commitments —The Company has commitments under the 2021 Credit Facility, its equipment line of credit and its lease obligations (Note 13 and 6). Contingencies —The Company is subject to purchase price contingencies related to earn-outs associated with certain acquisitions (Note 7). Legal —In the normal course of business, the Company is at times subject to pending and threatened legal actions. In management’s opinion, any potential loss resulting from the resolution of these matters is not expected to have a material effect on the unaudited condensed consolidated results of operations, financial position or cash flows of the Company.

Redeemable Series A-1 Preferred

Redeemable Series A-1 Preferred Stock	9 Months Ended
	Sep. 30, 2021
Temporary Equity Disclosure [Abstract]
Redeemable Series A-1 Preferred Stock	16. REDEEMABLE SERIES A-1 PREFERRED STOCK On October 19, 2018, the Company issued 12,000 shares of Redeemable Series A-1 Preferred Stock with a par value of $0.0001 per share and a detachable warrant to purchase 534,240 shares of the Company’s common stock. Each preferred share was issued as part of a unit, which consisted of one share of the Redeemable Series A-1 Preferred Stock at $0.01 million per share. On April 13, 2020, the Company amended and restated the certificate of designation of the Company’s Redeemable Series A-1 Preferred Stock. The most significant changes in the amendment included (i) the Redeemable Series A-1 Preferred Stock became pari passu with the Convertible and Redeemable Series A-2 Preferred Stock (Note 17), (ii) the maturity was extended to October 2024; (iii) the Company could use up to $50.0 million of indebtedness or cash on hand to redeem the Redeemable Series A-1 Preferred Stock, and (iv) upon an IPO, up to 50.0% of accumulated dividends could be paid in shares of common stock and (v) the Company could elect to reduce the three year make whole penalty to a two year make whole penalty if the warrant issued in connection with the issuance of the Redeemable Series A-1 Preferred Stock was redeemed in full at a share price of no less than $31.60. Following a partial redemption of outstanding Redeemable Series A-1 Preferred Stock, the dividend rate of the remaining Redeemable Series A-1 Preferred Stock would be reduced proportionally (between 15.0% and 9.0%) in relation to the proportion of Redeemable Series A-1 Preferred Stock redeemed, with the rate increasing by an additional 1.0% for dividends that are accrued versus paid in cash. Based on a qualitative assessment performed by the Company, the Redeemable Series A-1 Preferred Stock amendments did not represent a significant long-term change to the original terms of the instrument and, therefore, there was no change in the accounting of the instrument. On July 27, 2020, the Company redeemed in full the Redeemable Series A-1 Preferred Stock, including the guaranteed minimum two-year dividend. The Redeemable Series A-1 Preferred Stock contained restrictive covenants. Prior to its redemption, the Company was subject to a consolidated total leverage ratio (including the outstanding principal and accrued dividend on the Redeemable Series A-1 Preferred Stock) limit of less than 10.0 times as of the end of any fiscal quarter ending until maturity. Before redemption, the Redeemable Series A-1 Preferred Stock accrued dividends quarterly at an annual rate of 15.0% with respect to dividends that were paid in cash and at an annual rate of 14.2% with respect to dividends that were accrued. At issuance the Company determined that the detachable warrant (Note 12) and the contingent put option were required to be accounted for separately. The contingent put option change in value of $26.3 million and $19.2 million for the three and nine months ended September 30, 2020 was recorded to other expense.

Convertible and Redeemable Seri

Convertible and Redeemable Series A-2 Preferred Stock	9 Months Ended
	Sep. 30, 2021
Temporary Equity Disclosure [Abstract]
Convertible and Redeemable Series A-2 Preferred Stock	17. CONVERTIBLE AND REDEEMABLE SERIES A-2 PREFERRED STOCK On April 13, 2020, the Company entered into an agreement to issue 17,500 shares of the Convertible and Redeemable Series A-2 Preferred Stock with a par value of $0.0001 per share and a detachable warrant to purchase shares of the Company’s common stock with a 10-year life, in exchange for gross proceeds of $175.0 million, net of $1.3 million debt issuance costs. Before the Company’s IPO, each share of Convertible and Redeemable Series A-2 Preferred Stock accrued dividends at the rate of 15.0% per annum, with respect to dividends that were paid in cash, and 14.2% per annum, with respect to dividends that were accrued. The Company paid dividends on shares of the Convertible and Redeemable Series A-2 Preferred Stock of $4.1 million and $12.3 million during the three and nine months ended September 30, 2021, respectively. During the three and nine months ended September 30, 2020, the Company paid dividends on shares of the Convertible and Redeemable Series A-2 Preferred Stock of $2.9 million. At issuance, the Company determined that Convertible and Redeemable Series A-2 Preferred Stock and the detachable warrant (Note 12 ), were required to be accounted for separately. Upon the Company’s IPO, following which the Redeemable Series A-1 Preferred Stock was fully redeemed, the Convertible and Redeemable Series A-2 Preferred Stock terms automatically updated to the following: (i) no mandatory redemption, (ii) no stated value cash repayment obligation other than in the event of certain defined liquidation events, (iii) only redeemable at the Company’s option, (iv) the instrument became convertible into common stock beginning on the four year anniversary of issuance at a 15.0% discount to the common stock market price (with a limit of $60.0 million in stated value of Convertible and Redeemable Series A-2 Preferred Stock eligible to be converted in any 60-day period prior to the seventh anniversary of issuance and the amount of stated value of the Convertible and Redeemable Series A-2 Preferred Stock eligible for conversion limited to $60.0 million during year 5 and $120.0 million (which includes the aggregate amount of the stated value of the Convertible and Redeemable Series A-2 Preferred Stock and any accrued but unpaid dividends added to such stated value of any shares of Convertible and Redeemable Series A-2 Preferred Stock converted in year 5) during year 6), (v) the dividend rate stepped down to 9.0% per year with required quarterly cash payments, (vi) in an event of noncompliance, the dividend rate shall increase to 12.0% per annum for the first 90-day period from and including the date the noncompliance event occurred, and thereafter shall increase to 14.0% per annum, (vii) the debt incurrence test ratio increased to 4.5 times, (viii) the total leverage cap covenant was removed, and (ix) minimum repayment amount dropped down from $50.0 million to $25.0 million. The Company may, at its option on any one or more dates, redeem all or a minimum portion (the lesser of (i) $25.0 million in aggregate stated value of the Convertible and Redeemable Series A-2 Preferred Stock and (ii) all of the Convertible and Redeemable Series A-2 Preferred Stock then outstanding) of the outstanding Convertible and Redeemable Series A-2 Preferred Stock in cash. With respect to any redemption of any share of the Convertible and Redeemable Series A-2 Preferred Stock prior to the third-year anniversary, the Company is subject to a make whole penalty in which the holders of the Convertible and Redeemable Series A-2 Preferred Stock are guaranteed a minimum repayment equal to outstanding redeemed stated value plus three years of dividends accrued or accruable thereon. The Convertible and Redeemable Series A-2 Preferred Stock does not meet the definition of a liability pursuant to “ASC 480- Distinguishing Liabilities from Equity.” However, as (i) the instrument is redeemable upon a change of control as defined in the certificate of designations governing the terms of the Convertible and Redeemable Series A-2 Preferred Stock, and (ii) the Company cannot assert it would have sufficient authorized and unissued shares of common stock to settle all future conversion requests due to the variable conversion terms, the instrument is redeemable upon the occurrence of events that are not solely within the control of the Company, and therefore the Company classifies the Convertible and Redeemable Series A-2 Preferred Stock as mezzanine equity. Subsequent adjustment of the carrying value of the instrument is required if the instrument is probable of becoming redeemable. As of September 30, 2021, the Company has determined that a change of control is not probable. Additionally, as of September 30, 2021, the Company has determined that it is not probable that there will be a future conversion request that the Company is unable to settle with authorized and issued shares based on the Company’s current stock price and available shares as well as the Company’s monitoring efforts to ensure there are a sufficient number of shares available to settle any conversion request. Therefore, as of September 30, 2021, the Company has determined that the instrument is not probable of becoming redeemable, and does not believe subsequent adjustment of the carrying value of the instrument will be necessary. The Convertible and Redeemable Series A-2 Preferred Stock had an aggregate liquidation preference of $182.2 million as of September 30, 2021. The Convertible and Redeemable Series A-2 Preferred Stock contains embedded features that are required to be bifurcated and are subject to separate accounting treatment from the instrument itself. At issuance, these embedded features consisted of (i) a contingent dividend feature associated with the decrease in the dividend rate upon an IPO and (ii) a conversion option of the preferred shares to shares of common stock beginning on the fourth-year anniversary of the issuance date. Upon the Company’s IPO, the embedded derivative only consisted of the conversion option. As of September 30, 2021, this conversion embedded feature had a net fair value of $22.5 million. The change in value of $0.5 million and $1.7 million for the three and nine months ended September 30, 2021 was recorded to other expense. The Convertible and Redeemable Series A-2 Preferred Stock was not convertible into common stock until completion of the IPO in July 2020.

Stockholder's Equity (Deficit)

Stockholder's Equity (Deficit)	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Stockholders' Equity (Deficit)	18. STOCKHOLDERS’ EQUITY (DEFICIT) Authorized Capital Stock— The Company was authorized to issue 190,000,000 shares of common stock, with a par value of $0.000004 per share as of September 30, 2021 and December 31, 2020. Warrants— In May 2015, the Company issued warrants to acquire 116,350 shares of Common Stock at a price of approximately $17.19 per share to the placement agent as consideration for backstopping the financing completed in May 2015. These warrants were exercised in full as a cashless transaction during the first quarter of 2021. As a result of this cashless transaction, the resulting number of shares issued was 67,713 shares. Common Stock Issuances —The Company issued the following shares of common stock: Three Months Ended September 30, 2021 2020 Shares Average Price per Share Total Consideration (in thousands) Shares Average Price per Share Total Consideration (in thousands) Common stock issued in connection initial public offering — $ — $ — 11,500,000 $ 15.00 $ 172,500 Acquisitions 62,738 $ 52.18 3,274 — — — Redemption of series A-1 preferred stock — — — 1,786,739 15.00 26,801 Exercise of warrant options — — — 2,534,239 0.01 25 Exercise of options 354,918 8.30 2,946 16,850 8.92 150 Restricted shares, net — — — 8,333 24.00 — Cancellation of shares — — — (117,785 ) 17.15 — Total 417,656 $ 14.89 $ 6,220 15,728,376 $ 12.82 $ 199,476 Nine Months Ended September 30, 2021 2020 Shares Average Price per Share Total Consideration (in thousands) Shares Average Price per Share Total Consideration (in thousands) Common stock issued in connection initial public offering — $ — $ — 11,500,000 $ 15.00 $ 172,500 Acquisitions 143,800 41.86 6,020 791,139 31.60 25,000 Redemption of series A-1 preferred stock — — — 1,786,739 15.00 26,801 Exercise of warrant options 67,713 17.19 — 2,534,239 0.01 25 Exercise of options 779,068 7.74 6,032 20,350 9.27 189 Restricted shares, net 38,929 31.31 — 8,333 24.00 — Payment of earn-out liability 539,607 46.33 25,000 — — — Payment of purchase price true up 24,200 44.81 1,084 — — — Cancellation of shares — — — (117,785 ) 17.15 — Total 1,593,317 $ 25.43 $ 38,136 16,523,015 $ 13.72 $ 224,515 Employee Equity Incentive Plans —The Company has two plans under which stock-based awards have been issued: (i) the Montrose Amended & Restated 2017 Stock Incentive Plan (“2017 Plan”) and (ii) the Montrose Amended & Restated 2013 Stock Option Plan (“2013 Plan”) (collectively the “Plans”). As of September 30, 2021, and September 30, 2020 , there was $ million and $ 10.4 million, respectively, of total unrecognized stock compensation expense related to unvested options and restricted stock granted under the Plans. Such unrecognized expense is expected to be recognized over a weighted-average three year period . The following number of shares were authorized to be issued and available for grant: September 30, 2021 2017 Plan 2013 Plan Total Shares authorized to be issued 3,944,750 2,047,269 5,992,019 Shares available for grant 1,593,287 — 1,593,287 September 30, 2020 2017 Plan 2013 Plan Total Shares authorized to be issued 2,945,443 2,047,269 4,992,712 Shares available for grant 1,233,266 — 1,233,266 Total stock compensation expense for the Plans was as follows: Three Months Ended September 30, 2021 2020 2017 plan 2013 plan 2017 plan 2013 plan Options Restricted Stock Options Total Options Restricted Stock Options Total Cost of revenue $ 282 $ — $ — $ 282 $ 276 $ — $ (5 ) $ 271 Selling, general and administrative expense 1,844 237 2 2,083 592 233 53 878 Total $ 2,126 $ 237 $ 2 $ 2,365 $ 868 $ 233 $ 48 $ 1,149 Nine Months Ended September 30, 2021 2020 2017 plan 2013 plan 2017 plan 2013 plan Options Restricted Stock Options Total Options Restricted Stock Options Total Cost of revenue $ 1,178 $ — $ 10 $ 1,188 $ 973 $ — $ 126 $ 1,099 Selling, general and administrative expense 4,638 749 11 5,399 1,178 973 189 2,340 Total $ 5,816 $ 749 $ 21 $ 6,587 $ 2,151 $ 973 $ 315 $ 3,439 Montrose Amended & Restated 2017 Stock Incentive Plan Restricted Stock— The Company issues restricted stock to certain 2017 Plan participants as Director’s compensation. These shares of restricted stock granted in the nine months ended September 30, 2021 and September 30, 2020 vest one year from the date of grant, or, in each case, in full upon a change in control, subject to the participant’s continued service as a Director throughout such date, or upon retirement. Members of the Board of Directors that receive stock-based compensation are treated as employees for accounting purposes. Restricted stock activity was as follows: Nine Months Ended September 30, 2021 2020 Shares Average Price per Share Total (in thousands) Shares Average Price per Share Total (in thousands) Shares granted 19,066 $ 36.99 $ 705 33,229 $ 31.60 $ 1,050 There were no forfeitures of restricted shares during the three and nine months ended September 30, 2021. There were zero and 1,667 forfeitures of restricted shares during the three and nine months ended September 30, 2020. There were zero and 38,929 shares of restricted stock that became fully vested and were released as unrestricted shares of common stock during the three and nine months ended September 30, 2021, respectively. There were zero and 10,000 shares of restricted stock that became fully vested and were released as unrestricted shares of common stock during the three and nine months ended September 30, 2020, respectively. There was an aggregate of 286,239 and 267,173 restricted shares outstanding as of September 30, 2021 and September 30, 2020, respectively. Options —Options issued to all optionees under the 2017 Plan vest over four years from the date of issuance (or earlier vesting start date, as determined by the Board of Directors) as follows: one half half Options to Purchase Common Stock Weighted- Average Exercise Price per Share Weighted Average Grant Date Fair Value per Share Weighted Average Remaining Contract Life (in Years) Aggregate Intrinsic Value of In-The- Money Options (in Thousands) Outstanding at January 1, 2020 617,852 $ 24 $ 12 7.82 $ 4,693 Granted 851,877 32 12 — — Forfeited/ cancelled (14,375 ) 22 — — — Exercised (925 ) 14 — — 7,918 Outstanding at September 30, 2020 1,454,429 21 12 7.59 7,649 Outstanding at January 1, 2021 1,840,229 23 12 9.09 15,598 Granted 264,470 40 22 — — Forfeited/ cancelled (29,025 ) 28 — — — Expired (1,250 ) 18 — — — Exercised (66,875 ) 22 — — 2,078 Outstanding at September 30, 2021 2,007,549 25 14 8.53 73,724 Exercisable at September 30, 2021 320,861 28 — 7.68 10,755 Options vested and expected to vest 2,007,549 25 — 8.53 73,724 The following weighted-average assumptions were used in the Black-Sholes option-pricing model calculation: September 30, September 30, 2021 2020 Common stock value (per share) $ 39.83 $ 16.36 Expected volatility 58.34 46.14 Risk-free interest rate 0.77 0.41 Expected life (years) 5.5-7.0 7.0 Forfeiture rate None None Dividend rate None None Montrose Amended & Restated 2013 Stock Option Plan —The following summarizes the activity of the 2013 Plan: Options to Purchase Common Stock Weighted- Average Exercise Price per Share Weighted Average Grant Date Fair Value per Share Weighted Average Remaining Contract Life (in Years) Aggregate Intrinsic Value of In-The- Money Options (in Thousands) Outstanding at January 1, 2020 1,855,469 $ 6 $ 1 6.40 46,617 Forfeited/ cancelled (7,500 ) 10 — — — Expired (11,300 ) 6 — — — Exercised (19,175 ) 8 — — 354 Outstanding at September 30, 2020 1,817,494 6 1 5.63 33,772 Outstanding at January 1, 2021 1,787,869 6 1 5.40 43,867 Expired (625 ) 6 — — — Exercised (710,568 ) 6 — — 33,480 Outstanding at September 30, 2021 1,076,676 6 2 4.65 59,532 Exercisable at September 30, 2021 1,076,676 6 — 4.65 59,532 Options vested and expected to vest 1,076,676 6 — 4.65 59,532 Total shares outstanding from exercised options were 1,028,268 shares and 221,950 shares as of September 30, 2021 and September 30, 2020, respectively. Common Stock Reserved for Future Issuances —The Company has reserved certain stock of its authorized but unissued common stock for possible future issuance in connection with the following: September 30, 2021 2020 Warrants — 116,350 Montrose 2013 Stock Incentive Plan 2,047,269 2,047,269 Montrose 2017 Stock Incentive Plan 3,944,750 2,945,443 Common stock reserved for future issuance 5,992,019 5,109,062

Net Income (Loss) Per Share

Net Income (Loss) Per Share	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Net Income (Loss) Per Share	19. NET INCOME (LOSS) PER SHARE Basic net income (loss) per share is computed by dividing net income (loss) attributable to common stockholders by the weighted average number of common shares outstanding during each period. The Redeemable Series A-1 Preferred Stock, which was outstanding prior to its redemption on July 27, 2020, and the Convertible and Redeemable Series A-2 Preferred Stock are considered a participating security during the applicable period. Net losses are not allocated to the Redeemable Series A-1 Preferred stockholders nor the Convertible and Redeemable Series A-2 stockholders, as they were not contractually obligated to share in the Company’s losses. Diluted net income (loss) per share is computed by dividing net (income) loss attributable to common stockholders by the weighted average number of common and dilutive common equivalent shares outstanding for the period using the treasury-stock method or the as-converted method. During the three and nine months ended September 30, 2020, shares issuable in connection with the warrant options (Note 12) were considered outstanding common shares for purposes of calculating net (loss) income per share since they did not contain any conditions that must be satisfied for the holder to exercise the warrant. Potentially dilutive shares are comprised of restricted stock and shares of common stock underlying stock options outstanding under the Plans and warrants (other than warrant options) to purchase common stock. During the nine months ended September 30, 2021 and the three and nine months ended September 30, 2020, there is no difference in the number of shares used to calculate basic and diluted shares outstanding due to the Company’s net loss and potentially dilutive shares being anti-dilutive. The following table summarizes the computation of basic and diluted net (loss) income per share attributable to common stockholders of the Company: Three Months Ended Nine Months Ended September 30, September 30, (In thousands, except for net loss per share) 2021 2020 2021 2020 Net income (loss) $ 2,226 $ (30,737 ) $ (23,853 ) $ (58,761 ) Accretion of redeemable series A-1 preferred stock — (6,542 ) — (17,601 ) Redeemable series A-1 preferred stock deemed dividend (24,341 ) (24,341 ) Convertible and redeemable series A-2 preferred stock dividend (4,100 ) (2,870 ) (12,300 ) (2,870 ) Net loss attributable to common stockholders –basic and diluted (1,874 ) (64,490 ) (36,153 ) (103,573 ) Weighted-average common shares outstanding – basic and diluted 26,220 21,554 25,798 13,669 Net loss per share attributable to common stockholders –basic and diluted $ (0.07 ) $ (2.99 ) $ (1.40 ) $ (7.58 ) The following equity shares were excluded from the calculation of diluted net income (loss) per share attributable to common stockholders because their effect would have been anti-dilutive: September 30, 2021 2020 Stock options 1,686,688 2,423,900 Restricted stock 28,623 176,384 Warrants — 116,350

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2021
Segment Reporting [Abstract]
Segment Information	20. SEGMENT INFORMATION The Company has three operating and reportable segments: Assessment, Permitting and Response, Measurement and Analysis and Remediation and Reuse. These segments are monitored separately by management for performance against budget and prior year and are consistent with internal financial reporting. The Company’s operating segments are organized based upon primary services provided, the nature of the production process, their type of customers, methods used to distribute the products and the nature of the regulatory environment. Segment Adjusted EBITDA is the primary measure of operating performance for all three operating segments. Segment Adjusted EBITDA is the calculated Company’s Earnings before Interest, Tax, Depreciation and Amortization (“EBITDA”), adjusted to exclude certain transactions such as stock-based compensation, acquisition costs and fair value changes in financial instruments, amongst others. The Chief Operating Decision Maker (“CODM”) does not review segment assets as a measure of segment performance. Corporate and Other includes costs associated with general corporate overhead (including executive, legal, finance, safety, human resources, marketing and IT related costs) that are not directly related to supporting operations. Overhead costs that are directly related to supporting operations (such as insurance, software, licenses, shared services and payroll processing costs) are allocated to the operating segments on a basis that reasonably approximates an estimate of the use of these services. Segment revenues and Adjusted EBITDA consisted of the following: Three Months Ended September 30, 2021 2020 Segment Segment Segment Adjusted Segment Adjusted Revenues EBITDA Revenues EBITDA Assessment, Permitting and Response $ 63,412 $ 15,719 $ 26,593 $ 8,230 Measurement and Analysis 38,752 8,283 39,770 11,226 Remediation and Reuse 30,414 5,472 18,342 2,012 Total Operating Segments 132,578 29,474 84,705 21,468 Corporate and Other — (7,987 ) — (4,762 ) Total $ 132,578 $ 21,487 $ 84,705 $ 16,706 Nine Months Ended September 30, 2021 2020 Segment Segment Segment Adjusted Segment Adjusted Revenues EBITDA Revenues EBITDA Assessment, Permitting and Response $ 209,379 $ 46,379 $ 49,768 $ 14,665 Measurement and Analysis 111,309 22,634 113,130 30,422 Remediation and Reuse 81,931 12,262 56,604 6,495 Total Operating Segments 402,619 81,275 219,502 51,582 Corporate and Other — (22,026 ) — (15,425 ) Total $ 402,619 $ 59,249 $ 219,502 $ 36,157 Presented below is a reconciliation of the Company’s segment measure to income (loss) before (expense) benefit from income taxes: For the Three Months Ended September 30, For the Nine Months Ended September 30, 2021 2020 2021 2020 Total $ 21,487 $ 16,706 $ 59,249 $ 36,157 Interest expense, net (1,722 ) (3,043 ) (11,208 ) (10,896 ) Income tax (expense) benefit (902 ) (3,348 ) (648 ) 1,563 Depreciation and amortization (11,471 ) (9,740 ) (33,145 ) (27,084 ) Stock-based compensation (2,365 ) (1,149 ) (6,587 ) (3,439 ) Start-up losses and investment in new services (1,186 ) (602 ) (3,276 ) (1,283 ) Acquisition costs (913 ) (6 ) (1,656 ) (3,767 ) Fair value changes in financial instruments (531 ) (9,710 ) (1,651 ) (17,492 ) Fair value changes in business acquisitions contingent consideration — (13,404 ) (24,035 ) (17,387 ) Short term purchase accounting fair value adjustment to deferred revenue — — — (243 ) Initial public offering expense and related bonuses — (6,378 ) — (6,908 ) Discontinued services ( i ) — (30 ) — (7,526 ) Expenses related to financing transactions — — (50 ) (277 ) Other losses or expenses (171 ) (33 ) (846 ) (179 ) Net income (loss) $ 2,226 $ (30,737 ) $ (23,853 ) $ (58,761 ) (i) During the first quarter of 2020, the Company determined to reduce the footprint of its environmental lab in Berkeley, California, and to exit its non-specialized municipal water engineering service line and its food waste biogas engineering service line. As a part of discontinuing service lines, the Company made the decision to book an additional bad debt reserve related to the uncertainty around the ability to collect on receivables related to these service lines (Note 3). It was determined that the discontinuation of these service lines did not represent a strategic shift that had (or will have) a major effect on the Company’s operations and financial results therefore did not meet the requirements to be classified as discontinued operations.

Related-Party Transactions

Related-Party Transactions	9 Months Ended
	Sep. 30, 2021
Related Party Transactions [Abstract]
Related-Party Transactions	21. RELATED-PARTY TRANSACTIONS The Company engaged a related party to provide Quality of Earnings reports on acquisition targets. The Company paid this related party zero during the three and nine months ended September 30, 2021 and zero and $0.1 September 30, 2020, respectively The Company ceased using the services of this related party during 2020. During the year ended December 31, 2020, the holder of the Redeemable Series A-1 Preferred Stock and Convertible and Redeemable A-2 Preferred Stock became a stockholder in the Company. On the redemption date of the Redeemable Series A-1 Preferred Stock (Note 16), the Company issued 1,786,739 shares of common stock as dividend payment. Additionally, this related party exercised its warrant options (Note 12), becoming the holder of 2,534,239 additional common shares. To the Company’s knowledge, the related party has sold all of such shares of its Company common stock.

Defined Contribution Plan

Defined Contribution Plan	9 Months Ended
	Sep. 30, 2021
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	22. DEFINED CONTRIBUTION PLAN On January 1, 2014, the Company established the Montrose Environmental Group 401(k) Savings Plan (the “401(k) Savings Plan”). As of September 30, 2021, and December 31, 2020, plan participants may defer up to 85.0% of their eligible wages for the year, up to the Internal Revenue Service dollar limit and catch-up contribution allowed by law. Prior to May 22, 2020, the Company provided employer matching contributions equal to 100.0% of the first 3.0% of the participant’s compensation and 50.0% of the participant’s elective deferrals that exceed 3.0% but do not exceed 4.0% of the participant’s compensation. Beginning on May 22, 2020, the Company temporarily ceased making employer contributions. Employer contributions were reinstated beginning on April 23, 2021. Employer contributions under the 401(k) Savings Plan were $0.8 million and $1.6 million for the three and nine months ended September 30, 2021, respectively, and zero and $1.2 million for the three and nine months ended September 30, 2020, respectively,

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2021
Subsequent Events [Abstract]
Subsequent Events	23. SUBSEQUENT EVENTS Follow-on Offering— In October 2021, the Company completed a follow-on offering of common stock, in which it sold 2,500,000 shares at a public offering price of $62.00 per share less underwriting discounts. In addition, the Company granted the underwriters a 30-day option to purchase up to an additional 375,000 shares of common stock on the same terms and conditions (the “Option”). On October 5, 2021, the underwriters exercised the Option in full. Net proceeds to the Company from the follow-on offering, including the Option, were approximately $169.8 million Business Acquisitions —On October 1, 2021, t he Company completed the business acquisition of Environmental Chemistry, Inc. (“ECI”) by acquiring 100.0% of its common stock. ECI provides a full suite of environmental laboratory analytical services to industrial, governmental, and engineering/consulting clients. Combined with the Company’s existing Houston, TX laboratory, ECI (located also in Houston, TX) will enable Montrose to provide air, water and soil analytical services in the gulf coast region. On November 1, 2021, t he Company completed the business acquisition of Horizon Water and Environment, LLC (“Horizon”) by acquiring 100.0% of its membership interests. Horizon is an environmental consulting firm specializing in planning, watershed science, and environmental compliance for water and natural resource projects in California. Horizon will join Montrose’s Planning & Ecosystem Consulting segment These transactions qualified as an acquisition of a business and will be accounted for as a business combination. The following table summarizes the elements of the purchase price of these acquisitions: Other Purchase Total Cash (1) Common Stock (2) Price Component (3) Purchase Price ECI $ 4,000 — — $ 4,000 Horizon 10,580 2,300 — 12,880 (1) The cash portion of these acquisitions’ purchase price was funded through cash on hand. (2) The common stock component of Horizon was paid through the issuance of 34,921 shares of common stock. (3) The other purchase price components for ECI consist of a potential working capital target payment and a potential earnout amount, which are not finalized at this time. The other purchase price component of Horizon consists of a potential working capital target payment, which is not finalized at this time. The Company has not yet completed the initial purchase price allocation for these acquisitions, including obtaining all of the information required for the valuation of the acquired intangible assets, goodwill, assets and liabilities assumed, due to the timing of the close of the transaction.

Description of the Business a_2

Description of the Business and Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation —The unaudited condensed consolidated financial statements include the operations of the Company and its wholly-owned subsidiaries. These unaudited condensed consolidated financial statements are presented in conformity with accounting principles generally accepted in the United States (“U.S. GAAP”) and have been prepared pursuant to the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”) that permit reduced disclosure for interim periods. The unaudited condensed consolidated financial statements include all accounts of the Company and, in the opinion of management, include all recurring adjustments and normal accruals necessary for a fair statement of the Company’s financial position, results of operations and cash flows for the dates and periods presented. These unaudited condensed consolidated financial statements should be read in conjunction with the Company’s audited financial statements as of and for the year ended December 31, 2020. Results for interim periods are not necessarily indicative of the results to be expected during the remainder of the current year or for any future period. All intercompany transactions, accounts and profits, have been eliminated in the unaudited condensed consolidated financial statements.
Recently Adopted Accounting Pronouncements /Recently Issued Accounting Pronouncements Not Yet Adopted	Recently Adopted Accounting Pronouncements —The Company qualifies as an emerging growth company as defined in the Jumpstart Our Business Startups Act of 2012 (“JOBS Act”) and therefore intends to take advantage of certain exemptions from various public company reporting requirements, including delaying adoption of new or revised accounting standards until those standards apply to private companies. The Company has elected to use this extended transition period under the JOBS Act. The effective dates shown below reflect the election to use the extended transition period. However, as June 30, 2021, the end of the Company’s second fiscal quarter, the market value of the Company’s common stock held by non-affiliates exceeded $700.0 million and, as a result, the Company will no longer qualify as an emerging growth company at the end of the fiscal year ended December 31, 2021. The adoption dates discussed below are based on the Company no longer qualifying as an emerging growth company. In December 2019, the Financial Accounting Standards Board (“FASB”) issued Accounting Standard Update (“ASU”) 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. ASU 2019-12 removes certain exceptions to the general principles in Accounting Standard Codification (“ASC”) 740 and clarifies and amends certain guidance to promote consistent application. The standard was adopted as of January 1, 2021 and did not have a material impact on the Company’s unaudited condensed consolidated financial statements. In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation: Improvements to Nonemployee Share-Based Payment Accounting. Under the revised guidance, the accounting for awards issued to non-employees will be similar to the accounting for employee awards. The new guidance is effective for fiscal years beginning after December 15, 2019. The standard was adopted as of January 1, 2020 and did not have a material impact on the Company’s unaudited condensed consolidated financial statements. In January 2017, the FASB issued ASU 2017-04, Intangibles—Goodwill and Other (Topic 350): Simplifying the Accounting for Goodwill Impairment. The revised guidance eliminates Step 2 of the current goodwill impairment analysis test, which requires hypothetical purchase price allocation to measure goodwill impairment. A goodwill impairment loss will instead be measured at the amount by which a reporting unit’s carrying amount exceeds its fair value, not to exceed the carrying amount of goodwill. The revised guidance was adopted as of January 1, 2020 and did not have a material impact on the Company’s unaudited condensed consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), to improve financial reporting regarding leasing transactions. In June 2021, with an effective adoption date of January 1, 2021, the Company adopted ASU No. 2016-02, using the modified retrospective transition approach with a cumulative effect adjustment to the income statement as of the date of adoption (Note 6). As a result of electing the modified retrospective approach, the Company is not required to restate comparative periods for the effects of ASC 842. The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification determines whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability, regardless of lease classification. The Company elected the package of practical expedients, which allows it not to have to reassess previous conclusions about lease identification, lease classification and initial direct costs. In addition, the Company elected to not record operating lease right-of-use assets or operating lease liabilities for leases with an initial term of 12 months or less and elected the practical expedient to not separate lease and non-lease components for all of its leases. As of adoption date, the Company recognized operating lease right-of-use assets, current operating lease liabilities and operating lease liabilities, net of current portion of $24.6 million, $7.3 million and $17.3 million, respectively. The Company recognized finance lease right-of-use assets, current finance lease liabilities and finance lease liabilities, net of current portion of $7.2 million, $2.9 million and $4.6 million, respectively. Recently Issued Accounting Pronouncements Not Yet Adopted —In August 2020, the FASB issued ASU 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40)—Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity. The ASU simplifies accounting for convertible instruments by removing major separation models required under current U.S. GAAP. Consequently, more convertible debt instruments will be reported as a single liability instrument with no separate accounting for embedded conversion features. The ASU removes certain settlement conditions that are required for equity contracts to qualify for the derivative scope exception, which will permit more equity contracts to qualify for the exception. The ASU also simplifies the diluted net income per share calculation in certain areas. The new guidance is effective for fiscal years beginning after December 15, 2021, including interim periods within those fiscal years, and early adoption is permitted. The Company is currently evaluating the impact of the adoption of the standard on the unaudited condensed consolidated financial statements. In March 2020, the FASB issued ASU 2020-04, Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting. ASU 2020-04 provides optional expedients and exceptions for applying U.S. GAAP to contracts, hedging relationships and other transactions affected by the expected transition away from reference rates that are expected to be discontinued, such as LIBOR. ASU 2020-04 was effective upon issuance. The Company may elect to apply the guidance prospectively through December 31, 2022. The Company is currently evaluating the impact of the adoption of the standard on the unaudited condensed consolidated financial statements. In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326). The standard introduces a new model for recognizing credit losses on financial instruments based on an estimate of current expected credit losses and will apply to trade receivables. The new guidance will be effective for the Company’s annual and interim periods beginning after December 15, 2021. The Company does not anticipate that the adoption of this standard will have a material impact on the unaudited condensed consolidated financial statements.

Revenues and Accounts Receiva_2

Revenues and Accounts Receivable (Tables)	9 Months Ended
	Sep. 30, 2021
Revenues And Accounts Receivable [Abstract]
Schedule of Contract Balances	The following table presents the Company’s contract balances: September 30, December 31, 2021 2020 Contract assets $ 46,270 $ 38,576 Contract liabilities 8,037 6,114
Schedule of Accounts Receivable, Net	Accounts receivable, net consisted of the following: September 30, December 31, 2021 2020 Accounts receivable, invoiced $ 68,974 $ 57,228 Accounts receivable, other 1,876 1,139 Allowance for doubtful accounts (4,379 ) (4,265 ) Accounts receivable—net $ 66,471 $ 54,102
Schedule of Allowance for Doubtful Accounts	The allowance for doubtful accounts consisted of the following: Beginning Balance Bad Debt Expense Charged to Allowance Other ( 1) Ending Balance Nine months ended September 30, 2021 $ 4,265 $ 803 $ (1,229 ) $ 540 $ 4,379 Year ended December 31, 2020 1,327 4,532 (2,633 ) 1,039 4,265 ____________________ (1) This amount consists of additions to the allowance due to business acquisitions.

Prepaid and Other Current Ass_2

Prepaid and Other Current Assets (Tables)	9 Months Ended
	Sep. 30, 2021
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Schedule of Prepaid and Other Current Assets	Prepaid and other current assets consisted of the following: September 30, December 31, 2021 2020 Deposits $ 795 $ 708 Prepaid expenses 6,574 3,510 Supplies 2,470 2,491 Prepaid and other current assets $ 9,839 $ 6,709

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	9 Months Ended
	Sep. 30, 2021
Property Plant And Equipment [Abstract]
Property and Equipment, Net	Property and equipment, net, consisted of the following: Estimated September 30, December 31, Useful Life 2021 2020 Lab and test equipment 7 years $ 18,183 $ 18,631 Vehicles 5 years 5,147 13,320 Equipment 3-7 years 35,099 32,177 Furniture and fixtures 7 years 2,696 2,938 Leasehold improvements 7 years 6,807 6,767 Aircraft 10 years 931 834 Building 39 years 2,975 2,975 71,838 77,642 Land 725 725 Construction in progress 1,660 219 Less accumulated depreciation (43,145 ) (44,187 ) Total property and equipment— net $ 31,078 $ 34,399

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2021
Leases [Abstract]
Summary of Components of Lease Expense	The components of lease expense were as follows: Three Months Ended Nine Months Ended Statement of Operations Location September 30, 2021 September 30, 2021 Operating lease cost Lease cost Selling, general and administrative expense $ 2,203 $ 6,398 Variable lease cost Selling, general and administrative expense 120 329 Total operating lease cost 2,323 6,727 Finance lease cost Amortization of right of use assets Depreciation and amortization 825 2,391 Interest on lease liabilities Interest expense—net 97 300 Total finance lease cost 922 2,691 Total lease cost $ 3,245 $ 9,418
Summary of Supplemental Cash Flow Information Related To Leases	Supplemental cash flows information related to leases was as follows: Nine Months Ended September 30, 2021 Cash paid for amounts included in the measurement of lease liabilities Operating cash flows used in operating leases $ 6,273 Operating cash flows used in finance leases 300 Financing cash flows used in finance leases 2,367 Lease liabilities arising from new ROU assets Operating leases 5,078 Finance leases 2,719
Summary of Weighted Average Remaining Lease Terms and Weighted Average Discount Rates	Weighted average remaining lease terms and weighted average discount rates were: September 30, 2021 Operating Leases Finance Leases Weighted average remaining lease term (years) 5.22 2.80 Weighted average discount rate 2.62 % 4.95 %
Schedule of Maturities of Lease Liabilities	The following is a schedule by year of the maturities of lease liabilities with original terms in excess of one year: September 30, 2021 Operating Leases Finance Leases Remainder of 2021 $ 2,103 $ 923 2022 6,659 3,295 2023 4,557 2,316 2024 3,031 1,317 2025 and thereafter 8,637 484 Total undiscounted future minimum lease payments 24,987 8,335 Less imputed interest (1,688 ) (520 ) Total discounted future minimum lease payments $ 23,299 $ 7,815
Schedule of Future Minimum Rental Commitments for Capital Lease and Non-Cancelable Operating Leases	A schedule of the future minimum rental commitments under the Company’s capital lease agreements and non-cancelable operating lease agreements with an initial or remaining term in excess of one year as of December 31, 2020, in accordance with ASC 840, the predecessor to ASC 842, were as follows: December 31, 2020 Operating Leases Finance Leases 2021 $ 5,946 $ 2,652 2022 4,865 2,172 2023 3,146 1,444 2024 1,812 496 2025 and thereafter 4,954 69 Total $ 20,723 $ 6,833

Business Acquisitions (Tables)

Business Acquisitions (Tables)	9 Months Ended
	Sep. 30, 2021
Business Acquisition [Line Items]
Summary of Elements of Purchase Price of Acquisitions	The following table summarizes the elements of purchase price of the acquisitions completed during the year ended December 31, 2020: Cash Common Stock Other Purchase Price Components Current Other Purchase Price Components Long Term Contingent Consideration Current Contingent Consideration Long Term Total Purchase Price CTEH $ 175,000 $ 25,000 $ (1,939 ) $ — $ 34,451 $ 10,543 $ 243,055 Other acquisitions 450 — 50 100 210 — 810 Total $ 175,450 $ 25,000 $ (1,889 ) $ 100 $ 34,661 $ 10,543 $ 243,865
Summary of Purchase Price Attributable to Acquisitions	The purchase price attributable to the acquisitions was allocated as follows: CTEH Other 2020 Acquisitions Total Cash $ 1,527 $ — $ 1,527 Accounts receivable 17,059 — 17,059 Other current assets 1,265 — 1,265 Current assets 19,851 — 19,851 Property and equipment 7,042 75 7,117 Customer relationships 56,000 — 56,000 Trade names 4,200 — 4,200 Covenants not to compete 4,000 109 4,109 Proprietary software 14,700 — 14,700 Goodwill 146,983 626 147,609 Total assets 252,776 810 253,586 Current liabilities 9,721 — 9,721 Total liabilities 9,721 — 9,721 Purchase price $ 243,055 $ 810 $ 243,865
Summary of Supplemental Unaudited Pro-Forma Information	The unaudited condensed consolidated financial information summarized in the following table gives effect to the 2021 and the 2020 acquisitions discussed above assuming they occurred on January 1, 2020. These unaudited consolidated pro forma operating results do not assume any impact from revenue, cost or other operating synergies that are expected or may have been realized as a result of the acquisitions. These unaudited consolidated pro forma operating results are presented for illustrative purposes only and are not indicative of the operating results that would have been achieved had the acquisitions occurred on January 1, 2020, nor does the information purport to reflect results for any future period. For the Three Months Ended September 30, 2021 2020 As reported Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) As reported Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) Revenues $ 132,578 $ — $ 132,578 $ 84,705 $ 5,064 $ 89,769 Net (loss) income 2,226 — 2,226 (30,737 ) 628 (30,109 ) For the Nine Months Ended September 30, 2021 2020 As reported Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) As reported Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) Revenues $ 402,619 $ 12,454 $ 415,073 $ 219,502 $ 61,583 $ 281,085 Net (loss) income (23,853 ) 2,205 (21,648 ) (58,761 ) 15,847 (42,914 )
MSE Group and Vista
Business Acquisition [Line Items]
Summary of Elements of Purchase Price of Acquisitions	The following table summarizes the elements of the purchase price of the acquisitions completed during the nine months ended September 30, 2021: Cash Common Stock Other Purchase Price Components Current Contingent Consideration Current Contingent Consideration Long Term Total Purchase Price MSE $ 9,082 $ 2,271 $ 10,280 $ — $ 1,804 $ 23,437 Vista 9,025 475 172 — 1,000 10,672 EI 15,397 2,274 5,324 — — 22,995 Sensible 6,500 1,000 52 1,000 3,200 11,752 Total $ 40,004 $ 6,020 $ 15,828 $ 1,000 $ 6,004 $ 68,856
Summary of Purchase Price Attributable to Acquisitions	The preliminary purchase price attributable to the acquisitions was allocated as follows: MSE EI All other acquisitions Total Cash $ 2,810 $ 250 $ 464 $ 3,524 Accounts receivable 2,987 4,564 1,078 8,629 Other current assets 31 84 267 382 Current assets 5,828 4,898 1,809 12,535 Property and equipment 513 32 978 1,523 Customer relationships 8,720 10,265 5,336 24,321 Trade names 521 996 1,323 2,840 Covenants not to compete 922 511 248 1,681 Acquired Technology — — 321 321 Goodwill 7,747 8,944 12,879 29,570 Total assets 24,251 25,646 22,894 72,791 Current liabilities (814 ) (2,651 ) (470 ) (3,935 ) Total liabilities (814 ) (2,651 ) (470 ) (3,935 ) Purchase price $ 23,437 $ 22,995 $ 22,424 $ 68,856

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2021
Goodwill And Intangible Assets Disclosure [Abstract]
Schedule of Amounts Related to Goodwill	Amounts related to goodwill are as follows: Assessment, Permitting and Response Measurement and Analysis Remediation and Reuse Total Balance as of December 31, 2020 $ 162,156 $ 69,054 $ 43,457 $ 274,667 Goodwill acquired during the period 8,944 12,879 7,747 29,570 Balance as of September 30, 2021 $ 171,100 $ 81,933 $ 51,204 $ 304,237
Schedule of Amounts Related to Finite-Lived Intangible Assets	Amounts related to finite-lived intangible assets are as follows: September 30, 2021 Estimated Useful Life Gross Balance Accumulated Amortization Total Intangible Assets—Net Finite lived intangible assets Customer relationships 2-15 years $ 189,103 $ 67,328 $ 121,775 Covenants not to compete 4-5 years 31,623 24,832 6,791 Trade names 1-5 years 19,778 15,107 4,671 Proprietary software 3-5 years 21,595 9,853 11,742 Patent 16 years 17,479 2,219 15,260 Total other intangible assets —net $ 279,578 $ 119,339 $ 160,239 December 31, 2020 Estimated Useful Life Gross Balance Accumulated Amortization Total Intangible Assets—Net Finite lived intangible assets Customer relationships 7-15 years $ 164,782 $ 53,446 $ 111,336 Covenants not to compete 4-5 years 29,942 21,469 8,473 Trade names 1-5 years 16,938 12,849 4,089 Proprietary software 3-5 years 21,007 6,132 14,875 Patent 16 years 17,479 1,398 16,081 Total other intangible assets —net $ 250,148 $ 95,294 $ 154,854
Schedule of Future Amortization Expense	Future amortization expense is estimated to be as follows for each of the five following years and thereafter: December 31, 2021 (remaining) $ 8,463 2022 30,298 2023 24,407 2024 20,623 2025 14,225 2026 and thereafter 62,223 Total $ 160,239

Accounts Payable and Other Ac_2

Accounts Payable and Other Accrued Liabilities (Tables)	9 Months Ended
	Sep. 30, 2021
Payables And Accruals [Abstract]
Summary of Accounts Payable and Other Accrued Liabilities	Accounts payable and other accrued liabilities consisted of the following: September 30, December 31, 2021 2020 Accounts payable $ 17,233 $ 15,481 Accrued expenses 15,703 11,469 Other business acquisitions purchase price obligations 274 50 Contract liabilities 8,037 6,114 Other current liabilities 1,126 1,507 Income tax payable — 256 Total accounts payable and other accrued liabilities $ 42,373 $ 34,877

Accrued Payroll and Benefits (T

Accrued Payroll and Benefits (Tables)	9 Months Ended
	Sep. 30, 2021
Statement Of Financial Position [Abstract]
Schedule of Accrued Payroll and Benefits	Accrued payroll and benefits consisted of the following: September 30, December 31, 2021 2020 Accrued bonuses $ 8,233 $ 5,416 Accrued paid time off 1,990 2,067 Accrued payroll 7,472 9,133 Accrued other 4,790 4,565 Total accrued payroll and benefits $ 22,485 $ 21,181

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2021
Debt Disclosure [Abstract]
Schedule of Debt	Debt consisted of the following: September 30, December 31, 2021 2020 Term loan facility $ 175,000 $ 173,906 Revolving Line of Credit 37,000 — Capital leases — 3,088 Equipment line of credit — 3,018 Less deferred debt issuance costs (2,374 ) (4,108 ) Total debt 209,626 175,904 Less current portion of long-term debt (8,750 ) (5,583 ) Long-term debt, less current portion $ 200,876 $ 170,321
Summary of Term Loan Amortization	The 2021 Credit Facility term loan must be repaid in quarterly installments and Amortization Table Year 1 Year 2 Year 3 Year 4 Year 5 Term Loan 5.0 % 5.0 % 7.5 % 7.5 % 10.0 %
Summary of 2021 Credit Facility Interest Rate Subject to Leverage Ratio and LIBOR	The 2021 Credit Facility term loan and the revolver bear interest subject to the Company’s leverage ratio and LIBOR as follows: Pricing Tier Net Leverage Ratio 2021 Credit Facility Commitment Fee Letter of Credit Fee LIBOR Base Rate 1 ≥ 3.75x to 1.0 2.50 % 1.50 % 0.25 % 2.50 % 2 <3.75x to 1.0 but ≥ 3.25 to 1.0 2.25 1.25 0.23 2.25 3 <3.25 to 1.0 but ≥ 2.50 to 1.0 2.00 1.00 0.20 2.00 4 <2.50 to 1.0 but ≥ 1.75 to 1.0 1.75 0.75 0.15 1.75 5 <1.75 to 1.0 1.50 0.50 0.15 1.50
Schedule of Applicable Percentages	The applicable margin means a percentage per annum determined in accordance with the following table: Pricing Tier Consolidated Leverage Ratio Commitment Fee Eurodollar Rate Loans and LIBOR Letter of Credit Fee Daily Floating Rate Loans Rate Loans 1 > 3.75 to 1.0 0.50 % 4.00 % 4.00 % 3.00 % 2 ≤ 3.75 to 1.0 but > 3.00 to 1.0 0.50 3.50 3.50 2.50 3 ≤ 3.00 to 1.0 but > 2.25 to 1.0 0.40 3.00 3.00 2.00 4 < 2.25 to 1.0 0.30 2.50 2.50 1.50
Schedule of Aggregate Annual Maturities of Long-Term Debt	The following is a schedule of the aggregate annual maturities of long-term debt presented on the unaudited condensed consolidated statement of financial position, based on the terms of the 2021 Credit Facility and equipment line of credit: September 30, 2022 $ 8,750 2023 10,938 2024 13,125 2025 15,313 2026 163,874 Total $ 212,000

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Summary of Financial Liabilities Measured at Fair Value on Recurring Basis	The following financial liabilities are measured at fair value on a recurring basis using significant unobservable inputs (Level 3): September 30, December 31, 2021 2020 Business acquisitions contingent consideration, current $ 31,152 $ 49,902 Business acquisitions contingent consideration, long-term 4,200 4,565 Conversion option 22,537 20,886 Total $ 57,889 $ 75,353
Summary of Financial Instruments Measured at Fair Value on Recurring Basis	The following table sets forth the Company’s financial instruments that were measured at fair value on a recurring basis: Compound Embedded Option Business Acquisitions Contingent Consideration, Current Business Acquisitions Contingent Consideration, Long-term Conversion Option Contingent Put Option Warrant Options Total Liabilities Balance—at January 1, 2020 $ — $ 8,614 $ 379 $ — $ 7,100 $ 16,878 $ 32,971 Series A-2 compound embedded option (9,361 ) — — — — — $ (9,361 ) Issuance of warrant option — — — — — 30,097 $ 30,097 Acquisitions — 34,661 10,543 — — — $ 45,204 Changes in fair value included in earnings 27,420 18,387 (1,000 ) — (19,240 ) 9,312 $ 34,879 Payment of contingent consideration payable — (12,464 ) — — — — $ (12,464 ) Foreign currency translation of contingent consideration payment — (208 ) — — — — $ (208 ) Reclass of long term to short term contingent liabilities — 180 (180 ) — — — $ — Write off of the contingent put option — — — — 12,140 — $ 12,140 Exercise of warrant options — — — — — (56,287 ) $ (56,287 ) Balance—at September 30, 2020 $ 18,059 $ 49,170 $ 9,742 $ — $ — $ — $ 76,971 . Balance—at January 1, 2021 $ — $ 49,902 $ 4,565 $ 20,886 $ — $ — $ 75,353 Acquisitions — 1,000 6,004 — — — 7,004 Changes in fair value included in earnings — 13,774 10,261 1,651 — — 25,686 Payment of contingent consideration payable — (50,154 ) — — — — (50,154 ) Reclass of long term to short term contingent liabilities 16,630 (16,630 ) — Balance—at September 30, 2021 $ — $ 31,152 $ 4,200 $ 22,537 $ — $ — $ 57,889
Summary of Fair Value of Warrant Options Associated with Issuance of Redeemable Preferred Stock	The fair value of the warrant option associated with the issuance of the Redeemable Series A-1 Preferred Stock was calculated based on the Black-Sholes pricing model using the following assumptions: July 30, 2020 Common stock value (per share) $ 22.22 Expected volatility 44.35 % Risk-free interest rate 0.55 % Expected life (years) 10

Stockholder's Equity (Deficit)

Stockholder's Equity (Deficit) (Tables)	9 Months Ended
	Sep. 30, 2021
Summary of Issued Shares of Common Stock	The Company issued the following shares of common stock: Three Months Ended September 30, 2021 2020 Shares Average Price per Share Total Consideration (in thousands) Shares Average Price per Share Total Consideration (in thousands) Common stock issued in connection initial public offering — $ — $ — 11,500,000 $ 15.00 $ 172,500 Acquisitions 62,738 $ 52.18 3,274 — — — Redemption of series A-1 preferred stock — — — 1,786,739 15.00 26,801 Exercise of warrant options — — — 2,534,239 0.01 25 Exercise of options 354,918 8.30 2,946 16,850 8.92 150 Restricted shares, net — — — 8,333 24.00 — Cancellation of shares — — — (117,785 ) 17.15 — Total 417,656 $ 14.89 $ 6,220 15,728,376 $ 12.82 $ 199,476 Nine Months Ended September 30, 2021 2020 Shares Average Price per Share Total Consideration (in thousands) Shares Average Price per Share Total Consideration (in thousands) Common stock issued in connection initial public offering — $ — $ — 11,500,000 $ 15.00 $ 172,500 Acquisitions 143,800 41.86 6,020 791,139 31.60 25,000 Redemption of series A-1 preferred stock — — — 1,786,739 15.00 26,801 Exercise of warrant options 67,713 17.19 — 2,534,239 0.01 25 Exercise of options 779,068 7.74 6,032 20,350 9.27 189 Restricted shares, net 38,929 31.31 — 8,333 24.00 — Payment of earn-out liability 539,607 46.33 25,000 — — — Payment of purchase price true up 24,200 44.81 1,084 — — — Cancellation of shares — — — (117,785 ) 17.15 — Total 1,593,317 $ 25.43 $ 38,136 16,523,015 $ 13.72 $ 224,515
Schedule of Share Authorized to be Issue and Available for Grant	The following number of shares were authorized to be issued and available for grant: September 30, 2021 2017 Plan 2013 Plan Total Shares authorized to be issued 3,944,750 2,047,269 5,992,019 Shares available for grant 1,593,287 — 1,593,287 September 30, 2020 2017 Plan 2013 Plan Total Shares authorized to be issued 2,945,443 2,047,269 4,992,712 Shares available for grant 1,233,266 — 1,233,266
Schedule of Stock Compensation Expense	Total stock compensation expense for the Plans was as follows: Three Months Ended September 30, 2021 2020 2017 plan 2013 plan 2017 plan 2013 plan Options Restricted Stock Options Total Options Restricted Stock Options Total Cost of revenue $ 282 $ — $ — $ 282 $ 276 $ — $ (5 ) $ 271 Selling, general and administrative expense 1,844 237 2 2,083 592 233 53 878 Total $ 2,126 $ 237 $ 2 $ 2,365 $ 868 $ 233 $ 48 $ 1,149 Nine Months Ended September 30, 2021 2020 2017 plan 2013 plan 2017 plan 2013 plan Options Restricted Stock Options Total Options Restricted Stock Options Total Cost of revenue $ 1,178 $ — $ 10 $ 1,188 $ 973 $ — $ 126 $ 1,099 Selling, general and administrative expense 4,638 749 11 5,399 1,178 973 189 2,340 Total $ 5,816 $ 749 $ 21 $ 6,587 $ 2,151 $ 973 $ 315 $ 3,439
Summary of Weighted Average Assumptions Used in Black-Sholes Option-pricing Model	The following weighted-average assumptions were used in the Black-Sholes option-pricing model calculation: September 30, September 30, 2021 2020 Common stock value (per share) $ 39.83 $ 16.36 Expected volatility 58.34 46.14 Risk-free interest rate 0.77 0.41 Expected life (years) 5.5-7.0 7.0 Forfeiture rate None None Dividend rate None None
Schedule of Common Stock Reserved for Future Issuance	Common Stock Reserved for Future Issuances —The Company has reserved certain stock of its authorized but unissued common stock for possible future issuance in connection with the following: September 30, 2021 2020 Warrants — 116,350 Montrose 2013 Stock Incentive Plan 2,047,269 2,047,269 Montrose 2017 Stock Incentive Plan 3,944,750 2,945,443 Common stock reserved for future issuance 5,992,019 5,109,062
Montrose Amended & Restated 2017 Stock Incentive Plan
Schedule of Restricted Stock Activity	Restricted stock activity was as follows: Nine Months Ended September 30, 2021 2020 Shares Average Price per Share Total (in thousands) Shares Average Price per Share Total (in thousands) Shares granted 19,066 $ 36.99 $ 705 33,229 $ 31.60 $ 1,050
Summary of Stock Option Activity	The following summarizes the options activity of the 2017 Plan: Options to Purchase Common Stock Weighted- Average Exercise Price per Share Weighted Average Grant Date Fair Value per Share Weighted Average Remaining Contract Life (in Years) Aggregate Intrinsic Value of In-The- Money Options (in Thousands) Outstanding at January 1, 2020 617,852 $ 24 $ 12 7.82 $ 4,693 Granted 851,877 32 12 — — Forfeited/ cancelled (14,375 ) 22 — — — Exercised (925 ) 14 — — 7,918 Outstanding at September 30, 2020 1,454,429 21 12 7.59 7,649 Outstanding at January 1, 2021 1,840,229 23 12 9.09 15,598 Granted 264,470 40 22 — — Forfeited/ cancelled (29,025 ) 28 — — — Expired (1,250 ) 18 — — — Exercised (66,875 ) 22 — — 2,078 Outstanding at September 30, 2021 2,007,549 25 14 8.53 73,724 Exercisable at September 30, 2021 320,861 28 — 7.68 10,755 Options vested and expected to vest 2,007,549 25 — 8.53 73,724
Montrose Amended and Restated 2013 Stock Option Plan
Summary of Stock Option Activity	The following summarizes the activity of the 2013 Plan: Options to Purchase Common Stock Weighted- Average Exercise Price per Share Weighted Average Grant Date Fair Value per Share Weighted Average Remaining Contract Life (in Years) Aggregate Intrinsic Value of In-The- Money Options (in Thousands) Outstanding at January 1, 2020 1,855,469 $ 6 $ 1 6.40 46,617 Forfeited/ cancelled (7,500 ) 10 — — — Expired (11,300 ) 6 — — — Exercised (19,175 ) 8 — — 354 Outstanding at September 30, 2020 1,817,494 6 1 5.63 33,772 Outstanding at January 1, 2021 1,787,869 6 1 5.40 43,867 Expired (625 ) 6 — — — Exercised (710,568 ) 6 — — 33,480 Outstanding at September 30, 2021 1,076,676 6 2 4.65 59,532 Exercisable at September 30, 2021 1,076,676 6 — 4.65 59,532 Options vested and expected to vest 1,076,676 6 — 4.65 59,532

Net Income (Loss) Per Share (Ta

Net Income (Loss) Per Share (Tables)	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net (Loss) Income Per Share Attributable to Common Stockholders	The following table summarizes the computation of basic and diluted net (loss) income per share attributable to common stockholders of the Company: Three Months Ended Nine Months Ended September 30, September 30, (In thousands, except for net loss per share) 2021 2020 2021 2020 Net income (loss) $ 2,226 $ (30,737 ) $ (23,853 ) $ (58,761 ) Accretion of redeemable series A-1 preferred stock — (6,542 ) — (17,601 ) Redeemable series A-1 preferred stock deemed dividend (24,341 ) (24,341 ) Convertible and redeemable series A-2 preferred stock dividend (4,100 ) (2,870 ) (12,300 ) (2,870 ) Net loss attributable to common stockholders –basic and diluted (1,874 ) (64,490 ) (36,153 ) (103,573 ) Weighted-average common shares outstanding – basic and diluted 26,220 21,554 25,798 13,669 Net loss per share attributable to common stockholders –basic and diluted $ (0.07 ) $ (2.99 ) $ (1.40 ) $ (7.58 )
Equity Shares Excluded from Calculation of Diluted Net (Loss) Income per Share Attributable to Common Stockholders	The following equity shares were excluded from the calculation of diluted net income (loss) per share attributable to common stockholders because their effect would have been anti-dilutive: September 30, 2021 2020 Stock options 1,686,688 2,423,900 Restricted stock 28,623 176,384 Warrants — 116,350

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2021
Segment Reporting [Abstract]
Components of Segment Revenues and Adjusted EBITDA	Segment revenues and Adjusted EBITDA consisted of the following: Three Months Ended September 30, 2021 2020 Segment Segment Segment Adjusted Segment Adjusted Revenues EBITDA Revenues EBITDA Assessment, Permitting and Response $ 63,412 $ 15,719 $ 26,593 $ 8,230 Measurement and Analysis 38,752 8,283 39,770 11,226 Remediation and Reuse 30,414 5,472 18,342 2,012 Total Operating Segments 132,578 29,474 84,705 21,468 Corporate and Other — (7,987 ) — (4,762 ) Total $ 132,578 $ 21,487 $ 84,705 $ 16,706 Nine Months Ended September 30, 2021 2020 Segment Segment Segment Adjusted Segment Adjusted Revenues EBITDA Revenues EBITDA Assessment, Permitting and Response $ 209,379 $ 46,379 $ 49,768 $ 14,665 Measurement and Analysis 111,309 22,634 113,130 30,422 Remediation and Reuse 81,931 12,262 56,604 6,495 Total Operating Segments 402,619 81,275 219,502 51,582 Corporate and Other — (22,026 ) — (15,425 ) Total $ 402,619 $ 59,249 $ 219,502 $ 36,157
Reconciliation of Segment Measure to Income (Loss) Before (Expense) Benefit from Income Taxes	Presented below is a reconciliation of the Company’s segment measure to income (loss) before (expense) benefit from income taxes: For the Three Months Ended September 30, For the Nine Months Ended September 30, 2021 2020 2021 2020 Total $ 21,487 $ 16,706 $ 59,249 $ 36,157 Interest expense, net (1,722 ) (3,043 ) (11,208 ) (10,896 ) Income tax (expense) benefit (902 ) (3,348 ) (648 ) 1,563 Depreciation and amortization (11,471 ) (9,740 ) (33,145 ) (27,084 ) Stock-based compensation (2,365 ) (1,149 ) (6,587 ) (3,439 ) Start-up losses and investment in new services (1,186 ) (602 ) (3,276 ) (1,283 ) Acquisition costs (913 ) (6 ) (1,656 ) (3,767 ) Fair value changes in financial instruments (531 ) (9,710 ) (1,651 ) (17,492 ) Fair value changes in business acquisitions contingent consideration — (13,404 ) (24,035 ) (17,387 ) Short term purchase accounting fair value adjustment to deferred revenue — — — (243 ) Initial public offering expense and related bonuses — (6,378 ) — (6,908 ) Discontinued services ( i ) — (30 ) — (7,526 ) Expenses related to financing transactions — — (50 ) (277 ) Other losses or expenses (171 ) (33 ) (846 ) (179 ) Net income (loss) $ 2,226 $ (30,737 ) $ (23,853 ) $ (58,761 ) (i) During the first quarter of 2020, the Company determined to reduce the footprint of its environmental lab in Berkeley, California, and to exit its non-specialized municipal water engineering service line and its food waste biogas engineering service line. As a part of discontinuing service lines, the Company made the decision to book an additional bad debt reserve related to the uncertainty around the ability to collect on receivables related to these service lines (Note 3). It was determined that the discontinuation of these service lines did not represent a strategic shift that had (or will have) a major effect on the Company’s operations and financial results therefore did not meet the requirements to be classified as discontinued operations.

Subsequent Events (Tables)

Subsequent Events (Tables)	9 Months Ended
	Sep. 30, 2021
Subsequent Event [Line Items]
Summary of Elements of Purchase Price of Acquisitions	The following table summarizes the elements of purchase price of the acquisitions completed during the year ended December 31, 2020: Cash Common Stock Other Purchase Price Components Current Other Purchase Price Components Long Term Contingent Consideration Current Contingent Consideration Long Term Total Purchase Price CTEH $ 175,000 $ 25,000 $ (1,939 ) $ — $ 34,451 $ 10,543 $ 243,055 Other acquisitions 450 — 50 100 210 — 810 Total $ 175,450 $ 25,000 $ (1,889 ) $ 100 $ 34,661 $ 10,543 $ 243,865
EI and SL
Subsequent Event [Line Items]
Summary of Elements of Purchase Price of Acquisitions	These transactions qualified as an acquisition of a business and will be accounted for as a business combination. The following table summarizes the elements of the purchase price of these acquisitions: Other Purchase Total Cash (1) Common Stock (2) Price Component (3) Purchase Price ECI $ 4,000 — — $ 4,000 Horizon 10,580 2,300 — 12,880 (1) The cash portion of these acquisitions’ purchase price was funded through cash on hand. (2) The common stock component of Horizon was paid through the issuance of 34,921 shares of common stock. (3) The other purchase price components for ECI consist of a potential working capital target payment and a potential earnout amount, which are not finalized at this time. The other purchase price component of Horizon consists of a potential working capital target payment, which is not finalized at this time.

Description of the Business a_3

Description of the Business and Basis of Presentation - Additional Information (Details) $ / shares in Units, $ in Millions	Jul. 27, 2020USD ($)$ / sharesshares	Sep. 30, 2021Employee$ / sharesshares	Sep. 30, 2020$ / sharesshares	Sep. 30, 2021EmployeeOfficeSegment$ / sharesshares	Sep. 30, 2020$ / sharesshares
Description Of Business And Basis Of Presentation [Line Items]
Entity formation, month and year				2013-11
Number of offices in which entity operates | Office				80
Entity number of employees | Employee		2,300		2,300
Number of operating segments | Segment				3
Common stock issued, shares | shares		417,656	15,728,376	1,593,317	16,523,015
Share price | $ / shares		$ 39.83	$ 16.36	$ 39.83	$ 16.36
IPO
Description Of Business And Basis Of Presentation [Line Items]
Common stock issued, shares | shares	11,500,000
Share price | $ / shares	$ 15
Net proceeds from sale of common stock | $	$ 161.3
Underwriting discounts | $	11.2
Deferred offset | $	$ 4.4
Underwriters
Description Of Business And Basis Of Presentation [Line Items]
Common stock issued, shares | shares	1,500,000

Summary of New Accounting Pro_2

Summary of New Accounting Pronouncements - Additional Information (Details) - USD ($)	Sep. 30, 2021	Jan. 31, 2021
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating lease right-of-use asset—net	$ 23,111,000	$ 24,600,000
Current portion of operating lease liabilities	6,715,000	7,300,000
Operating lease liability—net of current portion	16,584,000	17,300,000
Finance lease right-of-use asset—net	7,493,000	7,200,000
Current portion of finance lease liabilities	$ 3,174,000	$ 2,900,000
Accounting Standards Update 2019-12
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2021
Change in accounting principle, accounting standards update, immaterial effect	true
Accounting Standards Update 2018-07
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2020
Change in accounting principle, accounting standards update, immaterial effect	true
Accounting Standards Update 2017-04
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2020
Change in accounting principle, accounting standards update, immaterial effect	true
Accounting Standards Update 2016-02
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2021
Operating lease right-of-use asset—net	$ 24,600
Current portion of operating lease liabilities	7,300
Operating lease liability—net of current portion	17,300
Finance lease right-of-use asset—net	7,200
Current portion of finance lease liabilities	2,900
Finance lease liabilities, net of current portion	$ 4,600

Revenues and Accounts Receiva_3

Revenues and Accounts Receivable - Schedule of Contract Balances (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Revenues And Accounts Receivable [Abstract]
Contract assets	$ 46,270	$ 38,576
Contract liabilities	$ 8,037	$ 6,114

Revenues and Accounts Receiva_4

Revenues and Accounts Receivable - Additional Information (Details) $ in Thousands	3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2021USD ($)Customer	Sep. 30, 2020Customer	Mar. 31, 2020USD ($)	Jun. 30, 2020USD ($)	Sep. 30, 2021USD ($)Customer	Sep. 30, 2020USD ($)Customer	Dec. 31, 2020USD ($)
Accounts Notes And Loans Receivable [Line Items]
Contracts assets acquired through business acquisitions					$ 400		$ 6,500
Contract liabilities acquired through business acquisitions					500		0
Contract with customer liabilities, revenue recognized	$ 1,300				3,800
Revenue remaining performance obligations	$ 64,100				64,100		24,400
Bad debt reserve			$ 6,300		$ 803	$ 6,445	$ 4,532
Remediation and Reuse Segment
Accounts Notes And Loans Receivable [Line Items]
Bad debt reserve				$ 5,500
Customer Concentration Risk | Accounts Receivable | Customer One
Accounts Notes And Loans Receivable [Line Items]
Concentration risk percentage					10.00%		10.20%
Customer Concentration Risk | Revenue
Accounts Notes And Loans Receivable [Line Items]
Number of customers | Customer	0	0			3	0
Customer Concentration Risk | Revenue | Customer One
Accounts Notes And Loans Receivable [Line Items]
Concentration risk percentage					12.10%
Customer Concentration Risk | Revenue | Significant Customer
Accounts Notes And Loans Receivable [Line Items]
Concentration risk percentage	10.00%	10.00%				10.00%
Customer Concentration Risk | Revenue | Customer Two
Accounts Notes And Loans Receivable [Line Items]
Concentration risk percentage					11.40%
Customer Concentration Risk | Revenue | Customer Three
Accounts Notes And Loans Receivable [Line Items]
Concentration risk percentage					10.70%

Revenues and Accounts Receiva_5

Revenues and Accounts Receivable - Additional Information (Details1) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue remaining performance obligations	$ 64.1	$ 24.4
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-10-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue remaining performance obligations	$ 51.7
Revenue remaining performance obligations, satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2022-01-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue remaining performance obligations	$ 12.4
Revenue remaining performance obligations, satisfaction period

Revenues and Accounts Receiva_6

Revenues and Accounts Receivable - Schedule of Accounts Receivable, Net (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenues And Accounts Receivable [Abstract]
Accounts receivable, invoiced	$ 68,974	$ 57,228
Accounts receivable, other	1,876	1,139
Allowance for doubtful accounts	(4,379)	(4,265)	$ (1,327)
Accounts receivable—net	$ 66,471	$ 54,102

Revenues and Accounts Receiva_7

Revenues and Accounts Receivable - Schedule of Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Revenues And Accounts Receivable [Abstract]
Beginning Balance	$ 1,327	$ 4,265	$ 1,327	$ 1,327
Bad Debt Expense	$ 6,300	803	$ 6,445	4,532
Charged to Allowance		(1,229)		(2,633)
Other		540		1,039
Ending Balance		$ 4,379		$ 4,265

Prepaid and Other Current Ass_3

Prepaid and Other Current Assets - Schedule of Prepaid and Other Current Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Deposits	$ 795	$ 708
Prepaid expenses	6,574	3,510
Supplies	2,470	2,491
Prepaid and other current assets	$ 9,839	$ 6,709

Property and Equipment, Net - P

Property and Equipment, Net - Property and Equipment, Net (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 71,838	$ 77,642
Less accumulated depreciation	(43,145)	(44,187)
Total property and equipment— net	$ 31,078	34,399
Lab and test equipment
Property Plant And Equipment [Line Items]
Estimated Useful Life	7 years
Property and equipment, gross	$ 18,183	18,631
Vehicles
Property Plant And Equipment [Line Items]
Estimated Useful Life	5 years
Property and equipment, gross	$ 5,147	13,320
Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 35,099	32,177
Equipment | Minimum
Property Plant And Equipment [Line Items]
Estimated Useful Life	3 years
Equipment | Maximum
Property Plant And Equipment [Line Items]
Estimated Useful Life	7 years
Furniture and fixtures
Property Plant And Equipment [Line Items]
Estimated Useful Life	7 years
Property and equipment, gross	$ 2,696	2,938
Leasehold improvements
Property Plant And Equipment [Line Items]
Estimated Useful Life	7 years
Property and equipment, gross	$ 6,807	6,767
Aircraft
Property Plant And Equipment [Line Items]
Estimated Useful Life	10 years
Property and equipment, gross	$ 931	834
Building
Property Plant And Equipment [Line Items]
Estimated Useful Life	39 years
Property and equipment, gross	$ 2,975	2,975
Land
Property Plant And Equipment [Line Items]
Property and equipment, gross	725	725
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 1,660	$ 219

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Property Plant And Equipment [Line Items]
Depreciation expense	$ 2,500	$ 2,100	$ 7,200	$ 6,100
Finance lease liability	7,815		7,815		$ 3,088
Accounting Standards Update 2016-02
Property Plant And Equipment [Line Items]
Finance lease liability	$ 7,200		$ 7,200

Leases - Additional Information

Leases - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Jan. 31, 2021
Lessee Lease Description [Line Items]
Operating lease right-of-use assets	$ 23,111	$ 23,111	$ 24,600
Current operating lease liabilities	6,715	6,715	7,300
Operating lease liabilities, net of current portion	16,584	16,584	17,300
Finance lease right-of-use assets	7,493	7,493	7,200
Current finance lease liabilities	3,174	3,174	2,900
Finance lease liabilities, net of current portion	4,641	4,641	$ 4,600
Rent expense	$ 2,200	$ 6,400
Minimum
Lessee Lease Description [Line Items]
Lessee operating and finance lease term		1 year
Maximum
Lessee Lease Description [Line Items]
Lessee operating and finance lease term		15 years

Leases - Summary of Components

Leases - Summary of Components of Lease Expense (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021
Lessee Lease Description [Line Items]
Lease cost	$ 2,200	$ 6,400
Total operating lease cost	2,323	6,727
Total finance lease cost	922	2,691
Total lease cost	3,245	9,418
Selling, General and Administrative Expenses
Lessee Lease Description [Line Items]
Lease cost	2,203	6,398
Variable lease cost	120	329
Depreciation and Amortization
Lessee Lease Description [Line Items]
Amortization of right of use assets	825	2,391
Interest Expense, Net
Lessee Lease Description [Line Items]
Interest on lease liabilities	$ 97	$ 300

Leases - Summary of Supplementa

Leases - Summary of Supplemental Cash Flow Information Related To Leases (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021USD ($)
Leases [Abstract]
Operating cash flows used in operating leases	$ 6,273
Operating cash flows used in finance leases	300
Financing cash flows used in finance leases	2,367
Operating leases	5,078
Finance leases	$ 2,719

Leases - Summary of Weighted Av

Leases - Summary of Weighted Average Remaining Lease Terms and Weighted Average Discount Rates (Details)	Sep. 30, 2021
Leases [Abstract]
Weighted average discount rate, Operating Leases	2.62%
Weighted average discount rate, Finance Leases	4.95%

Leases - Summary of Maturities

Leases - Summary of Maturities of Lease Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Leases [Abstract]
Operating Leases, Remainder of 2021	$ 2,103
Operating Leases, 2022	6,659
Operating Leases, 2023	4,557
Operating Leases, 2024	3,031
Operating Leases, 2025 and thereafter	8,637
Operating Leases, Total undiscounted future minimum lease payments	24,987
Operating Leases, Less imputed interest	(1,688)
Operating Leases, Total discounted future minimum lease payments	23,299
Finance Leases, Remainder of 2021	923
Finance Leases, 2022	3,295
Finance Leases, 2023	2,316
Finance Leases, 2024	1,317
Finance Leases, 2025 and thereafter	484
Finance Leases, Total undiscounted future minimum lease payments	8,335
Finance Leases, Less imputed interest	(520)
Finance Leases, Total discounted future minimum lease payments	$ 7,815	$ 3,088

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Rental Commitments for Capital Lease and Non-Cancelable Operating Leases (Details) $ in Thousands	Dec. 31, 2020USD ($)
Leases [Abstract]
Operating Leases, 2021	$ 5,946
Operating Leases, 2022	4,865
Operating Leases, 2023	3,146
Operating Leases, 2024	1,812
Operating Leases, 2025 and thereafter	4,954
Operating Leases, Total	20,723
Finance Leases, 2021	2,652
Finance Leases, 2022	2,172
Finance Leases, 2023	1,444
Finance Leases, 2024	496
Finance Leases, 2025 and thereafter	69
Finance Leases, Total	$ 6,833

Business Acquisitions - Additio

Business Acquisitions - Additional Information (Details) - USD ($)	1 Months Ended						3 Months Ended		9 Months Ended
	Aug. 31, 2021	Jul. 31, 2021	Jun. 30, 2021	Apr. 30, 2021	Jan. 31, 2021	Apr. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Business Acquisition [Line Items]
Transaction costs related to business combinations							$ 913,000	$ 6,000	$ 1,656,000	$ 3,767,000
Number of common stock shares issued in connection with business acquisition							62,738		143,800	791,139
Revenues from discontinued service lines							$ 0	$ 0	$ 0	$ 3,800,000
MSE Group
Business Acquisition [Line Items]
Percentage of interests acquired					100.00%
Number of common stock shares issued in connection with business acquisition									71,740
MSE Group | Customer Relationships
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets					7 years
MSE Group | Related Backlog
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets					2 years
MSE Group | Trade Names
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets					2 years
MSE Group | Covenants Not to Compete
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets					5 years
Vista
Business Acquisition [Line Items]
Percentage of interests acquired			100.00%
Number of common stock shares issued in connection with business acquisition									9,322
Vista | Customer Relationships
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets			10 years
Vista | Trade Names
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets			3 years
Vista | Covenants Not to Compete
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets			5 years
EI
Business Acquisition [Line Items]
Percentage of interests acquired		100.00%
Number of common stock shares issued in connection with business acquisition									43,100
EI | Customer Relationships
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets		10 years
EI | Trade Names
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets		5 years
EI | Covenants Not to Compete
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets		5 years
Sensible
Business Acquisition [Line Items]
Percentage of interests acquired	100.00%
Number of common stock shares issued in connection with business acquisition									19,638
Sensible | Customer Relationships
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets	10 years
Sensible | Acquired Technology
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets	5 years
MSE Group ,Vista and Sensible
Business Acquisition [Line Items]
Revenue							11,900,000		$ 19,700,000
Pre-tax loss							(200,000)		0
MSE Group ,Vista and Sensible | 2022 Earn Out
Business Acquisition [Line Items]
Business combination, earn-out payment, maximum							12,700,000		12,700,000
MSE Group ,Vista and Sensible | 2023 Earn Out
Business Acquisition [Line Items]
Business combination, earn-out payment, maximum							$ 12,700,000		$ 12,700,000
CTEH
Business Acquisition [Line Items]
Percentage of interests acquired						100.00%
Number of common stock shares issued in connection with business acquisition						791,139
CTEH | External Proprietary Software
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets						5 years
CTEH | Customer Relationships
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets						15 years
CTEH | Trade Names
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets						5 years
CTEH | Covenants Not to Compete
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets						5 years
CTEH | Proprietary Software
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets						3 years
CTEH | 2020 Earn Out
Business Acquisition [Line Items]
Business combination, earn-out payment, maximum						$ 50,000,000
Business combination, contingent consideration arrangements, basis for amount									The CTEH first year earn-out was calculated at twelve times CTEH’s 2020 EBITDA (as defined in the purchase agreement) in excess of $18.3 million, with a maximum first year earn-out payment of $50.0 million, which was fully achieved.
Business combination, contingent consideration arrangements, description									The 2020 earn-out was initially payable 100.0% in common stock, but as a result of the completion of the Company’s IPO (Note 1), 50.0% was payable in cash. In April 2021, the 2020 earn-out payment was made with 50.0% paid in cash and the remaining 50.0% paid in common stock of the Company (Notes 14 and 18).
Business combination, minimum EBITDA for contingent consideration arrangements						$ 18,300,000
Percentage of earn out payable in common stock						100.00%
Percentage of earn out paid in cash, as result of completing of IPO				50.00%
Percentage of earn out paid in common stock, as a result of completion of IPO				50.00%
CTEH | 2020 Earn Out | Minimum
Business Acquisition [Line Items]
Business combination, actual EBITDA						$ 18,300,000
CTEH | 2020 Earn Out | Maximum
Business Acquisition [Line Items]
Business combination, actual EBITDA						22,500,000
CTEH | 2021 Earn Out
Business Acquisition [Line Items]
Business combination, earn-out payment, maximum						$ 30,000,000
Business combination, contingent consideration arrangements, basis for amount									The second year earn-out is to be calculated at ten times CTEH’s 2021 EBITDA in excess of actual 2020 EBITDA (with actual 2020 EBITDA subject to a minimum of $18.3 million and a maximum of $22.5 million), with a maximum second year earn-out payment of $30.0 million.
Business combination, contingent consideration arrangements, description									The 2021 earn-out, if any, is payable 100.0% in cash.
Percentage of earn out payable in cash						100.00%
Other Acquisition | Covenants Not to Compete
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets						4 years

Business Acquisitions - Summary

Business Acquisitions - Summary of Elements of Purchase Price of Acquisitions (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Business Acquisition [Line Items]
Cash	$ 40,004	$ 175,450
Common Stock	6,020	25,000
Other Purchase Price Components Current	15,828	(1,889)
Contingent Consideration Current	1,000	34,661
Other Purchase Price Components Long Term		100
Contingent Consideration Long Term	6,004	10,543
Total Purchase Price	68,856	243,865
MSE Group
Business Acquisition [Line Items]
Cash	9,082
Common Stock	2,271
Other Purchase Price Components Current	10,280
Contingent Consideration Long Term	1,804
Total Purchase Price	23,437
Vista
Business Acquisition [Line Items]
Cash	9,025
Common Stock	475
Other Purchase Price Components Current	172
Contingent Consideration Long Term	1,000
Total Purchase Price	10,672
EI
Business Acquisition [Line Items]
Cash	15,397
Common Stock	2,274
Other Purchase Price Components Current	5,324
Total Purchase Price	22,995
Sensible
Business Acquisition [Line Items]
Cash	6,500
Common Stock	1,000
Other Purchase Price Components Current	52
Contingent Consideration Current	1,000
Contingent Consideration Long Term	3,200
Total Purchase Price	$ 11,752
CTEH
Business Acquisition [Line Items]
Cash		175,000
Common Stock		25,000
Other Purchase Price Components Current		(1,939)
Contingent Consideration Current		34,451
Contingent Consideration Long Term		10,543
Total Purchase Price		243,055
Other Acquisition
Business Acquisition [Line Items]
Cash		450
Other Purchase Price Components Current		50
Contingent Consideration Current		210
Other Purchase Price Components Long Term		100
Total Purchase Price		$ 810

Business Acquisitions - Summa_2

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

Business Acquisitions - Summa_3

Business Acquisitions - Summary of Supplemental Unaudited Pro-Forma Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Business Acquisition [Line Items]
Revenues	$ 132,578	$ 84,705	$ 402,619	$ 219,502
Net (loss) income	2,226	(30,737)	(23,853)	(58,761)
Acquisition Proforma
Business Acquisition [Line Items]
Revenues		5,064	12,454	61,583
Net (loss) income		628	2,205	15,847
Consolidated Proforma
Business Acquisition [Line Items]
Revenues	132,578	89,769	415,073	281,085
Net (loss) income	$ 2,226	$ (30,109)	$ (21,648)	$ (42,914)

Goodwill and Intangible Asset_2

Goodwill and Intangible Assets - Schedule of Amounts Related to Goodwill (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021USD ($)
Goodwill [Line Items]
Balance	$ 274,667
Goodwill acquired during the period	29,570
Balance	304,237
Assessment, Permitting and Response
Goodwill [Line Items]
Balance	162,156
Goodwill acquired during the period	8,944
Balance	171,100
Measurement and Analysis
Goodwill [Line Items]
Balance	69,054
Goodwill acquired during the period	12,879
Balance	81,933
Remediation and Reuse Segment
Goodwill [Line Items]
Balance	43,457
Goodwill acquired during the period	7,747
Balance	$ 51,204

Goodwill and Intangible Asset_3

Goodwill and Intangible Assets - Schedule of Amounts Related to Finite-Lived Intangible Assets (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross Balance	$ 279,578	$ 250,148
Accumulated Amortization	119,339	95,294
Total Intangible Assets—Net	160,239	154,854
Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Gross Balance	189,103	164,782
Accumulated Amortization	67,328	53,446
Total Intangible Assets—Net	$ 121,775	$ 111,336
Customer Relationships | Minimum
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	2 years	7 years
Customer Relationships | Maximum
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	15 years	15 years
Covenants Not to Compete
Finite-Lived Intangible Assets [Line Items]
Gross Balance	$ 31,623	$ 29,942
Accumulated Amortization	24,832	21,469
Total Intangible Assets—Net	$ 6,791	$ 8,473
Covenants Not to Compete | Minimum
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	4 years	4 years
Covenants Not to Compete | Maximum
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	5 years	5 years
Trade Names
Finite-Lived Intangible Assets [Line Items]
Gross Balance	$ 19,778	$ 16,938
Accumulated Amortization	15,107	12,849
Total Intangible Assets—Net	$ 4,671	$ 4,089
Trade Names | Minimum
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	1 year	1 year
Trade Names | Maximum
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	5 years	5 years
Proprietary Software
Finite-Lived Intangible Assets [Line Items]
Gross Balance	$ 21,595	$ 21,007
Accumulated Amortization	9,853	6,132
Total Intangible Assets—Net	$ 11,742	$ 14,875
Proprietary Software | Minimum
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	3 years	3 years
Proprietary Software | Maximum
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	5 years	5 years
Patent
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	16 years	16 years
Gross Balance	$ 17,479	$ 17,479
Accumulated Amortization	2,219	1,398
Total Intangible Assets—Net	$ 15,260	$ 16,081

Goodwill and Intangible Asset_4

Goodwill and Intangible Assets - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Goodwill And Intangible Assets Disclosure [Abstract]
Amortization expense	$ 8.9	$ 7.6	$ 25.9	$ 21

Goodwill and Intangible Asset_5

Goodwill and Intangible Assets - Schedule of Future Amortization Expense (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Goodwill And Intangible Assets Disclosure [Abstract]
2021 (remaining)	$ 8,463
2022	30,298
2023	24,407
2024	20,623
2025	14,225
2026 and thereafter	62,223
Total Intangible Assets—Net	$ 160,239	$ 154,854

Accounts Payable and Other Ac_3

Accounts Payable and Other Accrued Liabilities - Summary of Accounts Payable and Other Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Payables And Accruals [Abstract]
Accounts payable	$ 17,233	$ 15,481
Accrued expenses	15,703	11,469
Other business acquisitions purchase price obligations	274	50
Contract liabilities	8,037	6,114
Other current liabilities	1,126	1,507
Income tax payable		256
Total accounts payable and other accrued liabilities	$ 42,373	$ 34,877

Accrued Payroll and Benefits -

Accrued Payroll and Benefits - Schedule of Accrued Payroll and Benefits (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Accrued bonuses	$ 8,233	$ 5,416
Accrued paid time off	1,990	2,067
Accrued payroll	7,472	9,133
Accrued other	4,790	4,565
Total accrued payroll and benefits	$ 22,485	$ 21,181

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Annual effective tax rate ("ETR") from continuing operations	28.80%	(12.20%)	(2.80%)	2.60%
Federal statutory rate			21.00%
Income tax expense/(benefit)	$ 902,000	$ 3,348,000	$ 648,000	$ (1,563,000)
Uncertain tax positions	$ 0		$ 0

Warrant Options - Additional In

Warrant Options - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	Jul. 30, 2020	Jul. 27, 2020	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Apr. 30, 2020	Apr. 13, 2020	Oct. 19, 2018
Class Of Warrant Or Right [Line Items]
Period between issuance and expiration of outstanding warrant							10 years
Share price			$ 16.36	$ 39.83	$ 16.36
Common Stock
Class Of Warrant Or Right [Line Items]
Exercise of the series A-1 preferred stock warrant, shares			534,240
Redeemable Series A-1 Preferred Stock
Class Of Warrant Or Right [Line Items]
Warrants to purchase common stock								534,240
Warrant exercise price per share								$ 0.01
Period between issuance and expiration of outstanding warrant								10 years
Warrants exercised in full for cash, date				Jul. 30, 2020
Redeemable Series A-1 Preferred Stock | Other Expense
Class Of Warrant Or Right [Line Items]
Fair value gains/(losses) of warrants			$ 5		$ 5
Redeemable Series A-2 Preferred Stock
Class Of Warrant Or Right [Line Items]
Warrants to purchase common stock						1,351,960
Warrant exercise price per share						$ 0.01
Period between issuance and expiration of outstanding warrant						10 years
Aggregate value to determine number of shares underlying the warrant and issuable upon exercise of warrants						$ 30
Convertible And Redeemable Series A-2 Preferred Stock
Class Of Warrant Or Right [Line Items]
Warrants to purchase common stock		1,999,999
Share price		$ 15
Increase of shares		648,039
Redeemable Series A-1 Preferred Stock and Convertible and Redeemable Series A-2 Preferred Stock | Common Stock
Class Of Warrant Or Right [Line Items]
Warrant exercise price per share	$ 0.01
Exercise of the series A-1 preferred stock warrant, shares	2,534,239
Redeemable Series A-1 Preferred Stock and Convertible and Redeemable Series A-2 Preferred Stock | Other Expense
Class Of Warrant Or Right [Line Items]
Fair value gains/(losses) of warrants			$ (14.3)		$ (14.3)

Debt - Schedule of Debt (Detail

Debt - Schedule of Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Line Of Credit Facility [Line Items]
Capital leases	$ 7,815	$ 3,088
Less deferred debt issuance costs	(2,374)	(4,108)
Total debt	209,626	175,904
Less current portion of long-term debt	(8,750)	(5,583)
Long-term debt, less current portion	200,876	170,321
Term Loan Facility
Line Of Credit Facility [Line Items]
Total debt	175,000	173,906
Revolving Line of Credit
Line Of Credit Facility [Line Items]
Total debt	$ 37,000
Equipment Line Of Credit
Line Of Credit Facility [Line Items]
Total debt		$ 3,018

Debt - Additional Information (

Debt - Additional Information (Details) - USD ($)

Sep. 30, 2022

Sep. 30, 2021

Apr. 27, 2021

Oct. 06, 2020

Sep. 30, 2020

Apr. 13, 2020

Oct. 06, 2020

Sep. 30, 2021

Dec. 31, 2020

Dec. 31, 2022

Dec. 31, 2021

May 16, 2019

Mar. 12, 2019

Line Of Credit Facility [Line Items]

Loss on extinguishment

$ (4,052,000)

2021 Credit Facility

Line Of Credit Facility [Line Items]

Line of credit facility, maximum borrowing capacity

$ 300,000,000

Credit facility maturity date

Apr. 27, 2026

Percentage of interest rate adjustment

0.05%

Maximum net leverage ratio

425.00%

Maximum net leverage ratio year two

400.00%

Maximum net leverage ratio year three

375.00%

Minimum fixed charge coverage ratio

125.00%

Consolidated total leverage ratio

280.00%

Percentage of proceeds of debt, subject to customary exceptions

100.00%

Percentage of proceeds of certain dispositions, subject to customary reinvestment rights

100.00%

Percentage of proceeds of insurance or condemnation, subject to customary reinvestment rights

100.00%

Weighted average interest rate

2.10%

2.10%

Incremental Term Loans

Line Of Credit Facility [Line Items]

Line of credit facility, maximum borrowing capacity

$ 150,000,000

2020 Credit Facility

Line Of Credit Facility [Line Items]

Consolidated total leverage ratio

2.70%

Line of credit

$ 225,000,000

Credit facility maturity

2025-04

Debt instrument, repayments term

quarterly

Debt instrument, repayments

$ 1,100,000

$ 500,000

Maximum consolidated leverage ratio

425.00%

425.00%

Minimum consolidated fixed charge coverage ratio

125.00%

Loss on extinguishment

$ 4,100,000

Debt fee paid

1,000,000

Unamortized debt issuance costs

3,100,000

2020 Credit Facility | Forecast

Line Of Credit Facility [Line Items]

Debt instrument, repayments

$ 1,600,000

Maximum consolidated leverage ratio

375.00%

400.00%

Prior Senior Secured Credit Facility | LIBOR Plus

Line Of Credit Facility [Line Items]

Interest rate

1.00%

Prior Senior Secured Credit Facility | Federal Funds Rate Plus

Line Of Credit Facility [Line Items]

Interest rate

0.50%

Equipment Line Of Credit

Line Of Credit Facility [Line Items]

Line of credit facility, maximum borrowing capacity

$ 2,000,000

Canadian Equipment Line Of Credit

Line Of Credit Facility [Line Items]

Line of credit facility, maximum borrowing capacity

$ 1,000,000

Term Loan Facility

Line Of Credit Facility [Line Items]

Line of credit

$ 175,000,000

$ 175,000,000

$ 173,906,000

Term Loan Facility | 2021 Credit Facility

Line Of Credit Facility [Line Items]

Line of credit facility, maximum borrowing capacity

175,000,000

Term Loan Facility | 2020 Credit Facility

Line Of Credit Facility [Line Items]

Line of credit

175,000,000

Term Loan Facility | 2020 Credit Facility | LIBOR Plus

Line Of Credit Facility [Line Items]

Interest rate

4.50%

5.00%

Term Loan Facility | 2020 Credit Facility | Base Rate Plus

Line Of Credit Facility [Line Items]

Interest rate

4.00%

Term Loan Facility | 2020 Credit Facility | LIBOR Floor

Line Of Credit Facility [Line Items]

Interest rate

1.00%

1.00%

Term Loan Facility | Prior Senior Secured Credit Facility

Line Of Credit Facility [Line Items]

Line of credit

50,000,000

50,000,000

Revolving Line of Credit

Line Of Credit Facility [Line Items]

Line of credit

37,000,000

37,000,000

Revolving Line of Credit | 2021 Credit Facility

Line Of Credit Facility [Line Items]

Line of credit facility, maximum borrowing capacity

125,000,000

Revolving Line of Credit | 2020 Credit Facility

Line Of Credit Facility [Line Items]

Line of credit

$ 50,000,000

Line of credit facility unused commitment fee

0.35%

Revolving Line of Credit | 2020 Credit Facility | LIBOR Plus

Line Of Credit Facility [Line Items]

Interest rate

3.50%

Revolving Line of Credit | 2020 Credit Facility | Base Rate Plus

Line Of Credit Facility [Line Items]

Interest rate

2.50%

Revolving Line of Credit | Prior Senior Secured Credit Facility

Line Of Credit Facility [Line Items]

Line of credit

$ 130,000,000

$ 130,000,000

Revolving Line of Credit | Letter of Credit | 2021 Credit Facility

Line Of Credit Facility [Line Items]

Line of credit facility, maximum borrowing capacity

$ 20,000,000

Debt - Summary of Term Loan Amo

Debt - Summary of Term Loan Amortization (Details) - 2021 Credit Facility - Term Loan Facility	Apr. 27, 2021
Subsequent Event [Line Items]
Amortization percentage, Year 1	5.00%
Amortization percentage, Year 2	5.00%
Amortization percentage, Year 3	7.50%
Amortization percentage, Year 4	7.50%
Amortization percentage, Year 5	10.00%

Debt - Summary of 2021 Credit F

Debt - Summary of 2021 Credit Facility Interest Rate Subject to Leverage Ratio and LIBOR (Details) - 2021 Credit Facility	Apr. 27, 2021
Pricing Tier 1
Subsequent Event [Line Items]
Commitment Fee	0.25%
Letter of Credit Fee	2.50%
Pricing Tier 1 | Minimum
Subsequent Event [Line Items]
Net Leverage Ratio	375.00%
Pricing Tier 1 | LIBOR
Subsequent Event [Line Items]
Interest rate	2.50%
Pricing Tier 1 | Base Rate
Subsequent Event [Line Items]
Interest rate	1.50%
Pricing Tier 2
Subsequent Event [Line Items]
Commitment Fee	0.23%
Letter of Credit Fee	2.25%
Pricing Tier 2 | Maximum
Subsequent Event [Line Items]
Net Leverage Ratio	375.00%
Pricing Tier 2 | Minimum
Subsequent Event [Line Items]
Net Leverage Ratio	325.00%
Pricing Tier 2 | LIBOR
Subsequent Event [Line Items]
Interest rate	2.25%
Pricing Tier 2 | Base Rate
Subsequent Event [Line Items]
Interest rate	1.25%
Pricing Tier 3
Subsequent Event [Line Items]
Commitment Fee	0.20%
Letter of Credit Fee	2.00%
Pricing Tier 3 | Maximum
Subsequent Event [Line Items]
Net Leverage Ratio	325.00%
Pricing Tier 3 | Minimum
Subsequent Event [Line Items]
Net Leverage Ratio	250.00%
Pricing Tier 3 | LIBOR
Subsequent Event [Line Items]
Interest rate	2.00%
Pricing Tier 3 | Base Rate
Subsequent Event [Line Items]
Interest rate	1.00%
Pricing Tier 4
Subsequent Event [Line Items]
Commitment Fee	0.15%
Letter of Credit Fee	1.75%
Pricing Tier 4 | Maximum
Subsequent Event [Line Items]
Net Leverage Ratio	250.00%
Pricing Tier 4 | Minimum
Subsequent Event [Line Items]
Net Leverage Ratio	175.00%
Pricing Tier 4 | LIBOR
Subsequent Event [Line Items]
Interest rate	1.75%
Pricing Tier 4 | Base Rate
Subsequent Event [Line Items]
Interest rate	0.75%
Pricing Tier 5
Subsequent Event [Line Items]
Commitment Fee	0.15%
Letter of Credit Fee	1.50%
Pricing Tier 5 | Maximum
Subsequent Event [Line Items]
Net Leverage Ratio	175.00%
Pricing Tier 5 | LIBOR
Subsequent Event [Line Items]
Interest rate	1.50%
Pricing Tier 5 | Base Rate
Subsequent Event [Line Items]
Interest rate	0.50%

Debt - Schedule of Applicable P

Debt - Schedule of Applicable Percentages (Detail) - Prior Senior Secured Credit Facility	9 Months Ended
	Sep. 30, 2021
Pricing Tier 1
Line Of Credit Facility [Line Items]
Commitment Fee	0.50%
Pricing Tier 1 | Minimum
Line Of Credit Facility [Line Items]
Consolidated total leverage ratio	375.00%
Pricing Tier 1 | Eurodollar Rate Loans and LIBOR Letter of Credit Fee
Line Of Credit Facility [Line Items]
Interest rate	4.00%
Pricing Tier 1 | Daily Floating Rate Loans
Line Of Credit Facility [Line Items]
Interest rate	4.00%
Pricing Tier 1 | Rate Loans
Line Of Credit Facility [Line Items]
Interest rate	3.00%
Pricing Tier 2
Line Of Credit Facility [Line Items]
Commitment Fee	0.50%
Pricing Tier 2 | Maximum
Line Of Credit Facility [Line Items]
Consolidated total leverage ratio	375.00%
Pricing Tier 2 | Minimum
Line Of Credit Facility [Line Items]
Consolidated total leverage ratio	300.00%
Pricing Tier 2 | Eurodollar Rate Loans and LIBOR Letter of Credit Fee
Line Of Credit Facility [Line Items]
Interest rate	3.50%
Pricing Tier 2 | Daily Floating Rate Loans
Line Of Credit Facility [Line Items]
Interest rate	3.50%
Pricing Tier 2 | Rate Loans
Line Of Credit Facility [Line Items]
Interest rate	2.50%
Pricing Tier 3
Line Of Credit Facility [Line Items]
Commitment Fee	0.40%
Pricing Tier 3 | Maximum
Line Of Credit Facility [Line Items]
Consolidated total leverage ratio	300.00%
Pricing Tier 3 | Minimum
Line Of Credit Facility [Line Items]
Consolidated total leverage ratio	225.00%
Pricing Tier 3 | Eurodollar Rate Loans and LIBOR Letter of Credit Fee
Line Of Credit Facility [Line Items]
Interest rate	3.00%
Pricing Tier 3 | Daily Floating Rate Loans
Line Of Credit Facility [Line Items]
Interest rate	3.00%
Pricing Tier 3 | Rate Loans
Line Of Credit Facility [Line Items]
Interest rate	2.00%
Pricing Tier 4
Line Of Credit Facility [Line Items]
Commitment Fee	0.30%
Pricing Tier 4 | Maximum
Line Of Credit Facility [Line Items]
Consolidated total leverage ratio	225.00%
Pricing Tier 4 | Eurodollar Rate Loans and LIBOR Letter of Credit Fee
Line Of Credit Facility [Line Items]
Interest rate	2.50%
Pricing Tier 4 | Daily Floating Rate Loans
Line Of Credit Facility [Line Items]
Interest rate	2.50%
Pricing Tier 4 | Rate Loans
Line Of Credit Facility [Line Items]
Interest rate	1.50%

Debt - Schedule of Aggregate An

Debt - Schedule of Aggregate Annual Maturities of Long-Term Debt (Details) $ in Thousands	Sep. 30, 2021USD ($)
Debt Disclosure [Abstract]
2022	$ 8,750
2023	10,938
2024	13,125
2025	15,313
2026	163,874
Total	$ 212,000

Fair Value of Financial Instr_3

Fair Value of Financial Instruments - Summary of Financial Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Recurring - Level 3 - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Level 3 Liabilities	$ 57,889	$ 75,353	$ 76,971	$ 32,971
Business Acquisitions Contingent Consideration, Current
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Level 3 Liabilities	31,152	49,902	49,170	8,614
Business Acquisitions Contingent Consideration, Long-Term
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Level 3 Liabilities	4,200	4,565	$ 9,742	$ 379
Conversion Option
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Level 3 Liabilities	$ 22,537	$ 20,886

Fair Value of Financial Instr_4

Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - Fair Value, Recurring - Level 3 - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance	$ 75,353	$ 32,971
Series A-2 compound embedded option		(9,361)
Issuance of warrant option		30,097
Acquisitions	7,004	45,204
Changes in fair value included in earnings	25,686	34,879
Payment of contingent consideration payable	(50,154)	(12,464)
Foreign currency translation of contingent consideration payment		(208)
Write off of the contingent put option		12,140
Exercise of warrant options		(56,287)
Ending balance	57,889	76,971
Contingent Put Option
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance		7,100
Changes in fair value included in earnings		(19,240)
Write off of the contingent put option		12,140
Compound Embedded Option
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Series A-2 compound embedded option		(9,361)
Changes in fair value included in earnings		27,420
Ending balance		18,059
Business Acquisitions Contingent Consideration, Current
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance	49,902	8,614
Acquisitions	1,000	34,661
Changes in fair value included in earnings	13,774	18,387
Payment of contingent consideration payable	(50,154)	(12,464)
Foreign currency translation of contingent consideration payment		(208)
Reclass of long term to short term contingent liabilities	16,630	180
Ending balance	31,152	49,170
Business Acquisitions Contingent Consideration, Long-Term
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance	4,565	379
Acquisitions	6,004	10,543
Changes in fair value included in earnings	10,261	(1,000)
Reclass of long term to short term contingent liabilities	(16,630)	(180)
Ending balance	4,200	9,742
Conversion Option
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance	20,886
Changes in fair value included in earnings	1,651
Ending balance	$ 22,537
Warrant Option
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance		16,878
Issuance of warrant option		30,097
Changes in fair value included in earnings		9,312
Exercise of warrant options		$ (56,287)

Fair Value of Financial Instr_5

Fair Value of Financial Instruments - Summary of Fair Value of Warrant Options Associated with Issuance of Redeemable Preferred Stock (Details)	Jul. 30, 2020	Apr. 13, 2020	Oct. 19, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option		10 years
Redeemable Series A-1 Preferred Stock
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option			10 years
Common Stock Value (per Share) | Redeemable Series A-1 Preferred Stock
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option	22.22
Expected Volatility | Redeemable Series A-1 Preferred Stock
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option	0.4435
Risk-free Interest Rate | Redeemable Series A-1 Preferred Stock
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option	0.0055
Expected Life (Years) | Redeemable Series A-1 Preferred Stock
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option	10 years

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	9 Months Ended
	Sep. 30, 2021
Commitments And Contingencies [Line Items]
Lessee operating lease, expiration year	2030
Office Equipment
Commitments And Contingencies [Line Items]
Lessee operating lease, expiration year	2026

Redeemable Series A-1 Preferr_2

Redeemable Series A-1 Preferred Stock - Additional Information (Details) - USD ($)	Apr. 13, 2020	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Oct. 19, 2018
Temporary Equity [Line Items]
Share price		$ 16.36	$ 39.83	$ 16.36
Percentage of dividends paid in cash	14.20%
Redeemable Series A-1 Preferred Stock
Temporary Equity [Line Items]
Temporary equity, shares issued					12,000
Temporary equity, par value per share					$ 0.0001
Warrants to purchase common stock					534,240
Temporary equity, liquidation preference per share					$ 10,000
Temporary equity, maturity month and year	2024-10
Temporary equity, penalty term	3 years
Temporary equity, penalty term upon full exercise of warrant	2 years
Percentage of increase in accrued dividend	1.00%
Temporary equity, covenant description			The Redeemable Series A-1 Preferred Stock contained restrictive covenants. Prior to its redemption, the Company was subject to a consolidated total leverage ratio (including the outstanding principal and accrued dividend on the Redeemable Series A-1 Preferred Stock) limit of less than 10.0 times as of the end of any fiscal quarter ending until maturity.
Percentage of dividends paid in cash			15.00%
Percentage of dividends accrued			14.20%
Fair value changes in the contingent put option		$ 26,300,000		$ 19,200,000
Redeemable Series A-1 Preferred Stock | Maximum
Temporary Equity [Line Items]
Temporary equity, aggregate amount of redemption requirement	$ 50,000,000
Percentage of accumulated dividends payable in common stock upon IPO	50.00%
Share price	$ 31.60
Percentage of dividend payable in cash	15.00%
Redeemable Series A-1 Preferred Stock | Minimum
Temporary Equity [Line Items]
Percentage of dividend payable in cash	9.00%

Convertible and Redeemable Se_2

Convertible and Redeemable Series A-2 Preferred Stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Apr. 13, 2020	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Temporary Equity [Line Items]
Period between issuance and expiration of outstanding warrant	10 years
Proceeds from the Series A-2 and Warrant	$ 175,000
Percentage of dividends accrued	14.20%
Preferred stock, dividends paid		$ 4,100	$ 4,100	$ 4,100	$ 2,870
Compound embedded derivative, change in value						$ (1,651)	$ (17,492)
Convertible And Redeemable Series A-2 Preferred Stock
Temporary Equity [Line Items]
Number of shares issued	17,500	17,500				17,500		17,500
Par value per share	$ 0.0001	$ 0.0001				$ 0.0001		$ 0.0001
Debt issuance costs, net	$ 1,300
Percentage of dividends accrued	15.00%
Preferred stock, dividends paid		$ 4,100			$ 2,900	$ 12,300	$ 2,900
Percentage of discount on common stock market price						15.00%
Percentage of dividend rate stepped downs per year						9.00%
Percentage of dividend rate increase per annum in the event of noncompliance						12.00%
Number of days dividend increase rate applicable noncompliance event occurred						90 days
Percentage of dividend rate increase per annum upon noncompliance occurred and thereafter						14.00%
Number of times increase in debt incurrence test ratio						4.5
Minimum repayment amount						$ 50,000
Minimum repayment amount drop down						$ 25,000
Temporary equity description						The Company may, at its option on any one or more dates, redeem all or a minimum portion (the lesser of (i) $25.0 million in aggregate stated value of the Convertible and Redeemable Series A-2 Preferred Stock and (ii) all of the Convertible and Redeemable Series A-2 Preferred Stock then outstanding) of the outstanding Convertible and Redeemable Series A-2 Preferred Stock in cash.
Aggregate stated value of stock redeemed		25,000				$ 25,000
Aggregate liquidation preference		182,200				182,200		$ 182,200
Compound embedded derivative, fair value net		22,500				22,500
Convertible And Redeemable Series A-2 Preferred Stock | Other Expense
Temporary Equity [Line Items]
Compound embedded derivative, change in value		500				1,700
Convertible And Redeemable Series A-2 Preferred Stock | 60-Day Period Prior to Seventh Anniversary
Temporary Equity [Line Items]
Temporary equity convertible into common stock		60,000				60,000
Convertible And Redeemable Series A-2 Preferred Stock | Year 5
Temporary Equity [Line Items]
Temporary equity convertible into common stock		60,000				60,000
Convertible And Redeemable Series A-2 Preferred Stock | Year 6
Temporary Equity [Line Items]
Temporary equity convertible into common stock		$ 120,000				$ 120,000

Stockholder's Equity (Deficit_2

Stockholder's Equity (Deficit) - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	May 31, 2015
Class Of Stock [Line Items]
Common stock, shares authorized	190,000,000		190,000,000		190,000,000
Common stock, par value	$ 0.000004		$ 0.000004		$ 0.000004
Stock issued on exercise of warrants		2,534,239	67,713	2,534,239
Total unrecognized stock compensation expense related to unvested options and restricted stock granted under the Plans	$ 15	$ 10.4	$ 15	$ 10.4
Unrecognized expense expected to be recognized period			3 years
Montrose Amended & Restated 2017 Stock Incentive Plan
Class Of Stock [Line Items]
Restricted shares outstanding	286,239	267,173	286,239	267,173
Restricted shares forfeited	0	0	0	1,667
Montrose Amended & Restated 2017 Stock Incentive Plan | Restricted Stock
Class Of Stock [Line Items]
Restricted shares became fully vested and released as common stock	0	0	38,929	10,000
Montrose Amended & Restated 2017 Stock Incentive Plan | Stock Options | Board of Directors
Class Of Stock [Line Items]
Shares of vested over period			4 years
Montrose Amended & Restated 2017 Stock Incentive Plan | Stock Options | Executive Officers
Class Of Stock [Line Items]
Shares of vested over period			3 years
Vesting frequency of period			1 year
Montrose Amended & Restated 2017 Stock Incentive Plan | Stock Options | Second Anniversary | Board of Directors
Class Of Stock [Line Items]
Award vesting percentage			50.00%
Montrose Amended & Restated 2017 Stock Incentive Plan | Stock Options | Fourth Anniversary | Board of Directors
Class Of Stock [Line Items]
Award vesting percentage			50.00%
Montrose Amended and Restated 2013 Stock Option Plan
Class Of Stock [Line Items]
Shares outstanding from exercised options	1,028,268	221,950	1,028,268	221,950
Private Offering
Class Of Stock [Line Items]
Warrants to purchase common stock						116,350
Warrant exercise price per share						$ 17.19

Stockholder's Equity (Deficit_3

Stockholder's Equity (Deficit) - Summary of Issued Shares of Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended					9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Equity [Abstract]
Common stock issued in connection initial public offering, Shares				11,500,000			11,500,000
Acquisitions, Shares	62,738					143,800	791,139
Redemption of the series A-1, shares				1,786,739			1,786,739
Exercise of warrant options, Shares				2,534,239		67,713	2,534,239
Exercise of options, Shares	(354,918)			(16,850)		(779,068)	(20,350)
Restricted shares, net, Shares				8,333		38,929	8,333
Payment of earn-out liability						539,607
Payment of purchase price true up						24,200
Cancellation of shares				(117,785)			(117,785)
Total, Shares	417,656			15,728,376		1,593,317	16,523,015
Common stock issued in connection initial public offering, Average Price per Share				$ 15			$ 15
Acquisitions, Average Price per Share	$ 52.18					$ 41.86	31.60
Redemption of series A-1 preferred stock, Average Price per Share				15			15
Exercise of warrant options, Average Price per Share				0.01		17.19	0.01
Exercise of options, Average Price per Share	8.30			8.92		7.74	9.27
Restricted shares, net, Average Price per Share				24		31.31	24
Payment of earn-out liability						46.33
Payment of purchase price true up						44.81
Cancellation of shares, Average Price per Share				17.15			17.15
Total, Average Price per Share	$ 14.89			$ 12.82		$ 25.43	$ 13.72
Issuance of common stock in connection with the initial public offering, net of issuance costs of $15.6 million				$ 156,881
Acquisitions, Total Consideration	$ 3,274					$ 6,020	$ 25,000
Redemption of the series A-1				26,801			26,801
Exercise of warrant options, Total Consideration				25			25
Exercise of options, Total Consideration	2,946			150		6,032	189
Payment of earn-out liability						25,000
Payment of purchase price true up						1,084
Total Consideration	$ 6,220	$ 27,460	$ 4,456	150	$ 25,021	38,136	224,515
Common stock issued in connection initial public offering, Total Consideration				172,500			172,500
Redemption of series A-1 preferred stock, Total Consideration				26,801			$ 26,801
Payment of earn-out liability						25,000
Payment of purchase price true up						$ 1,084
Total Consideration				$ 199,476

Stockholder's Equity (Deficit_4

Stockholder's Equity (Deficit) - Summary of Number of Shares Authorized to be Issued and Available for Grant (Details) - shares	Sep. 30, 2021	Sep. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized to be issued	5,992,019	4,992,712
Shares available for grant	1,593,287	1,233,266
2017 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized to be issued	3,944,750	2,945,443
Shares available for grant	1,593,287	1,233,266
2013 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized to be issued	2,047,269	2,047,269

Stockholder's Equity (Deficit_5

Stockholder's Equity (Deficit) - Schedule of Stock Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	$ 2,365	$ 1,149	$ 6,587	$ 3,439
2017 Plan | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	2,126	868	5,816	2,151
2017 Plan | Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	237	233	749	973
2013 Plan | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	2	48	21	315
Cost of Revenue
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	282	271	1,188	1,099
Cost of Revenue | 2017 Plan | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	282	276	1,178	973
Cost of Revenue | 2013 Plan | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense		(5)	10	126
Selling, General and Administrative Expenses
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	2,083	878	5,399	2,340
Selling, General and Administrative Expenses | 2017 Plan | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	1,844	592	4,638	1,178
Selling, General and Administrative Expenses | 2017 Plan | Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	237	233	749	973
Selling, General and Administrative Expenses | 2013 Plan | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	$ 2	$ 53	$ 11	$ 189

Stockholder's Equity (Deficit_6

Stockholder's Equity (Deficit) - Schedule of Restricted Stock Activity (Details) - $ / shares	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Average Price per Share	$ 24	$ 31.31	$ 24
Montrose Amended & Restated 2017 Stock Incentive Plan | Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares		19,066	33,229
Average Price per Share		$ 36.99	$ 31.60
Total (in thousands)		705,000	1,050,000

Stockholder's Equity (Deficit_7

Stockholder's Equity (Deficit) - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Exercise of options, Shares	(354,918)	(16,850)	(779,068)	(20,350)
Montrose Amended & Restated 2017 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Outstanding Beginning Balance			1,840,229	617,852	617,852
Options to Purchase Common Stock Granted			264,470	851,877
Options to Purchase Common Stock Forfeited/cancelled			(29,025)	(14,375)
Options to Purchase Common Stock Expired			(1,250)
Exercise of options, Shares			(66,875)	(925)
Options to Purchase Common Stock Outstanding Ending Balance	2,007,549	1,454,429	2,007,549	1,454,429	1,840,229	617,852
Options to Purchase Common Stock Exercisable	320,861		320,861
Options to Purchase Common Stock Options vested and expected to vest	2,007,549		2,007,549
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-Average Exercise Price per Share Outstanding Beginning Balance			$ 23	$ 24	$ 24
Weighted-Average Exercise Price per Share Granted			40	32
Weighted-Average Exercise Price per Share Forfeited/cancelled			28	22
Weighted-Average Exercise Price per Share Expired			18
Weighted-Average Exercise Price per Share Exercised			22	14
Weighted-Average Exercise Price per Share Outstanding Ending Balance	$ 25	$ 21	25	21	23	$ 24
Weighted-Average Exercise Price per Share Exercisable	28		28
Weighted-Average Exercise Price per Share Options vested and expected to vest	25		25
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Weighted Average Grant Date Fair Value per Share Outstanding Beginning Balance			12	12	12
Weighted Average Grant Date Fair Value per Share Granted			22	12
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 14	$ 12	$ 14	$ 12	$ 12	$ 12
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life (in Years) Outstanding			8 years 6 months 10 days	7 years 7 months 2 days	9 years 1 month 2 days	7 years 9 months 25 days
Weighted Average Remaining Contract Life (in Years) Exercisable			7 years 8 months 4 days
Weighted Average Remaining Contract Life (in Years) Options vested and expected to vest			8 years 6 months 10 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 73,724	$ 7,649	$ 73,724	$ 7,649	$ 15,598	$ 4,693
Aggregate Intrinsic Value of In-The-Money, Options Exercised			2,078	$ 7,918
Aggregate Intrinsic Value of In-The-Money Options Exercisable	10,755		10,755
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 73,724		$ 73,724
Montrose Amended and Restated 2013 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Outstanding Beginning Balance			1,787,869	1,855,469	1,855,469
Options to Purchase Common Stock Forfeited/cancelled				(7,500)
Options to Purchase Common Stock Expired			(625)	(11,300)
Exercise of options, Shares			(710,568)	(19,175)
Options to Purchase Common Stock Outstanding Ending Balance	1,076,676	1,817,494	1,076,676	1,817,494	1,787,869	1,855,469
Options to Purchase Common Stock Exercisable	1,076,676		1,076,676
Options to Purchase Common Stock Options vested and expected to vest	1,076,676		1,076,676
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-Average Exercise Price per Share Outstanding Beginning Balance			$ 6	$ 6	$ 6
Weighted-Average Exercise Price per Share Forfeited/cancelled				10
Weighted-Average Exercise Price per Share Expired			6	6
Weighted-Average Exercise Price per Share Exercised			6	8
Weighted-Average Exercise Price per Share Outstanding Ending Balance	$ 6	$ 6	6	6	6	$ 6
Weighted-Average Exercise Price per Share Exercisable	6		6
Weighted-Average Exercise Price per Share Options vested and expected to vest	6		6
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Weighted Average Grant Date Fair Value per Share Outstanding Beginning Balance			1	1	1
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 2	$ 1	$ 2	$ 1	$ 1	$ 1
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life (in Years) Outstanding			4 years 7 months 24 days	5 years 7 months 17 days	5 years 4 months 24 days	6 years 4 months 24 days
Weighted Average Remaining Contract Life (in Years) Exercisable			4 years 7 months 24 days
Weighted Average Remaining Contract Life (in Years) Options vested and expected to vest			4 years 7 months 24 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 59,532	$ 33,772	$ 59,532	$ 33,772	$ 43,867	$ 46,617
Aggregate Intrinsic Value of In-The-Money, Options Exercised			33,480	$ 354
Aggregate Intrinsic Value of In-The-Money Options Exercisable	59,532		59,532
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 59,532		$ 59,532

Stockholder's Equity (Deficit_8

Stockholder's Equity (Deficit) - Summary of Weighted Average Assumptions Used in Black-Sholes Option-pricing Model (Details) - $ / shares	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock value (per share)	$ 39.83	$ 16.36
Expected volatility	58.34%	46.14%
Risk-free interest rate	0.77%	0.41%
Expected life (years)		7 years
Forfeiture rate	0.00%	0.00%
Dividend rate	0.00%	0.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected life (years)	5 years 6 months
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected life (years)	7 years

Stockholder's Equity (Deficit_9

Stockholder's Equity (Deficit) - Schedule of Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2021	Sep. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	5,992,019	5,109,062
Warrants
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance		116,350
Montrose 2013 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	2,047,269	2,047,269
Montrose 2017 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	3,944,750	2,945,443

Net Income (Loss) Per Share - C

Net Income (Loss) Per Share - Computation of Basic and Diluted Net (Loss) Income Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net income (loss)	$ 2,226	$ (14,483)	$ (11,596)	$ (30,737)	$ 13,224	$ (41,248)	$ (23,853)	$ (58,761)
Accretion of redeemable series A-1 preferred stock				(6,542)				(17,601)
Redeemable series A-1 preferred stock deemed dividend				(24,341)				(24,341)
Convertible and redeemable series A-2 preferred stock dividend	(4,100)			(2,870)			(12,300)	(2,870)
Net loss attributable to common stockholders –basic and diluted	$ (1,874)			$ (64,490)			$ (36,153)	$ (103,573)
Weighted-average common shares outstanding – basic and diluted	26,220			21,554			25,798	13,669
NET LOSS PER SHARE ATTRIBUTABLE TO COMMON STOCKHOLDERS— BASIC AND DILUTED	$ (0.07)			$ (2.99)			$ (1.40)	$ (7.58)

Net Income (Loss) Per Share - E

Net Income (Loss) Per Share - Equity Shares Excluded from Calculation of Diluted Net (Loss) Income Per Share Attributable to Common Stockholders (Details) - shares	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of loss per share amount	1,686,688	2,423,900
Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of loss per share amount	28,623	176,384
Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of loss per share amount		116,350

Segment Information - Additiona

Segment Information - Additional Information (Details)	9 Months Ended
	Sep. 30, 2021Segment
Segment Reporting [Abstract]
Number of operating segments	3
Number of reportable segments	3

Segment Information - Component

Segment Information - Components of Segment Revenues and Adjusted EBITDA (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Segment Revenues	$ 132,578	$ 84,705	$ 402,619	$ 219,502
Segment Adjusted EBITDA	21,487	16,706	59,249	36,157
Operating Segments
Segment Reporting Information [Line Items]
Segment Revenues	132,578	84,705	402,619	219,502
Segment Adjusted EBITDA	29,474	21,468	81,275	51,582
Assessment, Permitting and Response | Operating Segments
Segment Reporting Information [Line Items]
Segment Revenues	63,412	26,593	209,379	49,768
Segment Adjusted EBITDA	15,719	8,230	46,379	14,665
Measurement and Analysis | Operating Segments
Segment Reporting Information [Line Items]
Segment Revenues	38,752	39,770	111,309	113,130
Segment Adjusted EBITDA	8,283	11,226	22,634	30,422
Remediation and Reuse | Operating Segments
Segment Reporting Information [Line Items]
Segment Revenues	30,414	18,342	81,931	56,604
Segment Adjusted EBITDA	5,472	2,012	12,262	6,495
Corporate and Other
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	$ (7,987)	$ (4,762)	$ (22,026)	$ (15,425)

Segment Information - Reconcili

Segment Information - Reconciliation of Segment Measure to Income (Loss) Before (Expense) Benefit from Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting [Abstract]
Total	$ 21,487	$ 16,706	$ 59,249	$ 36,157
Interest expense, net	(1,722)	(3,043)	(11,208)	(10,896)
Income tax (expense) benefit	(902)	(3,348)	(648)	1,563
Depreciation and amortization	(11,471)	(9,740)	(33,145)	(27,084)
Stock-based compensation	(2,365)	(1,149)	(6,587)	(3,439)
Start-up losses and investment in new services	(1,186)	(602)	(3,276)	(1,283)
Acquisition costs	(913)	(6)	(1,656)	(3,767)
Fair value changes in financial instruments	(531)	(9,710)	(1,651)	(17,492)
Fair value changes in business acquisitions contingent consideration		(13,404)	(24,035)	(17,387)
Short term purchase accounting fair value adjustment to deferred revenue				(243)
Initial public offering expense and related bonuses		(6,378)		(6,908)
Discontinued services		(30)		(7,526)
Expenses related to financing transactions			(50)	(277)
Other losses or expenses	(171)	(33)	(846)	(179)
Net income (loss)	$ 2,226	$ (30,737)	$ (23,853)	$ (58,761)

Related-Party Transactions - Ad

Related-Party Transactions - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Related Party Transaction [Line Items]
Amount due to related party	$ 0		$ 0		$ 0
Number of shares issued as dividend payment					1,786,739
Number of additional common stock outstanding owned by related party					2,534,239
Acquisition Targets | Selling, General and Administrative Expenses
Related Party Transaction [Line Items]
Payment to related party	$ 0	$ 0	$ 0	$ 100,000

Defined Contribution Plan - Add

Defined Contribution Plan - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		5 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	May 22, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, description				401(k) Savings Plan
Defined contribution plan, maximum annual contributions per employee, percent				85.00%		85.00%
Defined contribution plan, employer matching contribution, percent of match			100.00%
Defined contribution plan participant's compensation percent			3.00%
Defined contribution plan percentage of participant's elective deferrals			50.00%
Selling, General and Administrative Expenses
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, employer discretionary contribution amount	$ 0.8	$ 0		$ 1.6	$ 1.2
Minimum
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan participant's compensation percent			3.00%
Maximum
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan participant's compensation percent			4.00%

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - Subsequent Event - USD ($) $ / shares in Units, $ in Millions	Oct. 05, 2021	Oct. 31, 2021	Nov. 01, 2021	Oct. 01, 2021
ECI
Subsequent Event [Line Items]
Percentage of interests acquired				100.00%
Horizon
Subsequent Event [Line Items]
Percentage of interests acquired			100.00%
Follow-on Offering
Subsequent Event [Line Items]
Number of shares issued		2,500,000
Offer price per share		$ 62
Underwriters option to purchase additional shares		375,000
Net proceeds	$ 169.8

Subsequent Events - Schedule of

Subsequent Events - Schedule of Elements of Purchase Price of Acquisitions (Details) - USD ($) $ in Thousands	Nov. 01, 2021	Oct. 01, 2021	Sep. 30, 2021	Dec. 31, 2020
Subsequent Event [Line Items]
Cash			$ 40,004	$ 175,450
Common Stock			6,020	25,000
Total Purchase Price			$ 68,856	$ 243,865
ECI | Subsequent Event
Subsequent Event [Line Items]
Cash		$ 4,000
Total Purchase Price		$ 4,000
Horizon | Subsequent Event
Subsequent Event [Line Items]
Cash	$ 10,580
Common Stock	2,300
Total Purchase Price	$ 12,880

Subsequent Events - Schedule _2

Subsequent Events - Schedule of Elements of Purchase Price of Acquisitions (Parenthetical) (Details) - shares	Nov. 01, 2021	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2020
Subsequent Event [Line Items]
Number of common stock shares issued in connection with business acquisition		62,738	143,800	791,139
Horizon | Subsequent Event
Subsequent Event [Line Items]
Number of common stock shares issued in connection with business acquisition	34,921