Montrose Environmental (MEG) S-1 23 Nov 20 IPO registration Financial data

Cover

Cover	9 Months Ended
	Sep. 30, 2020
Cover [Abstract]
Document Type	S-1
Amendment Flag	false
Entity Registrant Name	Montrose Environmental Group, Inc.
Entity Central Index Key	0001643615
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false

CONSOLIDATED STATEMENTS OF FINA

CONSOLIDATED STATEMENTS OF FINANCIAL POSITION - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
CURRENT ASSETS:
Cash		$ 6,411	$ 2,489
Cash and restricted cash	$ 38,377	6,884
Restricted cash		473
Accounts receivable-net	44,785	45,927	38,468
Contract assets	35,441	13,605	10,112
Prepaid and other current assets	8,585	6,823	2,598
Income tax receivable	1,182	1,205	332
Total current assets	127,188	73,239	53,999
NON-CURRENT ASSETS:
Property and equipment-net	34,559	27,036	20,792
Goodwill	274,309	127,058	90,498
Other intangible assets-net	162,201	102,549	68,563
Other assets	4,081	1,956	519
TOTAL ASSETS	602,338	331,838	234,371
CURRENT LIABILITIES:
Accounts payable and other accrued liabilities	33,897	29,585	19,008
Accounts payable to related party			70
Accrued payroll and benefits	17,957	11,032	8,207
Warrant option		16,878	12,818
Business acquisitions contingent consideration, current	49,170	8,614
Current portion of long-term debt	5,034	7,143	2,262
Total current liabilities	106,058	73,252	42,365
Total liabilities	310,643	229,307	118,265
NON-CURRENT LIABILITIES:
Business acquisitions contingent consideration, long-term	9,742	379
Other non-current liabilities	3,431	379	67
Deferred tax liabilities-net	1,936	3,530	4,868
Conversion option	18,059
Long-term debt-net of deferred financing fees	171,417	145,046	70,965
Total liabilities	310,643	229,307	118,265
COMMITMENTS AND CONTINGENCIES
STOCKHOLDERS' (DEFICIT) EQUITY:
Common stock	0	0	0
Additional paid-in capital	261,945	38,153	47,869
Accumulated deficit	(123,165)	(64,404)	(40,847)
Notes receivable from stockholders			(122)
Accumulated other comprehensive loss	(13)	(40)
Total stockholders' (deficit) equity	138,767	(26,291)	6,900
TOTAL LIABILITIES, REDEEMABLE SERIES A-1 PREFERRED STOCK AND STOCKHOLDERS' (DEFICIT) EQUITY	602,338	331,838	234,371
Redeemable Series A-1 Preferred Stock
NON-CURRENT LIABILITIES:
Preferred stock		128,822	$ 109,206
Convertible And Redeemable Series A-2 Preferred Stock
NON-CURRENT LIABILITIES:
Preferred stock	$ 152,928
Previously Reported
CURRENT ASSETS:
Prepaid and other current assets		5,618
CURRENT LIABILITIES:
Accounts payable and other accrued liabilities		38,199
Contingent Put Option
NON-CURRENT LIABILITIES:
Contingent put option		$ 7,100

CONSOLIDATED STATEMENTS OF FI_2

CONSOLIDATED STATEMENTS OF FINANCIAL POSITION (Parenthetical) - USD ($)	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Common stock, par value	$ 0.000004	$ 0.000004	$ 0.000004
Common stock, shares authorized	190,000,000	25,000,000	25,000,000
Common stock, shares issued	24,893,122	8,370,107	8,137,771
Common stock, shares outstanding	24,893,122	8,370,107	8,137,771
Redeemable Series A-1 Preferred Stock
Temporary equity, par value	$ 0.0001	$ 0.0001	$ 0.0001
Temporary equity, shares authorized	0	12,000	12,000
Temporary equity, shares issued	0	12,000	12,000
Temporary equity, shares outstanding	0	12,000	12,000
Temporary equity, aggregate liquidation preference	$ 0	$ 141,898	$ 123,417
Convertible And Redeemable Series A-2 Preferred Stock
Temporary equity, par value	$ 0.0001	$ 0.0001
Temporary equity, shares authorized	17,500	0
Temporary equity, shares issued	17,500	0
Temporary equity, shares outstanding	17,500	0
Temporary equity, aggregate liquidation preference	$ 182,200,000	$ 0

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
REVENUES	$ 84,705	$ 57,623	$ 219,502	$ 165,978	$ 233,854	$ 188,805
COST OF REVENUES (exclusive of depreciation and amortization shown below)	51,828	39,804	142,115	116,248	163,983	134,734
SELLING, GENERAL AND ADMINISTRATIVE EXPENSE	24,442	12,656	64,810	35,464	50,663	38,615
FAIR VALUE CHANGES IN BUSINESS ACQUISITIONS CONTINGENT CONSIDERATION	13,404	256	17,387	(670)	1,392	(158)
RELATED- PARTY EXPENSE					448	2,180
DEPRECIATION AND AMORTIZATION	9,740	7,412	27,084	20,262	27,705	23,915
LOSS FROM OPERATIONS	(14,709)	(2,505)	(31,894)	(5,326)	(8,945)	(10,639)
OTHER EXPENSE
Other (expense) income	(9,637)	(2,460)	(17,534)	(3,639)	(10,978)	265
Interest expense-net	(3,043)	(2,130)	(10,896)	(4,590)	(6,755)	(11,085)
Total other expenses-net	(12,680)	(4,590)	(28,430)	(8,229)	(17,733)	(10,820)
LOSS BEFORE BENEFIT FROM INCOME TAXES	(27,389)	(7,095)	(60,324)	(13,555)	(26,678)	(21,459)
INCOME TAXES BENEFIT	3,348	(412)	(1,563)	(1,308)	(3,121)	(4,968)
NET LOSS	(30,737)	(6,683)	(58,761)	(12,247)	(23,557)	(16,491)
EQUITY ADJUSTMENT FROM FOREIGN CURRENCY TRANSLATION	80	(20)	27	3	(40)
CONVERTIBLE PREFERRED STOCK DEEMED DIVIDEND, NET OF RETURN FROM HOLDERS						(932)
COMPREHENSIVE LOSS	(30,657)	(6,703)	(58,734)	(12,244)
ACCRETION OF REDEEMABLE SERIES A- 1 PREFERRED STOCK	(6,542)	(5,030)	(17,601)	(14,341)	(19,616)	(3,605)
REDEEMABLE SERIES A-1PREFERRED STOCK DEEMED DIVIDEND	(24,341)		(24,341)
CONVERTIBLE AND REDEEMABLE SERIES A-2 PREFERRED STOCK DIVIDEND	(2,870)		(2,870)
NET LOSS ATTRIBUTABLE TO COMMON STOCKHOLDERS	$ (64,490)	$ (11,713)	$ (103,573)	$ (26,588)	$ (43,173)	$ (21,028)
WEIGHTED AVERAGE COMMON SHARES OUTSTANDING- BASIC AND DILUTED	21,554	8,718	13,669	8,640	8,789	7,533
NET LOSS PER SHARE ATTRIBUTABLE TO COMMON STOCKHOLDERS- BASIC AND DILUTED	$ (2.99)	$ (1.34)	$ (7.58)	$ (3.08)	$ (4.91)	$ (2.79)

CONDENSED CONSOLIDATED STATEMEN

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	Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Notes Receivable from Stockholders	Accumulated Other Comprehensive Income (Loss)
Beginning balance at Dec. 31, 2017				$ 45,017
Beginning balance, shares at Dec. 31, 2017				69,817
Convertible preferred stock deemed dividend, net of return from holders	$ 932			$ 932
Conversion of convertible preferred stock into cash	30,986			$ (30,986)
Conversion of convertible preferred stock into cash, shares				(48,075)
Conversion of convertible preferred stock into common stock				$ (14,963)
Conversion of convertible preferred stock into common stock				(21,742)
Issuance of redeemable series A-1 preferred stock, net of transaction costs and warrant		$ 105,601
Issuance of redeemable series A-1 preferred stock, net of transaction costs and warrant, shares		12,000
Accretion of the redeemable series A-1 preferred stock to redeemable value		$ 3,605
Ending balance at Dec. 31, 2018		$ 109,206
Ending balance, shares at Dec. 31, 2018		12,000
Beginning balance at Dec. 31, 2017	5,601					$ 30,079	$ (24,356)	$ (122)
Beginning balance, shares at Dec. 31, 2017					7,287,495
Net income (loss)	(16,491)						(16,491)
Convertible preferred stock deemed dividend, net of return from holders	(932)					(932)
Conversion of convertible preferred stock into common stock	14,963					14,963
Conversion of convertible preferred stock into common stock, shares					674,008
Accretion of the redeemable series A-1 preferred stock to redeemable value	(3,605)					(3,605)
Repurchase of common stock	(2,722)					(2,722)
Repurchase of common stock, shares					(277,714)
Repurchase of options	(1,372)					(1,372)
Stock-based compensation	5,794					5,794
Common stock issued	5,664					5,664
Common stock issued, shares					453,982
Ending balance at Dec. 31, 2018	6,900					47,869	(40,847)	(122)
Ending balance, shares at Dec. 31, 2018					8,137,771
Accretion of the redeemable series A-1 preferred stock to redeemable value		$ 4,534
Ending balance at Mar. 31, 2019		$ 113,740
Ending balance, shares at Mar. 31, 2019		12,000
Net income (loss)	(5,242)						(5,242)
Accretion of the redeemable series A-1 preferred stock to redeemable value	(4,534)					(4,534)
Stock-based compensation	1,228					1,228
Common stock issued	6					6
Common stock issued, shares					1,775
Ending balance at Mar. 31, 2019	(1,642)					44,569	(46,089)	(122)
Ending balance, shares at Mar. 31, 2019					8,139,546
Beginning balance at Dec. 31, 2018		$ 109,206
Beginning balance, shares at Dec. 31, 2018		12,000
Ending balance at Sep. 30, 2019		$ 123,547
Ending balance, shares at Sep. 30, 2019		12,000
Beginning balance at Dec. 31, 2018	6,900					47,869	(40,847)	(122)
Beginning balance, shares at Dec. 31, 2018					8,137,771
Net income (loss)	(12,247)
Accumulated other comprehensive income (loss)	3
Ending balance at Sep. 30, 2019	(11,922)					41,291	(53,094)	(122)	$ 3
Ending balance, shares at Sep. 30, 2019					8,310,885
Beginning balance at Dec. 31, 2018		$ 109,206
Beginning balance, shares at Dec. 31, 2018		12,000
Accretion of the redeemable series A-1 preferred stock to redeemable value		$ 19,616
Ending balance at Dec. 31, 2019		$ 128,822
Ending balance, shares at Dec. 31, 2019		12,000	0
Beginning balance at Dec. 31, 2018	6,900					47,869	(40,847)	(122)
Beginning balance, shares at Dec. 31, 2018					8,137,771
Net income (loss)	(23,557)						(23,557)
Accretion of the redeemable series A-1 preferred stock to redeemable value	(19,616)					(19,616)
Collection of notes receivable from stockholders	122							122
Stock-based compensation	4,345					4,345
Common stock issued	5,555					5,555
Accumulated other comprehensive income (loss)	$ (40)								(40)
Common stock issued, shares	42,415				232,336
Ending balance at Dec. 31, 2019	$ (26,291)					38,153	(64,404)		(40)
Ending balance, shares at Dec. 31, 2019					8,370,107
Beginning balance at Mar. 31, 2019		$ 113,740
Beginning balance, shares at Mar. 31, 2019		12,000
Accretion of the redeemable series A-1 preferred stock to redeemable value		$ 4,777
Ending balance at Jun. 30, 2019		$ 118,517
Ending balance, shares at Jun. 30, 2019		12,000
Beginning balance at Mar. 31, 2019	(1,642)					44,569	(46,089)	(122)
Beginning balance, shares at Mar. 31, 2019					8,139,546
Net income (loss)	(322)						(322)
Accretion of the redeemable series A-1 preferred stock to redeemable value	(4,777)					(4,777)
Stock-based compensation	1,294					1,294
Common stock issued	3,452					3,452
Accumulated other comprehensive income (loss)	23								23
Common stock issued, shares					138,781
Ending balance at Jun. 30, 2019	(1,972)					44,538	(46,411)	(122)	23
Ending balance, shares at Jun. 30, 2019					8,278,327
Accretion of the redeemable series A-1 preferred stock to redeemable value		$ 5,030
Ending balance at Sep. 30, 2019		$ 123,547
Ending balance, shares at Sep. 30, 2019		12,000
Net income (loss)	(6,683)						(6,683)
Accretion of the redeemable series A-1 preferred stock to redeemable value	(5,030)					(5,030)
Stock-based compensation	1,055					1,055
Common stock issued	728					728
Accumulated other comprehensive income (loss)	(20)								(20)
Common stock issued, shares					32,558
Ending balance at Sep. 30, 2019	(11,922)					41,291	(53,094)	$ (122)	3
Ending balance, shares at Sep. 30, 2019					8,310,885
Beginning balance at Dec. 31, 2019		$ 128,822
Beginning balance, shares at Dec. 31, 2019		12,000	0
Accretion of the redeemable series A-1 preferred stock to redeemable value		$ 5,415
Ending balance at Mar. 31, 2020		$ 134,237
Ending balance, shares at Mar. 31, 2020		12,000
Beginning balance at Dec. 31, 2019	(26,291)					38,153	(64,404)		(40)
Beginning balance, shares at Dec. 31, 2019					8,370,107
Net income (loss)	(41,248)						(41,248)
Accretion of the redeemable series A-1 preferred stock to redeemable value	(5,415)					(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					8,370,107
Beginning balance at Dec. 31, 2019		$ 128,822
Beginning balance, shares at Dec. 31, 2019		12,000	0
Ending balance at Sep. 30, 2020			$ 152,928
Ending balance, shares at Sep. 30, 2020		0	17,500
Beginning balance at Dec. 31, 2019	(26,291)					38,153	(64,404)		(40)
Beginning balance, shares at Dec. 31, 2019					8,370,107
Net income (loss)	$ (58,761)
Redemption of the series A-1, shares	1,786,739
Dividend payment to the series A-2 preferred shareholders	$ (2,870)		$ (2,900)
Cancellation of shares	117,785
Accumulated other comprehensive income (loss)	$ 27
Ending balance at Sep. 30, 2020	138,767					261,945	(123,165)		(13)
Ending balance, shares at Sep. 30, 2020					24,893,122
Beginning balance at Mar. 31, 2020		$ 134,237
Beginning balance, shares at Mar. 31, 2020		12,000
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
Ending balance at Jun. 30, 2020		$ 139,881	$ 152,928
Ending balance, shares at Jun. 30, 2020		12,000	17,500
Beginning balance at Mar. 31, 2020	(71,767)					33,888	(105,652)		(3)
Beginning balance, shares at Mar. 31, 2020					8,370,107
Net income (loss)	13,224						13,224
Accretion of the redeemable series A-1 preferred stock to redeemable value	(5,644)					(5,644)
Stock-based compensation	1,140					1,140
Common stock issued	25,021					25,021
Accumulated other comprehensive income (loss)	(90)								(90)
Common stock issued, shares					794,639
Ending balance at Jun. 30, 2020	(38,116)					54,405	(92,428)		(93)
Ending balance, shares at Jun. 30, 2020					9,164,746
Accretion of the redeemable series A-1 preferred stock to redeemable value		$ 6,542
Series A-1 preferred stock deemed dividend		24,341
Redemption of the series A-1		$ (170,764)
Redemption of the series A-1, shares		(12,000)
Ending balance at Sep. 30, 2020			$ 152,928
Ending balance, shares at Sep. 30, 2020		0	17,500
Series A-1 preferred stock deemed dividend	(24,341)					(24,341)
Net income (loss)	(30,737)						(30,737)
Redemption of the series A-1	$ 26,801					26,801
Redemption of the series A-1, shares	1,786,739				1,786,739
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
Accretion of the redeemable series A-1 preferred stock to redeemable value	(6,542)					(6,542)
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 initial public offering	$ 156,881					156,881
Issuance of common stock in connection with initial public offering, shares					11,500,000
Cancellation of shares	117,785				(119,452)
Stock-based compensation	$ 1,149					1,149
Common stock issued	150					150
Accumulated other comprehensive income (loss)	80								80
Common stock issued, shares					26,850
Ending balance at Sep. 30, 2020	$ 138,767					$ 261,945	$ (123,165)		$ (13)
Ending balance, shares at Sep. 30, 2020					24,893,122

CONDENSED CONSOLIDATED STATEM_2

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

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
OPERATING ACTIVITIES:
Net loss	$ (58,761)	$ (12,247)	$ (23,557)	$ (16,491)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Provision for bad debt	6,445	590	1,246
DEPRECIATION AND AMORTIZATION	27,084	20,262	27,705	23,915
Gain on asset disposition			(388)	(126)
Lease abandonment				76
Stock-based compensation expense	3,439	3,577	4,345	5,794
Amortization of deferred financing costs			560	1,672
Deferred income taxes	(1,563)	(1,308)	(3,121)	(4,968)
Fair value changes in business acquisitions contingent consideration	17,387	(670)	1,392	(158)
Fair value changes in derivative				(352)
Fair value changes in warrant option			4,060
Fair value changes in the contingent put option	(19,240)		7,100
Gain on the extinguishment of the subordinated debt				(356)
Fair value changes in the compound embedded option	27,420
Fair value changes in the warrant options	9,312	4,059
Cloud computing costs	(2,350)	(247)	(1,609)
Foreign currency realized exchange loss			25
Other	1,170	97
Changes in operating assets and liabilities-net of acquisitions:
Accounts receivable and contract assets	(7,736)	(2,732)	(6,588)	(9,997)
Prepaid expenses and other current assets	(1,349)	(179)	(2,461)	(101)
Accounts payable and other accrued liabilities	(4,829)	(95)	6,155	(2,202)
Accounts payable to related party			(70)
Accrued payroll and benefits	6,084	1,440	2,248	449
Payment of contingent consideration and other assumed purchase price obligations	(6,390)		(1,113)	(2,009)
Net cash (used in) provided by operating activities	(3,877)	12,547	17,042	(2,845)
INVESTING ACTIVITIES:
Proceeds from property insurance			360	43
Purchases of property and equipment	(5,366)	(2,765)	(4,692)	(3,772)
Proceeds received from the sale of property and equipment			260	166
Proprietary software development	(370)	(6)	(21)	(337)
Payment of assumed purchase price obligations			(1,520)	(548)
Proceeds from net working capital adjustment related to acquisitions	2,819
Cash paid for acquisitions-net of cash acquired	(173,923)	(81,370)	(81,370)	(45,835)
Net cash used in investing activities	(176,840)	(84,141)	(86,983)	(50,283)
FINANCING ACTIVITIES:
Proceeds from line of credit	104,390	149,744	146,338	109,383
Payments on line of credit	(201,980)	(69,744)	(68,747)	(89,383)
Proceeds from term loans	175,000			35,333
Repayment of term loan	(49,297)		(1,250)	(71,590)
Payment of contingent consideration and other purchase price obligations	(6,004)	(1,136)
Repayment of capital leases	(2,257)	(1,304)	(1,972)	(1,010)
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)	(29)
Debt issuance cost	(4,866)	(417)	(435)	(889)
Debt extinguishment costs	(351)			(410)
Repayment of the subordinated debt				(11,944)
Proceeds from issuance of common stock	171	139	1,509	30
Proceeds from issuance of the Redeemable Series A-1 Preferred Stock-net of transaction costs				118,419
Issuance of convertible and redeemable Series A-2 preferred stock and warrant	173,664
Repurchase of options				(1,372)
Redemption of the Series A-1 Preferred Stock	(131,821)
Conversion of convertible preferred stock into cash				(30,986)
Dividend payment to the Series A-2 shareholders	(2,870)
Repurchase of common stock				(2,722)
Exercise of warrant options	25
Payment of contingent consideration and other assumed purchase price obligations	(6,390)		(1,113)	(2,009)
Collection of notes receivable from stockholders			122
Net cash provided by financing activities	212,167	77,253	74,452	50,850
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	31,450	5,659	4,511	(2,278)
Foreign exchange impact on cash balance	43	(5)	(116)
CASH, CASH EQUIVALENTS AND RESTRICTED CASH:
Beginning of year	6,884	2,489	2,489	4,767
End of period	38,377	8,143	6,884	2,489
SUPPLEMENTAL DISCLOSURES OF CASH FLOWS INFORMATION:
Cash paid for interest	9,368	2,689	5,891	7,924
Cash paid for income tax	171	878	1,205	596
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING ACTIVITIES:
Series A-1 preferred stock deemed dividend-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	486	932	975	793
Property and equipment acquired under capital leases	1,753	2,757	4,347	3,116
Write off of the derivative liability				2,295
Convertible preferred stock deemed dividend-net of return from holders				932
Conversion of convertible preferred stock into common stock				14,963
Detachable warrant issued in connection with Redeemable Series A-1 Preferred Stock				12,818
Acquisitions unpaid contingent consideration	58,912	5,401	5,402	4,760
Accretion of the redeemable series A-1 preferred stock to redeemable value	17,601	14,341	19,616	3,605
Common stock issued to acquire new businesses	25,000	$ 4,047	4,047	5,634
Offering costs included in accounts payable and other accrued liabilities	$ 1,237		$ 1,240	$ 154

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net loss	$ (23,557)	$ (16,491)
Equity adjustment from foreign currency translation	(40)
Comprehensive loss	$ (23,597)	$ (16,491)

Description of the Business and

Description of the Business and Basis of Presentation	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
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 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 Measurement and Analysis Remediation and Reuse Initial Public Offering Basis of Presentation	1. DESCRIPTION OF THE BUSINESS AND BASIS OF PRESENTATION Description of the Business 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 government through the following three segments: Assessment, Permitting and Response Measurement and Analysis Remediation and Reuse Basis of Presentation

Summary of New Accounting Prono

Summary of New Accounting Pronouncements	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
New Accounting Pronouncements And Changes In Accounting Principles Abstract
Summary of New Accounting Pronouncements	2. SUMMARY OF NEW ACCOUNTING PRONOUNCEMENTS Recently Adopted Accounting Pronouncements In June 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standard Update (“ASU”) 2018-07, Compensation—Stock to non-employees will In January 2017, the FASB issued ASU 2017-04, Intangibles—Goodwill Recently Issued Accounting Pronouncements Not Yet Adopted 2020-06, 470-20) 815-40)—Accounting In March 2020, the FASB issued ASU 2020-04, Reference ASU 2020-04 provides ASU 2020-04 was In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. ASU 2019-12 removes certain exceptions to the general principles in ASC 740 and clarifies and amends certain guidance to promote consistent application. ASU 2019-12 is effective for the Company’s annual and interim periods beginning on January 1, 2021, with early adoption permitted. Depending on the amendment, adoption may be applied on a retrospective, modified retrospective or prospective basis. The Company is currently evaluating the impact of the adoption of the standard on the condensed consolidated financial statements and does not anticipate the standard to have a significant impact. In June 2016, the FASB issued ASU 2016-13, Financial In February 2016, the FASB issued ASU 2016-2, Leases	3. SUMMARY OF NEW ACCOUNTING PRONOUNCEMENTS Recently Adopted Accounting Pronouncements Between May 2014 and May 2016, the FASB issued three Accounting Standard Updates (ASUs) that changed the requirements for recognizing and reporting revenue (together, herein referred to as the “Revenue ASUs”): ASU No. 2014-09, No. 2016-08, No. 2016-12, 2014-09 2016-08 2016-12 2014-09. In August 2018, the FASB issued Accounting Standards Update (“ASU”) No. 2018-15, Intangibles—Goodwill and Other—Internal-Use Software: Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract (a consensus of the FASB Emerging Issues Task Force). The FASB’s new guidance aligns the requirements for capitalizing implementation costs in a cloud computing arrangement service contract with the requirements for capitalizing implementation costs incurred for an internal-use software license. The Company early adopted the new guidance prospectively on January 1, 2018. Recently Issued Accounting Pronouncements 2018-07, non-employees In January 2017, the FASB issued ASU 2017-04, In June 2016, the FASB issued ASU 2016-13, In February 2016, the FASB issued ASU 2016-2, There have been no other new accounting pronouncements that, once adopted, are expected to have a material impact on the Company’s consolidated financial statements of notes thereto.

Revenues and Accounts Receivabl

Revenues and Accounts Receivable	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
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 Measurement and Analysis Revenues or time-and-materials based. Remediation and Reuse Revenues long-term fixed-fee contracts. time-and-materials based. Disaggregation of Revenue . Contract Balances weekly, bi-weekly or The following table presents the Company’s contract balances: September 30, 2020 December 31, 2019 Contract assets $ 35,441 $ 13,605 Contract liabilities 9,335 3,314 Contracts assets acquired through business acquisitions completed in the nine months ended September 30, 2020 and the twelve months ended December 31, 2019, amounted to $6.5 million and $0.7 million, respectively. Contract liabilities acquired through business acquisitions completed in the nine months ended September 30, 2020 and twelve months ended December 31, 2019, amounted to zero and $2.2 million. Revenue recognized during the three and nine months ended September 30, 2020, included in the contract liabilities balance at December 31, 2019 was $0.5 million and $2.1 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, 2020 was not material. Remaining Unsatisfied Performance Obligations Accounts Receivable, Net September 30, December 31, Accounts receivable, invoiced $ 52,372 $ 46,643 Accounts receivable, other 929 611 Allowance for doubtful accounts (8,516 ) (1,327 ) Accounts receivable—net $ 44,785 $ 45,927 The Company extends non-interest-bearing trade coronavirus, COVID-19. The COVID-19 pandemic The allowance for doubtful accounts as of September 30, 2020 and December 31, 2019 consisted of the following: Beginning Balance Bad Debt Expense Charged to Allowance Other (1) Ending Balance Nine months ended September 30, 2020 $ 1,327 $ 6,445 $ (294 ) $ 1,038 $ 8,516 Year ended December 31, 2019 453 1,246 (556 ) 184 1,327 (1) This amount consists of additions to the allowance due to business acquisitions.	4. REVENUES AND ACCOUNTS RECEIVABLE The Company’s main revenue sources derive from the following revenue streams: Assessment, Permitting and Response Revenues Measurement and Analysis Revenues time-and-materials Remediation and Reuse Revenues fixed-fee fixed-fee time-and-materials Disaggregation of Revenue . Contract Balances bi-weekly The following table presents the Company’s contract balances as of December 31, 2019 and 2018: 2019 2018 Contract assets $ 13,605 $ 10,112 Contract liabilities 3,314 2,395 Contracts assets acquired through business acquisitions amounted to $0.7 million as of December 31, 2019 and 2018. Contract liabilities acquired through business acquisitions amounted to $2.2 million and zero as of December 31, 2019 and 2018, respectively. Revenue recognized during the year ended December 31, 2019, included in the contract liability balance at the beginning of the year was $0.6 million. 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 year ended December 31, 2019 was not material. Remaining Unsatisfied Performance Obligations Accounts Receivable, Net 2019 2018 Accounts receivable, invoiced $ 46,643 $ 38,786 Accounts receivable, other 611 135 Allowance for doubtful accounts (1,327 ) (453 ) Accounts receivable—net $ 45,927 $ 38,468 The Company extends non-interest Beginning Bad Debt Charged to Other (1) Ending Year ended December 31, 2018 $ 573 $ (624 ) $ 504 $ 453 Year ended December 31, 2019 453 1,246 (556 ) 184 1,327 (1) This amount consists of additions to the allowance due to business acquisitions. Collection of trade receivables may be affected by changes in economic or other industry conditions and may, accordingly, impact the Company’s overall credit risk. Although the Company generally does not require collateral, the Company performs ongoing credit evaluations of customers and maintains reserves for potential credit losses. For all periods presented, no customer accounted for more than 10% of revenue or accounts receivable.

Prepaid and Other Current Asset

Prepaid and Other Current Assets	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
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 as of September 30, 2020 and December 31, 2019 consisted of the following: September 30, December 31, Deposits $ 733 $ 605 Prepaid expenses 1,665 1,235 Prepaid insurance 2,577 170 Supplies 2,428 2,368 Offering costs — 1,240 Income tax receivable 1,182 1,205 Prepaid and other current assets $ 8,585 $ 6,823	5. PREPAID AND OTHER CURRENT ASSETS Prepaid and other current assets as of December 31, 2019 and 2018 consisted of the following: 2019 2018 Deposits $ 605 $ 598 Prepaid expenses 1,235 1,016 Prepaid insurance 170 85 Supplies 2,368 745 Offering costs 1,240 Other 154 Prepaid and other current assets $ 5,618 $ 2,598

Property and Equipment

Property and Equipment	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Property and Equipment	5. PROPERTY AND EQUIPMENT Property and equipment are stated at cost or estimated fair value for assets acquired through business combinations. Depreciation and amortization is 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, as of September 30, 2020 and December 31, 2019 consisted of the following: Estimated September 30, December 31, Lab and test equipment 7 years $ 17,755 $ 14,810 Vehicles 5 years 12,869 11,073 Equipment 3–7 years 31,539 29,922 Furniture and fixtures 7 years 2,854 1,119 Leasehold improvements 7 years 6,758 5,954 Aircraft 10 years 834 — Building 39 years 2,975 — 75,584 62,878 Land 725 — Construction in progress 156 796 Less accumulated depreciation (41,906 ) (36,638 ) Total property and equipment—net $ 34,559 $ 27,036 Total depreciation expense included on the condensed consolidated statements of operations was $2.1 million and $6.1 million for the three and nine months ended September 30, 2020, respectively. Total depreciation expense included on the condensed consolidated statements of operations was $2.1 million and $5.9 million for the three and nine months ended September 30, 2019, respectively.	6. PROPERTY AND EQUIPMENT, NET Property and equipment are stated at cost or estimated fair value for assets acquired through business combinations. Depreciation and amortization is 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, as of December 31, 2019 and 2018 consisted of the following: Estimated 2019 2018 Lab and test equipment 7 years $ 14,810 $ 10,275 Vehicles 5 years 11,073 8,122 Equipment 3-7 years 29,922 25,468 Furniture and fixtures 7 years 1,119 1,004 Leasehold improvements 7 years 5,954 5,137 62,878 50,006 Construction in progress 796 Less accumulated depreciation and amortization (36,638 ) (29,214 ) Total property and equipment—net $ 27,036 $ 20,792 Total depreciation expense included on the consolidated statements of operations was $7.7 million and $7.6 million for the years ended December 31, 2019 and 2018, respectively.

Business Acquisitions

Business Acquisitions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Business Combinations [Abstract]
Business Acquisitions	6. BUSINESS ACQUISITIONS In line with the Company’s strategic growth initiatives, the Company acquired several businesses during the nine months ended September 30, 2020 and September 30, 2019. The results of each of those acquired businesses are included in the condensed consolidated financial statements beginning on the respective acquisition dates. Each transaction qualified as an acquisition of a business and was accounted for as a business combination. All acquisitions resulted in the recognition of goodwill. The Company paid these premiums resulting in such goodwill for a number of reasons, including expected synergies from combining operations of the acquiree and the Company while also growing the Company’s customer base, acquiring assembled workforces, expanding its presence in certain markets and expanding and advancing its product and service offerings. The Company recorded the assets acquired and liabilities assumed at their acquisition date fair value, with the difference between the fair value of the net assets acquired and the acquisition consideration reflected as goodwill. 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. Other purchase price obligations (primarily deferred purchase price liabilities and target working capital liabilities or receivables) are included on the 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 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 $3.8 million and $2.4 million for the nine months ended September 30, 2020 and September 30, 2019, respectively, and zero and $1.3 million for the three months ended September 30, 2020 and September 30, 2019, respectively. These costs are expensed within the selling, general and administrative expense in the accompanying condensed consolidated statements of operations. The cash payment made to acquire The Center for Toxicology and Environmental Health, L.L.C. (“CTEH”) was funded through the issuance of the Convertible and Redeemable Series A-2 Acquisitions completed during the nine months ended September 30, 2020 The Center for Toxicology and Environmental Health, L.L.C. Leed Environmental Inc. American Environmental Testing Co. The following table summarizes the elements of purchase price of the acquisitions completed during the nine months ended September 30, 2020: Cash Common Stock Other Purchase Price Component Current Other Purchase Price Component Long Term Contingent Consideration Current Contingent Consideration Long Term Total Purchase Price CTEH $ 175,000 $ 25,000 $ (2,297 ) $ — $ 34,451 $ 10,543 $ 242,697 All other acquisitions 450 — 50 100 210 — 810 $ 175,450 $ 25,000 $ (2,247 ) $ 100 $ 34,661 $ 10,543 $ 243,507 The contingent consideration elements of the purchase price of the acquisitions is 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. CTEH first year earnout is to be 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 year earn-out is year earn-out payment The preliminary purchase price attributable to the acquisitions was allocated as follows: CTEH All Other 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 146,625 626 147,251 Total assets 252,418 810 253,228 Current liabilities 9,721 — 9,721 Total liabilities 9,721 — 9,721 Purchase price $ 242,697 $ 810 $ 243,507 For the acquisitions completed during the nine months ended September 30, 2020, the results of operations have been combined with those of the Company. The Company’s condensed consolidated statement of operations for the three and nine months ended September 30, 2020 includes revenue and pre-tax income 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. The weighted average useful lives for the acquired covenants not to compete for the other acquisitions is 4 years. Goodwill associated with the CTEH, LEED and AETC acquisitions is deductible for income tax purposes. Acquisitions completed during the nine months ended September 30, 2019 Golden Specialty, Inc. — Target Emission Services Inc. Target Emission Services USA LP Air Water & Soil Laboratories, Inc. the mid-Atlantic region. Advanced Environmental Compliance LLC LEHDER Environmental Services Ltd Emerging Compounds Treatments Technologies, Inc. The following table summarizes the elements of purchase price of the acquisitions completed during the nine months ended September 30, 2019: Cash Common Stock Other Purchase Price Components Contingent Consideration Total Purchase Price Golden $ 1,500 — — $ 477 $ 1,977 TES 2,359 322 25 4,911 7,617 TESUS 18,683 3,041 1,495 — 23,219 AWS 6,020 — 150 — 6,170 AEC 808 — — — 808 LEHDER 3,878 684 — 13 4,575 ECT2 54,037 — (220 ) — 53,817 $ 87,285 $ 4,047 $ 1,450 $ 5,401 $ 98,183 Contingent consideration elements of the purchase price of these acquisitions are related to earn-outs which are based on the expected achievement of revenue or earnings thresholds by the applicable business as of the date of the acquisition and for which the maximum potential amount to be earned is generally not limited. The purchase price attributable to the acquisitions was allocated as follows: ECT2 All Other Acquisitions Total Cash $ 3,149 $ 2,137 $ 5,286 Restricted cash 629 — 629 Accounts receivable 1,707 3,751 5,458 Other current assets 498 61 559 Current assets 5,983 5,949 11,932 Property and equipment 776 3,288 4,064 Customer relationships 13,840 12,748 26,588 Trade names 1,008 659 1,667 Covenants not to compete 3,360 2,083 5,443 Proprietary software — 2,560 2,560 Patent 17,479 — 17,479 Goodwill 16,395 20,227 36,622 Total assets 58,841 47,514 106,355 Current liabilities 5,024 977 6,001 Non- — 2,171 2,171 Total liabilities 5,024 3,148 8,172 Purchase price $ 53,817 $ 44,366 $ 98,183 The weighted average useful lives for the acquired customer relationships, trade names, covenants not to compete, proprietary software and patent for these acquisitions are 9.5 years, 1.5 years, 4 years, 3 years and 16 years, respectively. For the acquisitions completed during the nine months ended September 30, 2019, the results of operations since the acquisition dates have been combined with those of the Company. The Company’s three and nine months ended September 30, 2019 condensed consolidated statement of operations includes revenue of $7.7 million and pre-tax income Goodwill associated with the acquisitions of Golden, AEC and ECT2 is deductible for income tax purposes. Supplemental Unaudited Pro-Forma For the Three Months Ended September 30, 2020 2019 As Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) As Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) Revenues $ 84,705 $ — $ 84,705 $ 57,623 $ 18,551 $ 76,174 Net (loss) income (30,737 ) — (30,737 ) (6,683 ) 4,042 (2,641 ) For the Nine Months Ended September 30, 2020 2019 As Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) As Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) Revenues $ 219,502 $ 31,253 $ 250,755 $ 165,978 $ 107,459 $ 273,437 Net (loss) income (58,761 ) 10,288 (48,473 ) (12,247 ) 30,002 17,755	7. BUSINESS ACQUISITIONS In line with the Company’s strategic growth initiatives, the Company acquired several businesses during the years ended December 31, 2019 and 2018. The results of each of those acquired businesses are included in the consolidated financial statements beginning on the respective acquisition dates. Each transaction qualified as an acquisition of a business and was accounted for as a business combination. All acquisitions resulted in the recognition of goodwill. The Company paid these premiums resulting in such goodwill for a number of reasons, including expected synergies from combining operations of the acquiree and the Company while also growing the Company’s customer base, acquiring assembled workforces, expanding its presence in certain markets and expanding and advancing its product and service offerings. The Company recorded the assets acquired and liabilities assumed at their acquisition date fair value, with the difference between the fair value of the net assets acquired and the acquisition consideration reflected as goodwill. The identifiable intangible assets for acquisitions occurring in 2019 and 2018 were valued using the excess earnings method discounted cash flow approach for customer relationships, the relief from royalty method for trade names and the patent, and the “with and without” method for covenants not to compete 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 Other purchase price obligations (primarily deferred purchase price liabilities and target working capital liabilities) and contingent consideration outstanding from 2019 and 2018 acquisitions are included on the consolidated statements of financial position in accounts payable and other accrued liabilities, other non-current receivable-net 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. External transaction costs related to business combinations totaled $2.4 million and $1.2 million for the years ended December 31, 2019 and 2018, respectively. These costs are expensed within the selling, general and administrative expense in the accompanying consolidated statements of operations. Cash payments made to acquire these businesses were funded primarily through the Company’s Credit Facilities. 2019 Acquisitions Golden Specialty, Inc. Target Emission Services Inc. Target Emission Services USA LP Air Water & Soil Laboratories, Inc. mid-Atlantic Advanced Environmental Compliance LLC LEHDER Environmental Services Ltd Emerging Compounds Treatments Technologies, Inc. The following table summarizes the elements of purchase price of the 2019 acquisitions: Cash Common Other Contingent Total Golden $ 1,500 $ $ $ 477 $ 1,977 TES 2,359 322 25 4,911 7,617 TESUS 18,683 3,041 1,495 23,219 AWS 6,020 150 6,170 AEC 808 808 LEHDER 3,878 684 13 4,575 ECT2 54,037 (220 ) 53,817 $ 87,285 $ 4,047 $ 1,450 $ 5,401 $ 98,183 Contingent consideration elements of the purchase price of the Company’s 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 to be earned is generally not limited. The purchase price attributable to each acquisition was allocated as follows: ECT2 All Other Total Cash $ 3,149 $ 2,137 $ 5,286 Restricted cash 629 629 Accounts receivable 1,707 3,751 5,458 Other current assets 498 61 559 Current assets 5,983 5,949 11,932 Property and equipment 776 3,288 4,064 Customer relationships 13,840 12,748 26,588 Trade names 1,008 659 1,667 Covenants not to compete 3,360 2,083 5,443 Proprietary software 2,560 2,560 Patent 17,479 17,479 Goodwill 16,395 20,227 36,622 Total assets 58,841 47,514 106,355 Current liabilities 5,024 977 6,001 Non- 2,171 2,171 Total liabilities 5,024 3,148 8,172 Purchase price $ 53,817 $ 44,366 $ 98,183 The weighted average useful lives for the acquired customer relationships, trade names, covenants not to compete, proprietary software and patent for these acquisitions are 9.5 years, 1.5 years, 4 years, 3 years and 16 years, respectively. For the acquisitions completed during 2019, the results of operations since the acquisition dates have been combined with those of the Company. The Company’s 2019 consolidated statement of operations includes revenue and pre-tax pre-tax Goodwill associated with the acquisitions of Golden, AEC and ECT2 is deductible for income tax purposes. 2018 Acquisitions Southern Environmental Sciences, Inc. First Analytical Laboratories NC, LLC — Advanced GeoServices Corp. Streamline Environmental, Inc. Leymaster Environmental Consulting, LLC Analytical Environmental Services, Corp. Environmental Planning Specialists, Inc. The following table summarizes the elements of purchase price of the 2018 acquisitions: Cash Common Other Contingent Total SES $ 450 $ $ 50 $ $ 500 FA 1,096 360 372 1,828 AGC 7,400 925 95 871 9,291 Streamline 5,678 631 180 6,489 Leymaster 2,465 435 250 434 3,584 AES 21,877 2,028 (16 ) 1,274 25,163 EPS 9,995 1,615 (71 ) 11,539 $ 48,961 $ 5,634 $ 848 $ 2,951 $ 58,394 Contingent consideration elements of the purchase price of the Company’s 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 to be earned is generally not limited. The purchase price attributable to each acquisition was allocated as follows: AES All Other Total Cash $ 2,746 $ 380 $ 3,126 Accounts receivable 1,404 5,045 6,449 Other current assets 32 288 320 Current assets 4,182 5,713 9,895 Property and equipment 532 1,002 1,534 Customer relationships 11,296 15,543 26,839 Trade names 241 812 1,053 Covenants not to compete 961 1,813 2,774 Goodwill 11,019 13,661 24,680 Total assets 28,231 38,544 66,775 Current liabilities 3,068 2,141 5,209 Non- 3,172 3,172 Total liabilities 3,068 5,313 8,381 Purchase price $ 25,163 $ 33,231 $ 58,394 The weighted average useful lives for the acquired customer relationships, trade names, and covenants not to compete for all acquisitions are 7 years, 1.5 years and 4 years, respectively. For the acquisitions completed during 2018, the results of operations since the acquisition dates have been combined with those of the Company. The Company’s 2018 consolidated statement of operations includes revenue and pre-tax pre-tax Goodwill associated with the acquisitions of FA, SES, Leymaster, AES and EPS is deductible for income tax purposes. Supplemental Unaudited Pro-Forma As reported Acquisitions Pro-Forma Consolidated Pro-Forma 2019 Revenues $ 233,854 $ 25,446 $ 259,300 Net income (loss) (23,557 ) 5,199 (18,358 ) 2018 Revenues $ 188,805 $ 58,526 $ 247,331 Net income (loss) (16,491 ) 14,580 (1,911 )

Goodwill and Intangible Assets

Goodwill and Intangible Assets	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	7. GOODWILL AND INTANGIBLE ASSETS Amounts related to goodwill as of September 30, 2020 and December 31, 2019 are as follows: Assessment, Permitting and Measurement and Analysis Remediation and Reuse Total Balance as of December 31, 2019 $ 15,173 $ 68,628 $ 43,257 $ 127,058 Goodwill acquired during the period 146,625 426 200 147,251 Balance as of September 30, 2020 $ 161,798 $ 69,054 $ 43,457 $ 274,309 Amounts related to finite-lived intangible assets as of September 30, 2020 and December 31, 2019 are as follows: September 30, 2020 Estimated Useful Life Gross Balance Accumulated Amortization Total Intangible Assets—Net Finite lived intangible assets Customer relationships 7–15 years $ 164,782 $ 49,068 $ 115,714 Covenants not to compete 4–5 years 29,942 20,444 9,498 Trade names 1–5 years 16,938 12,196 4,742 Proprietary software 3-5 20,808 4,915 15,893 Patent 16 years 17,479 1,125 16,354 Total other intangible assets—net $ 249,949 $ 87,748 $ 162,201 December 31, 2019 Estimated Useful Life Gross Balance Accumulated Amortization Total Intangible Assets—Net Finite lived intangible assets Customer relationships 7–10 years $ 108,782 $ 36,700 $ 72,082 Covenants not to compete 4–5 years 25,832 17,572 8,260 Trade names 1–5 years 12,738 10,230 2,508 Proprietary software 3 years 3,885 1,359 2,526 Patent 16 years 17,479 306 17,173 Total other intangible assets—net $ 168,716 $ 66,167 $ 102,549 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. Amortization expense was $7.6 million and $21.0 million for the three and nine months ended September 30, 2020, respectively and $5.3 million and $14.4 million for the three and nine months ended September 30, 2019, respectively. Future amortization expense is estimated to be as follows for each of the five following years and thereafter: December 31, 2020 (remaining) $ 7,543 2021 28,140 2022 24,053 2023 20,073 2024 16,951 Thereafter 65,441 $ 162,201	8. GOODWILL AND INTANGIBLE ASSETS Amounts related to goodwill as of December 31, 2019 and 2018 are as follows: Assessment, Measurements Remediation Total Balance as of January 1, 2018 $ — $ 44,123 $ 21,695 $ 65,818 Goodwill acquired during the year 15,173 4,340 5,167 24,680 Balance as of December 31, 2018 15,173 48,463 26,862 90,498 Goodwill acquired during the year 20,165 16,395 36,560 Balance as of December 31, 2019 $ 15,173 $ 68,628 $ 43,257 $ 127,058 Goodwill balances as of December 31, 2019 include $0.06 million of foreign currency translation adjustment related to TES and LEHDER acquisitions (Note 7). There was no translation adjustment as of December 31, 2018. Amounts related to finite-lived intangible assets as of December 31, 2019 and 2018 are as follows: 2019 Estimated Gross Accumulated Total Finite lived intangible assets Customer relationships 7–10 years $ 108,782 $ 36,700 $ 72,082 Covenants not to compete 4–5 years 25,832 17,572 8,260 Trade names 1–5 years 12,738 10,230 2,508 Proprietary software 3 years 3,885 1,359 2,526 Patent 16 years 17,479 306 17,173 Total other intangible assets—net $ 168,716 $ 66,167 $ 102,549 2018 Estimated Gross Accumulated Total Finite lived intangible assets Customer relationships 7–10 years $ 82,176 $ 23,932 $ 58,244 Covenants not to compete 4–5 years 20,390 13,785 6,605 Trade names 1–5 years 11,072 8,282 2,790 Proprietary software 3 years 1,303 379 924 Total other intangible assets—net $ 114,941 $ 46,378 $ 68,563 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. During 2018, Company capitalized certain costs incurred to develop extranet software for internal and client use. The capitalized costs included payments to vendors and consultants for the development and implementation or modification of internal use software. During 2019, the Company acquired a custom software platform as part of the TES acquisition (Note 7) to service upstream and midstream oil and gas producers and assist with their LDAR requirements related to government regulations of fugitive and greenhouse gas emissions. These assets are considered proprietary software. During 2019, as part of ECT2 acquisition (Note 7), the Company acquired a patent for the removal of contaminants in water. The Company evaluates intangible assets subject to amortization for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. Recoverability of intangible assets is measured by a comparison of the carrying amount of the intangible asset to the future undiscounted operating cash flows expected to be generated by the asset. As of December 31, 2019 and 2018, there was no impairment of intangible assets subject to amortization. Amortization expense for the years ended December 31, 2019 and 2018 was $20.0 million and $16.3 million, respectively. Future amortization expense is estimated to be as follows for each of the five following years and thereafter ending December 31, 2019: 2020 $ 21,473 2021 18,895 2022 14,810 2023 12,025 2024 9,357 Thereafter 25,989 $ 102,549

Accounts Payable and Other Accr

Accounts Payable and Other Accrued Liabilities	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Accounts Payable and Other Accrued Liabilities	8. ACCOUNTS PAYABLE AND OTHER ACCRUED LIABILITIES Accounts payable and other accrued liabilities consisted of the following as of September 30, 2020 and December 31, 2019: September 30, December 31, Accounts payable $ 13,360 $ 15,034 Accrued expenses 10,395 10,733 Other purchase obligations 572 — Contract liabilities 9,335 3,314 Other current liabilities 235 504 Total accounts payable and other accrued liabilities $ 33,897 $ 29,585	9. ACCOUNTS PAYABLE AND OTHER ACCRUED LIABILITIES Accounts payable and other accrued liabilities consisted of the following as of December 31, 2019 and 2018: 2019 2018 Accounts payable $ 15,034 $ 8,679 Accrued expenses 10,733 4,414 Business acquisitions contingent consideration 8,614 2,754 Other business acquisitions purchase price obligations 525 Contract liabilities 3,314 2,395 Other current liabilities 504 241 Total accounts payable and other accrued liabilities $ 38,199 $ 19,008

Accrued Payroll and Benefits

Accrued Payroll and Benefits	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Accrued Payroll and Benefits	9. ACCRUED PAYROLL AND BENEFITS Accrued payroll and benefits consisted of the following as of September 30, 2020 and December 31, 2019: September 30, December 31, Accrued bonuses $ 7,854 $ 3,449 Accrued paid time off 2,157 2,154 Accrued payroll 5,536 4,470 Accrued other 2,410 959 Total accrued payroll and benefits $ 17,957 $ 11,032	10. ACCRUED PAYROLL AND BENEFITS Accrued payroll and benefits consisted of the following as of December 31, 2019 and 2018: 2019 2018 Accrued bonuses $ 3,449 $ 1,585 Accrued paid time off 2,154 2,289 Accrued payroll 4,470 3,496 Accrued other 959 837 Total accrued payroll and benefits $ 11,032 $ 8,207

Income Taxes

Income Taxes	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	10. INCOME TAXES The Company calculates its interim income tax provision in accordance with Accounting Standard Codification Topic 270, Interim Reporting (“ASC 270”), and ASC 740. The Company’s effective tax rate (“ETR”) from continuing operations was (12.2%) and 5.8% for the three months ended September 30, 2020 and September 30, 2019, respectively, and 2.6% and 9.6% for the nine months ended September 30, 2020 and September 30, 2019, respectively, and the Company recorded income tax expense/(benefit) of $3.3 million and $(0.4) million for the three months ended September 30, 2020 and September 30, 2019, respectively, and income tax benefits of $1.6 million and $1.3 million for the nine months ended September 30, 2020 and September 30, 2019, respectively. The difference between the ETR and federal statutory rate of 21.0% is primarily attributable to items recorded for GAAP but permanently disallowed for U.S. federal income tax purposes, recognition of a US 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 more-likely-than-not The Company records uncertain tax positions in accordance with ASC 740, Income Taxes, on the basis of a two-step more-likely-than-not On March 27, 2020, the Coronavirus Aid, Relief, and Economic Security Act (“CARES Act”) was enacted. The CARES Act includes several significant provisions for corporations, including those pertaining to net operating losses, interest deductions and payroll tax benefits. Under ASC 740, the effects of new legislation are recognized upon enactment. Accordingly, the effects of the CARES Act have been incorporated into the income tax provision computation for the nine months ended September 30, 2020. These provisions did not have a material impact on the income tax provision. The Company anticipates deferring the employer side social security payments for payroll paid for the remainder of 2020 as permitted by the CARES Act.	11. INCOME TAXES Income tax benefit for the years ended December 31, 2019 and 2018 is comprised of the following: 2019 2018 Current: Federal $ 4 $ 7 State 289 245 Foreign 41 Total 334 252 Deferred: Federal (2,323 ) (3,639 ) State (1,132 ) (1,581 ) Total (3,455 ) (5,220 ) Income tax benefit $ (3,121 ) $ (4,968 ) The Company’s deferred taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Significant components of the Company’s deferred tax assets and liabilities as of December 31, 2019 and 2018 are as follows: 2019 2018 Deferred tax assets: Net operating losses $ 3,850 $ 2,570 Allowance for bad debts 331 122 Employee related 1,348 1,834 Interest expense 588 1,472 Other 318 Total deferred tax asset 6,435 5,998 Deferred tax liabilities: Intangible assets (4,825 ) (7,032 ) Property and equipment (4,589 ) (3,172 ) Other (551 ) (662 ) Total deferred tax liability (9,965 ) (10,866 ) Net deferred tax liability $ (3,530 ) $ (4,868 ) A reconciliation of the federal statutory income tax rate to the Company’s effective income tax rate for the years ended December 31, 2019 and 2018 is as follows: 2019 2018 Tax completed at federal statutory rate 21.00 % 21.00 % State tax net of federal benefit 2.51 4.96 Non- (1.38 ) (0.91 ) Equity compensation (2.41 ) (0.83 ) Contingent put option (5.59 ) 0.00 Foreign taxes (0.15 ) 0.00 Warrant option (3.20 ) Other 0.92 (1.03 ) Effective income tax rate 11.70 % 23.19 % The Company elected to account for the global intangible low-taxed The Company’s policy is to record any penalties or interest related to any unrecognized tax benefits as a component of the income tax provision. As of December 31, 2019 and 2018, the Company does not have any unrecognized tax benefits. In December 2017 new federal tax reform legislation was enacted in the United States resulting in significant changes from previous tax law. As a result, the Company previously provided a provisional estimate of the effect of the Tax Act in its financial statements. In the fourth quarter of 2018, the Company completed its analysis to determine the effect of the Tax Act and recorded immaterial adjustments as of December 31, 2018. As of December 31, 2019, federal and state net operating loss carryforwards of approximately $15.7 million and $10.4 million are available to offset future federal and state taxable income, respectively. Federal net operating loss carryforwards will begin to expire during 2035 while the Company’s state net operating loss carryforwards will begin to expire during various years, dependent on the jurisdiction. Due to the enactment of the Tax Act, federal net operating losses generated beginning in 2018 and thereafter are limited to 80% of annual taxable income. Such Federal NOLs cannot be carried back and are carried forward indefinitely. Therefore, $7.0 million of Federal net operating loss carryforwards will not expire. The Company is subject to audit by federal and state tax authorities in the ordinary course of business. The Company’s federal income tax returns remain subject to examination for the 2016 through 2019 tax years. The Company files in multiple state jurisdictions which remain subject to examination for the 2015 through 2019 tax years.

Warrant Options

Warrant Options	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Warrants And Rights Note Disclosure Abstract
Warrant Options	11. WARRANT OPTIONS In October 2018, in connection with the issuance of the Redeemable Series A-1 Preferred On July 30, 2020, the Redeemable Series A-1 In April 2020, in connection with the issuance of the Convertible and Redeemable Series A-2 Preferred A-2 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 On July 30, 2020, the Convertible and Redeemable Series A-2	12. WARRANT OPTION In October 2018, in connection with the issuance of the Redeemable Series A-1 The warrant is classified as a liability in accordance with ASC 815, Derivatives and Hedging, as the agreement provides for net cash settlement upon a change in control, which is outside the control of the Company.

Debt

Debt	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Debt Disclosure [Abstract]
Debt	12. DEBT Debt as of September 30, 2020 and December 31, 2019 consisted of the following: September 30, December 31, Term Loan Facility $ 174,453 $ 48,750 Revolving Line of Credit — 97,590 Capital leases 3,127 3,765 Other leases 2 12 Equipment line of credit 3,214 3,124 Less deferred debt issuance costs (4,345 ) (1,052 ) Total debt 176,451 152,189 Less current portion of long term debt (5,034 ) (7,143 ) Long-term debt, less current portion $ 171,417 $ 145,046 Deferred Financing Costs Revolving Line of Credit and Term Loan Facility The New Credit Facility includes a number of covenants imposing certain restrictions on the Company’s business, including, among other things, restrictions on the Company’s ability to incur indebtedness, prepay or amend other indebtedness, create liens, make certain fundamental changes including mergers or dissolutions, pay dividends and make other payments in respect of capital stock, make certain investments, sell assets, change the Company’s lines of business, enter into transactions with affiliates and other corporate actions. The New Credit Facility also contains financial covenants requiring the Company to remain below a maximum consolidated total leverage ratio of 4.25 times, which steps down to 4.00 times beginning December 31, 2021 and then to 3.75 times beginning December 31, 2022, and a minimum consolidated fixed charge coverage ratio of 1.25 times. The New Credit Facility contains mandatory prepayment features upon a number of events, including with the proceeds of certain asset sales, proceeds from the issuance of any debt and proceeds of the capital contribution amounts contributed to cure a financial covenant default. The New Credit Facility also includes mandatory prepayments of 50.0% of excess cash flow minus voluntary prepayments of the term loan and, solely to the extent accompanied by a permanent reduction in the revolving commitment, the revolver, if the Company’s consolidated total leverage ratio for the year ending December 31, 2020 is greater than or equal to 3.25 times and, for any year thereafter, the amount of any such mandatory prepayment shall be reduced to 25.0% of excess cash flow if the leverage ratio is less than 3.00 times. The weighted average interest rate on the New Credit Facility as of September 30, 2020 was 6.0%. The New Credit Facility contains a number of customary events of default related to, among other things, the non-payment The Company’s obligations under the 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. Prior Senior Secured Credit Facility Borrowings under the 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 Pricing Consolidated Leverage Ratio Commitment Fee Eurodollar Rate Loans and LIBOR Letter of Credit Fee Daily Floating Rate Rate Loans 1 > 3.75 to 1.0 0.50 % 4.00 % 4.00 % 3.00 % 2 £ 0.50 3.50 3.50 2.50 3 £ 0.40 3.00 3.00 2.00 4 < 2.25 to 1.0 0.30 2.50 2.50 1.50 As of December 31, 2019, the Company fell within Pricing Tier 2 and the Company was subject to a fixed charge coverage ratio of greater than 1.25 and a consolidated total leverage ratio of lower than 4.00. The Prior Senior Secured Credit Facility was repaid in full on April 13, 2020 via proceeds from the issuance of the New Credit Facility. The resulting loss on extinguishment amounted to $1.4 million, of which $0.4 million was related to fees paid and $1.0 million related to unamortized debt issuance costs. Total loss on extinguishment is recorded in interest expense-net Equipment Line of Credit Capital Lease Obligations September 30, Payments Interest Principal 2021 $ 2,670 $ 368 $ 2,302 2022 2,178 265 1,913 2023 1,582 139 1,443 2024 667 32 635 2025 49 1 48 $ 7,146 $ 805 $ 6,341 The following is a schedule of the aggregate annual maturities of long-term debt presented on the condensed consolidated statement of financial position, based on the terms of the credit facility, capital lease obligations and equipment line of credit as of September 30, 2020: September 30, 2021 $ 5,034 2022 6,839 2023 8,006 2024 7,198 2025 153,719 Total $ 180,796	14. DEBT Debt as of December 31, 2019 and 2018 consists of the following: 2019 2018 Term Loan Facility $ 48,750 $ 50,000 Revolving Line of Credit 97,590 20,000 Capital leases 3,765 2,640 Other leases 12 51 Equipment line of credit 3,124 1,712 Less deferred debt issuance costs (1,052 ) (1,176 ) Total debt 152,189 73,227 Less current portion of long term debt (7,143 ) (2,262 ) Long- term debt, less current portion $ 145,046 $ 70,965 Deferred Financing Costs Revolving Line of Credit and Term Loan Facility During 2019 and 2018, the Company’s Credit Facility was used for working capital, capital expenditures and to fund acquisitions. The Company’s obligations under the Credit Facility are guaranteed by each of its existing and future direct and indirect subsidiaries, and such obligations are secured by substantially all of the assets of the Company. Borrowings under the Credit Facility bear 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 Pricing Consolidated Leverage Ratio Commitment Eurodollar Daily Rate 1 > 3.75 to 1.0 0.50 % 4.00 % 4.00 % 3.00 % 2 £ 0.50 3.50 3.50 2.50 3 £ 0.40 3.00 3.00 2.00 4 £ 0.30 2.50 2.50 1.50 As of December 31, 2019 and 2018, the Company fell within Pricing Tier 2. As of December 31, 2019 and 2018 the Company was subject to a fixed charge coverage ratio of greater than 1.25. As of December 31, 2019 and 2018, the Company was subject to a consolidated total leverage ratio of lower than 4.00 and 3.75, respectively. The Revolving Line of Credit is due and payable at maturity on October 19, 2021, the total outstanding amount as of December 31, 2019 and 2018 was $97.6 million and $20.0 million, respectively. Beginning on December 31, 2019 the $50.0 million Term Loan Facility requires quarterly repayments of $1.3 million. The remaining principal balance on the Term Facility will be due on the Term Loan maturity on October 19, 2021. The Credit Facility contains mandatory prepayment features upon the following: 100% of the excess of the total revolving outstanding amount whenever it exceeds the aggregate revolving commitments then in effect; 100% of net cash proceeds of asset sales (to the extent not reinvested in eligible assets with 180 days and proceeds exceed $1.0 million in the aggregated in any fiscal year); 100% of the proceeds from the issuance of any debt; beginning with the fiscal year ending December 31, 2019, 50% of excess cash flow if the consolidated total leverage ratio is greater than 2.0 times; and within five days of a qualifying IPO, but prior to or contemporaneously with any permitted redemption of the Redeemable Series A-1 pro-forma The Credit Facility also restricts the Company’s ability (subject to certain exceptions) to incur indebtedness, prepay or amend other indebtedness, create liens, make certain fundamental changes including mergers or dissolutions, pay dividends and make other payments in respect of capital stock, make certain investments, sell assets, change its lines of business, enter into transactions with affiliates and other corporate actions. In the event of a default, subject to varying cure periods and rights for certain events of default, the administrative agent may, at its option, declare the commitments to fund the Credit Facility to be terminated. Additionally, all amounts accrued under the Credit Facility would be accelerated causing such obligations to be due and payable immediately, which could materially and adversely affect the Company. During the years ended December 31, 2019 and 2018, the Company made a number of amendments to the credit facility: In October 2019, the Company’s Credit Facility was amended to: • Increase the Revolving Credit Facility to $130.0 million. In July 2019, the Company’s Credit Facility was amended to: • Increase the Revolving Credit Facility to $110.0 million. • Amend the quarterly Term Loan repayment provision to $1.3 million beginning on December 31, 2019 and each fiscal quarter thereafter. In October 2018, the Company’s Credit Facility was amended to: • Decrease the Term Loan Facility capacity to $50.0 million; • Defer the maturity date for the Revolving Credit Line from September 30, 2019 to October 19, 2021; • Amend the quarterly Term Loan repayment rates to 2.50% beginning on December 31, 2019 and each fiscal quarter thereafter, and • Amend the covenant ratios as follows: (i) remove the Consolidated Senior Leverage ratio; (ii) increase the Consolidated Fixed Charge Coverage ratio to greater than 1.25 from 1.125; and (iii) decrease the Consolidated Total Leverage ratio to below 3.75 times beginning with the first fiscal quarter ending September 30, 2018 and lower than 4.00 beginning with the first fiscal quarter during which a pro forma compliance certificate is delivered in accordance with the agreement’s terms, from a 4.25 rate in 2017. The 2019 and 2018 amendments to the Term Loan Facility were treated as debt modifications for accounting purposes. For all amendments, the amended Revolving Credit Facility’s borrowing capacity was determined to be greater than the borrowing capacity of the old arrangement. Previously unamortized deferred financing fees continued to be deferred and amortized until the instrument’s maturity for syndicate lenders who remained the same and written off for syndicate lenders that exited the syndicate in 2018. Debt issuance costs written off in 2018 amounted to $0.2 million and were expensed in interest expense-net The weighted average interest rate on the Credit Facility as of December 31, 2019 and 2018 was 5.41% and 6.31%, respectively. Second Lien Term Loan Facility A-1 expense-net Equipment Line of Credit Capital Lease Obligations Payments Interest Principal 2020 $ 2,531 $ 372 $ 2,159 2021 2,230 298 1,932 2022 1,750 199 1,551 2023 1,041 73 968 2024 286 7 279 $ 7,838 $ 949 $ 6,889 Convertible Subordinated Debt pro-forma 12-month in-kind. On October 19, 2018, the Convertible Subordinated Debt was paid out in full through funds received from the issuance of the Redeemable Series A-1 expense-net The Convertible Subordinated Debt contained an automatic conversion feature which allowed for the conversion of the Subordinated Debt into shares of the Company’s common stock upon the consummation of any IPO at a conversion price equal to a 30% discount to the IPO share price. This automatic conversion feature met the definition of a derivative. The derivative liability was fair valued at each reporting period until its related host instrument was paid in full. During 2018, the outstanding derivative liability was written off as the Convertible Subordinated Debt was paid in full. Fair value adjustments during 2018 were not significant. The following is a schedule of the aggregate annual maturities of long-term debt presented on the consolidated statement of financial position, based on the terms of the Credit Facility, operating and capital lease obligations as of December 31, 2019: 2020 $ 7,143 2021 143,300 2022 1,551 2023 968 2024 279 Total $ 153,241

Fair Value of Financial Instrum

Fair Value of Financial Instruments	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	13. FAIR VALUE OF FINANCIAL INSTRUMENTS As of September 30, 2020 and December 31, 2019, the following financial liabilities are measured at fair value on a recurring basis using significant unobservable inputs (Level 3). September 30, December 31, Business acquisitions contingent consideration, current $ 49,170 $ 8,614 Business acquisitions contingent consideration, long-term 9,742 379 Contingent put option — 7,100 Warrant option — 16,878 Conversion option 18,059 — Total $ 76,971 $ 32,971 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 for the nine months ended September 30, 2020 and September 30, 2019: Level 3 Embedded Option Business Consideration, (1) Business Long-term (2) Contingent Put Option Warrant Options Total Balance—at December 31, 2018 $ — $ 2,754 $ — $ — $ 12,818 $ 15,572 Acquisitions 31 — — — 31 Changes in fair value included in earnings — (670 ) — — 4,059 3,389 Payment of contingent consideration payable — (554 ) — — — (554 ) Balance—at September 30, 2019 $ — $ 1,561 $ — $ — $ 16,877 $ 18,438 Balance—at December 31, 2019 $ — $ 8,614 $ 379 $ 7,100 $ 16,878 $ 32,971 Foreign currency translation of contingent consideration payment — (208 ) — — — (208 ) Payment of contingent consideration payable — (12,464 ) — — — (12,464 ) Acquisitions 34,661 10,543 45,204 Series A-2 (9,361 ) — — — — (9,361 ) Issuance of warrant option — — — — 30,097 30,097 Reclass of long term to short term contingent consideration — 180 (180 ) — — — Changes in fair value included in earnings 27,420 18,387 (1,000 ) (19,240 ) 9,312 34,879 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 (1) Current portion of the contingent consideration is recorded in business acquisitions contingent consideration, current. (2) Long term portion of the contingent consideration is recorded in business acquisitions contingent consideration, long-term. Quantitative Information about Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3): Business Acquisitions Contingent Consideration 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 earnout thresholds. Conversion Option A-2 “with-and-without” “with-and-without” as-is A-2. Contingent Put Option A-1 “with-and-without” “with-and-without” as-is A-1 A-1 Warrant Options Series A-1 Preferred A-2 Series A-1 July 30, September 30, Common stock value (per share) $ 22.22 $ 31.60 Expected volatility 44.35 % 47.63 % Risk-free interest rate 0.55 % 1.68 % Expected life (years) 10 10 Series A-2 July 30, September 30, Common stock value (per share) $ 22.22 n/a Expected volatility 44.35 % n/a Risk-free interest rate 0.55 % n/a Expected life (years) 10 n/a 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.	15. FAIR VALUE OF FINANCIAL INSTRUMENTS As of December 31, 2019 and 2018, the following financial liabilities are measured at fair value on a recurring basis using significant unobservable inputs (Level 3). Liabilities at Fair Value 2019 Level 1 Level 2 Level 3 Total Contingent consideration payable (Note 7) $ $ $ 8,993 $ 8,993 Warrant option (Note 12) 16,878 16,878 Contingent put option (Note 13) 7,100 7,100 Total $ $ $ 32,971 $ 32,971 Liabilities at Fair Value 2018 Level 1 Level 2 Level 3 Total Contingent consideration payable (Note 7) $ $ $ 2,754 $ 2,754 Warrant option (Note 12) 12,818 12,818 Total $ $ $ 15,572 $ 15,572 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. There were no changes in valuation techniques or inputs utilized or transfers between fair value measurement levels during the years ended December 31, 2019 and 2018. The following table sets forth the Company’s financial instruments that were measured at fair value on a recurring basis at December 31, 2019 and 2018: Level 3 Contingent Contingent Consideration Derivative Warrant Total Balance—at January 1, 2018 $ 1,345 $ $ 2,647 $ $ 3,992 Acquisitions 2,951 2,951 Issuance of financial instruments 12,818 12,818 Change in fair value included in earnings (158 ) (352 ) (510 ) Payment of contingent consideration payable (1,384 ) (1,384 ) Write off of financial instruments (2,295 ) (2,295 ) Balance—at December 31, 2018 2,754 12,818 15,572 Acquisitions 5,022 379 5,401 Issuance of financial instruments Change in fair value included in earnings 1,392 7,100 4,060 12,552 Payment of contingent consideration payable (554 ) (554 ) Balance—at December 31, 2019 $ 8,614 $ 379 $ 7,100 $ 16,878 $ 32,971 Quantitative Information about Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3): Contingent Consideration Payable Contingent Put Option A-1 “with-and-without” “with-and-without” as-is A-1 Warrant Option A-1 Common stock value (per share) $ 31.60 Expected volatility 47.51 % Risk-free interest rate 1.92 % Expected life (years) 10.00 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	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	14. COMMITMENTS AND CONTINGENCIES Operating Leases September 30, Rent Office Equipment Total 2021 $ 5,863 $ 332 $ 6,195 2022 4,865 327 5,192 2023 2,961 129 3,090 2024 1,410 39 1,449 2025 and thereafter 1,385 1 1,386 $ 16,484 $ 828 $ 17,312 Total rent expense under operating leases was $2.2 and $6.4 for the three and nine months ended September 30, 2020, respectively, and $2.0 and $5.6 for the three and nine months ended September 30, 2019, respectively Other Commitments Contingencies Legal	16. COMMITMENTS AND CONTINGENCIES Operating Leases Rent Office Total 2020 $ 6,123 $ 299 $ 6,422 2021 4,983 252 5,235 2022 3,957 232 4,189 2023 2,567 96 2,663 2024 and thereafter 2,281 28 2,309 $ 19,911 $ 907 $ 20,818 Total rent expense under operating leases for the years ended December 31, 2019 and 2018 was $7.6 million and $6.7 million, respectively. Other Commitments Contingencies Legal

Redeemable Series A-1 Preferred

Redeemable Series A-1 Preferred Stock	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Temporary Equity Disclosure [Abstract]
Redeemable Series A-1 Preferred Stock	15. REDEEMABLE SERIES A-1 PREFERRED On October 19, 2018, the Company issued 12,000 shares of Redeemable Series A-1 Preferred Series A-1 Preferred On April 13, 2020, the Company amended and restated the certificate of designation of the Company’s Redeemable Series A-1 Preferred Series A-1 Preferred Series A-2 Preferred Series A-1 Preferred Series A-1 Preferred Series A-1 Preferred Series A-1 Preferred Series A-1 Preferred A-1 On July 27, 2020, the Company redeemed in full the Redeemable Series A-1 two-year A-1 A-1 The Redeemable Series A-1 Preferred Series A-1 Preferred Before redemption, the Redeemable Series A-1 Preferred At issuance the Company determined that the detachable warrant (Note 11) and the contingent put option were required to be accounted for separately. As of July 27, 2020 and December 31, 2019, the contingent put option issued in connection with the Redeemable Series A-1 Preferred As of September 30, 2019, the Company determined that the fair value of the contingent put option related to the Redeemable Series A-1 Preferred	17. REDEEMABLE SERIES A-1 On October 19, 2018, the Company issued 12,000 shares of Redeemable Series A-1 A-1 In the event of the occurrence of (i) a change of control with respect to the Company, (ii) a sale of the Company, (iii) an IPO, (iv) any recapitalization of the Company or other similar transaction in each case to the extent financed by third party capital, (v) an event of noncompliance or (vi) the fifth anniversary of the closing date (each, a “Mandatory Redemption Event”), the Company shall, at the option of the holder(s) of a majority of the outstanding Redeemable Series A-1 A-1 A-1 A-1 The Redeemable Series A-1 A-1 A-1 Total proceeds received and transaction costs incurred from the issuance of the Redeemable Series A-1 A-1 The Redeemable Series A-1 The Company classified the Redeemable Series A-1

Convertible and Redeemable Seri

Convertible and Redeemable Series A-2 Preferred Stock	9 Months Ended
	Sep. 30, 2020
Text Block [Abstract]
Convertible and Redeemable Series A-2 Preferred Stock	16. CONVERTIBLE AND REDEEMABLE SERIES A-2 PREFERRED On April 13, 2020, the Company entered into an agreement to issue 17,500 shares of the Convertible and Redeemable Series A-2 10-year A-2 At issuance, the Company determined that Convertible and Redeemable Series A-2 Preferred Stock and the detachable warrant (Note 11), were required to be accounted for separately. Upon the Company’s IPO, following which the Redeemable Series A-1 A-2 A-2 60-day A-2 A-2 A-2 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 A-2 A-2 A-2 With respect to any redemption of any share of the Convertible and Redeemable Series A-2 A-2 The Convertible and Redeemable Series A-2 Preferred 480- Series A-2 Preferred The Convertible and Redeemable Series A-2

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Equity [Abstract]
Stockholders' Equity (Deficit)	17. STOCKHOLDERS’ EQUITY (DEFICIT) Authorized Capital Stock— Warrants— Common Stock Issuances and Cancellations Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Shares Average Shares Average Shares Average Shares Average Common stock issued in connection with initial public offering 11,500,000 $ 15.00 — $ — 11,500,000 $ 15.00 — $ — Acquisitions — — 25,408 26.90 791,139 31.60 150,439 22.41 Redemption of series A-1 1,786,739 15.00 — — 1,786,739 15.00 — — Exercise of warrant options 2,534,239 0.01 — — 2,534,239 0.01 — — Exercise of options 16,850 8.92 7,150 6.03 20,350 9.27 22,675 18.01 Restricted shares, net 8,333 24.00 — — 8,333 24.00 — — Cancellation of shares (117,785 ) 17.15 — — (117,785 ) 17.15 — — 15,728,376 $ 12.82 32,558 $ 22.32 16,523,015 $ 13.72 173,114 $ 21.83 Employee Equity Incentive Plans As of September 30, 2020 and September 30, 2019, there was $10.4 million and $6.7 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 the next two years. The following number of shares were authorized to be issued and available for grant as of September 30, 2020 and September 30, 2019: September 30, 2020 2017 Plan 2013 Plan Shares authorized to be issued 2,945,443 2,047,269 Shares available for grant 1,233,266 — September 30, 2019 2017 Plan 2013 Plan Shares authorized to be issued 981,800 2,074,604 Shares available for grant 159,428 — Total stock compensation expense for the Plans was as follows: Three Months Ended September 30, 2020 2019 2017 plan 2013 plan 2017 plan 2013 plan Options Restricted Stock Options Total Options Restricted Stock Options Total Cost of revenue $ 276 $ — $ (5 ) $ 271 $ 278 $ — $ 97 $ 375 Selling, general and administrative expense 592 233 53 878 185 111 384 680 $ 868 $ 233 $ 48 $ 1,149 $ 463 $ 111 $ 481 $ 1,055 Nine Months Ended September 30, 2020 2019 2017 plan 2013 plan 2017 plan 2013 plan Options Restricted Stock Options Total Options Restricted Stock Options Total Cost of revenue $ 973 $ — $ 126 $ 1,099 $ 623 $ — $ 390 $ 1,013 Selling, general and administrative expense 1,178 973 189 2,340 457 323 1,784 2,564 $ 2,151 $ 973 $ 315 $ 3,439 $ 1,080 $ 323 $ 2,174 $ 3,577 Montrose Amended & Restated 2017 Stock Incentive Plan Restricted Stock Options 3-year 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 December 31, 2018 257,762 $ 20 $ 10 — $ 1,151 Granted 356,470 27 14 — — Forfeited/ cancelled (17,700 ) 19 — — — Outstanding at September 30, 2019 596,532 24 12 8.07 4,693 Outstanding at December 31, 2019 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 Exercised at September 30, 2020 925 14 — 7.33 7,649 Options vested and expected to vest 1,454,429 21 9.04 7,649 The following weighted-average assumptions were used in the Black-Sholes option-pricing model calculation for the nine months ended September 30, 2020 and September 30, 2019: September 30, September 30, Common stock value (per share) $ 16.36 $ 27.30 Expected volatility 46.14 47.94 Risk-free interest rate 0.41 2.15 Expected life (years) 7 7 Forfeiture rate None None Dividend rate None None Montrose Amended & Restated 2013 Stock Option Plan Options to Purchase Common Stock Weighted- Average Exercise Price per Share Weighted Average Grant Date Fair Value per Share Weighted Average Remaining Contract (in Years) Aggregate Intrinsic Value of In-The- Money Options (in Thousands) Outstanding at December 31, 2018 1,900,404 $ 6 $ 1 7.30 33,290 Forfeited/ cancelled (14,500 ) 8 — — — Expired (5,400 ) 8 — — — Exercised (19,010 ) 6 — — 466 Outstanding at September 30, 2019 1,861,494 6 1 6.65 47,381 Outstanding at December 31, 2019 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 Exercised at September 30, 2020 221,025 6 — 5.14 33,772 Options vested and expected to vest 1,817,494 6 — 5.17 33,772 Total shares outstanding from exercised options were 221,950 shares and 201,600 shares as of September 30, 2020 and December 31, 2019. Common Stock Reserved for Future Issuances September 30, December 31, Warrants 116,350 650,590 Montrose 2013 Stock Incentive Plan 2,047,269 2,058,619 Montrose 2017 Stock Incentive Plan 2,945,443 997,785 Common stock reserved for future issuance 5,109,062 3,706,994	19. STOCKHOLDERS’ EQUITY (DEFICIT) Authorized Capital Stock Warrants Common Stock Issuances and Repurchases 2019 Number of Average Price Acquisitions 150,439 $ 26.90 Exercise of options 27,350 6.16 Shares sold 42,415 31.60 Release of restricted stock 12,132 18.82 232,336 2018 Number of Average Price Acquisitions 264,582 $ 20.26 Exercise of options 5,000 6.03 Release of restricted stock 184,400 13.64 453,982 Furthermore, during 2018, certain shares of common stock were repurchased from an investor. This repurchase transaction was accounted for as treasury stock by the Company and recorded as a reduction to additional paid in capital. The treasury stock is carried at cost at the purchase price of $9.80 per share. There are no commitments to repurchase capital stock nor any restrictions imposed by state law related to this transaction. Amounts paid to repurchase these shares were funded through the issuance of the Redeemable Series A-1 Employee Equity Incentive Plans In October 2018, the Company offered to existing holders of stock options under the Plans to partially repurchase some of their options. As a result, on November 30, 2018, a total of 86,884 options were repurchased by the Company at the fair value price of $22.20 per option, less the aggregate applicable exercise price of each option for a total purchase price of $1.4 million. Total amount paid for these retired options was funded through the issuance of the Redeemable Series A-1 Total stock-based compensation for the Plans was $4.3 million and $5.8 million for the years ended December 31, 2019 and 2018. Of this amount, $1.4 million ($1.0 million related to the 2017 Plan and $0.4 million related to the 2013 Plan) and $0.8 million ($0.2 million related to the 2017 plan and $0.6 million related to the 2013 plan) was recorded in cost of revenues, $2.9 million ($1.1 million related to the 2017 Plan and $1.8 million related to the 2013 Plan) and $3.1 million ($0.2 million related to the 2017 plan and $2.9 million related to the 2013 plan) was recorded in selling, general and administrative expense and zero and $1.9 million (all related to the 2017 plan) was recorded in related party expense on the consolidated statements of operations for the years ended December 31, 2019 and 2018, respectively. As of December 31, 2019, there was $6.3 million of total unrecognized stock compensation expense related to unvested options and restricted stock granted under the Plans. That expense is expected to be recognized over the next two years. Montrose 2017 Stock Incentive Plan The Company was authorized to issue up to 997,785 and 600,900 shares as equity awards to participants under the 2017 Stock Incentive Plan as of December 31, 2019 and 2018, respectively. There were 137,908 and 131,113 shares available for grant at December 31, 2019 and 2018. Restricted Stock As of December 31, 2019 and 2018, the Company issued 40,000 and 26,400 shares of restricted stock with a fair market value of $24.00 and $13.64 per share, respectively, to certain plan participants as Director’s compensation. These shares of restricted stock vest over three years, 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. During 2019 and 2018, a total of 12,132 and 8,775 of these restricted shares became fully vested and released as common stock. There were 239,893 and 212,025 restricted shares outstanding as of December 31, 2019 and 2018. There were no forfeitures, cancellations or expirations of restricted shares during 2019 or 2018. Total stock compensation expense for restricted stock amounted to $0.4 million and $2.0 million for the years ended December 31, 2019 and 2018, respectively. In 2019, this amount consists of restricted stock granted to advisors of the Board. In 2018, this amount consists of $0.2 million pertaining to the restricted stock granted to Board Directors and $1.8 million pertaining to the restricted stock granted to advisors to the Board. Options Weighted- Weighted Weighted Aggregate Options to Average Average Average Intrinsic Value Purchase Exercise Grant Date Remaining of In-The-Money Common Price per Fair Value Contract Life Options Stock Share per Share (in Years) (in Thousands) Outstanding at January 1, 2018 49,875 $ 14 $ 7 $ 40 Granted 220,900 21 11 Forfeitured/cancelled (6,000 ) 18 Expired (7,013 ) 14 Outstanding at December 31, 2018 257,762 20 10 1,151 Granted 380,820 27 14 Forfeitured/cancelled (20,730 ) 20 Outstanding at December 31, 2019 617,852 24 12 7.82 4,696 Options vested and expected to vest 617,852 24 8.79 4,696 The following weighted-average assumptions were used in the Black-Sholes option-pricing model calculation for the years ended December 31, 2019 and 2018: 2019 2018 Common stock value (per share) $24.00–$31.60 $18.00–$24.00 Expected volatility 48.13% 48.69% Risk- free interest rate 1.43%–2.63% 2.86%–2.89% Expected life (years) 7.00 7.00 Forfeiture rate None None Dividend rate None None Total stock compensation expense for options granted under the Montrose 2017 Stock Incentive Plan was $2.1 million and $2.3 million for the years ended December 31, 2019 and 2018, respectively. Montrose Amended & Restated 2013 Stock Option Plan As of December 31, 2019 and 2018, the Company was authorized to issue up to 2,058,619 and 2,154,600 shares, respectively, as equity awards to employees and non-employee Weighted- Weighted Weighted Aggregate Options to Average Average Average Intrinsic Value Purchase Exercise Grant Date Remaining of In-The-Money Common Price per Fair Value Contract Life Options Stock Share per Share (in Years) (in Thousands) Outstanding at January 1, 2018 2,066,000 $ 6 $ 1 7.83 $ 16,534 Forfeitured/cancelled (43,225 ) 7 Expired (117,371 ) 6 Exercised (5,000 ) 6 60 Outstanding at December 31, 2018 1,900,404 6 1 7.30 33,290 Forfeitured/cancelled (10,635 ) 8 Expired (6,950 ) 8 Exercised (27,350 ) 6 Outstanding at December 31, 2019 1,855,469 6 1 6.40 46,617 Exercised at December 31, 2019 201,600 6 5.90 46,617 Options vested and expected to vest 1,855,494 6 5.86 46,617 Stock compensation expense related to the Montrose Amended & Restated 2013 Stock Option Plan was $2.2 million and $3.5 million for the years ended December 31, 2019 and 2018, respectively. Key Assumptions Used by Management Expected volatility represents the estimated volatility of the shares over the expected life of the options. The Company has estimated the expected volatility based on the weighted average historical volatilities of a pool of public companies that are comparable to Montrose since Montrose’s common stock is not publicly traded and does not have a readily determinable fair value. The Company uses an expected dividend yield of zero since no dividends are expected to be paid. The risk-free interest rate for periods within the expected life of the option is derived from the U.S. treasury interest rates in effect at the date of grant. The expected option life represents the period of time the option is expected to be outstanding. The simplified method is used to estimate the term since the Company does not have sufficient exercise history to calculate the expected life of the options. Forfeitures are recognized when they occur. Previously recognized expense is reversed for the portion of awards forfeited prior to vesting as and when forfeitures occur. Unless otherwise approved by the Board of Directors, options must generally be exercised while the individual is an employee. The expiration date of newly issued options is ten years after grant date unless earlier terminated as provided for in the Plan. Common Stock Reserved for Future Issuances 2019 2018 Warrants 650,590 650,590 Montrose 2013 Stock Incentive Plan 2,058,619 2,154,600 Montrose 2017 Stock Incentive Plan 997,785 600,900 Common stock reserved for future issuance 3,706,994 3,406,090

Net Loss per Share

Net Loss per Share	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Earnings Per Share [Abstract]
Net Loss per Share	18. NET LOSS PER SHARE Basic net loss per share is computed by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding during each period. The Redeemable Series A-1 A-2 A-1 Diluted net loss per share is computed by dividing net 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. The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company for the three and nine months ended September 30, 2020 and September 30, 2019: Three Months Ended Nine Months Ended In thousands, except for net loss per share 2020 2019 2020 2019 Net loss $ (30,737 ) $ (6,683 ) $ (58,761 ) $ (12,247 ) Accretion of redeemable series A-1 (6,542 ) (5,030 ) (17,601 ) (14,341 ) Redeemable series A-1 (24,341 ) — (24,341 ) — Convertible and redeemable series A-2 preferred stock dividend (2,870 ) — (2,870 ) — Net loss attributable to common stockholders—basic and diluted (64,490 ) (11,713 ) (103,573 ) (26,588 ) Weighted-average common shares outstanding—basic and diluted 21,554 8,718 13,669 8,640 Net loss per share attributable to common stockholders—basic and diluted $ (2.99 ) $ (1.34 ) $ (7.58 ) $ (3.08 ) The following equity shares were excluded from the calculation of diluted net loss per share attributable to common stockholders because their effect would have been anti-dilutive for the three and nine months ended September 30, 2020 and September 30, 2019: September 30, 2020 2019 Stock options 2,423,900 2,062,825 Restricted stock 176,384 158,426 Warrants 116,350 116,350	20. NET LOSS PER SHARE Basic net loss per share is computed by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding during each period. The Series A Convertible Preferred Stock, which was outstanding prior to its redemption and conversion on October 19, 2018, is considered a participating security. Therefore, the Company applies the two-class Diluted net loss per share is computed by dividing net 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 The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company for the years ended December 31, 2019 and 2018: In thousands, except for net loss per share 2019 2018 Net loss $ (23,557 ) $ (16,491 ) Convertible preferred stock deemed dividend, net of return from holders (932 ) Accretion of redeemable series A- (19,616 ) (3,605 ) Net loss attributable to common stockholders—basic and diluted (43,173 ) (21,028 ) Weighted- average common shares outstanding—basic and diluted 8,789 7,533 Net loss per share attributable to common stockholders—basic and diluted $ (4.91 ) $ (2.79 ) The following equity shares were excluded from the calculation of diluted net loss per share attributable to common stockholders because their effect would have been anti-dilutive for the years ended December 31, 2019 and 2018: 2019 2018 Stock options 2,048,738 1,321,890 Restricted stock 153,352 112,637 Warrants 116,350 108,267

Segment Information

Segment Information	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Segment Reporting [Abstract]
Segment Information	19. 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 for the three and nine months ended September 30, 2020 and September 30, 2019 were as follows: Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Segment Segment Segment Segment Segment Segment Segment Segment Assessment, Permitting and Response $ 26,593 $ 8,230 $ 5,413 $ 1,695 $ 49,768 $ 14,665 $ 15,017 $ 5,693 Measurement and Analysis 39,770 11,226 35,672 7,691 113,130 30,422 98,521 19,761 Remediation and Reuse 18,342 2,012 16,538 2,400 56,604 6,495 52,440 7,317 Total Operating Segments 84,705 21,468 57,623 11,786 219,502 51,582 165,978 32,771 Corporate and Other — (4,762 ) — (3,567 ) — (15,425 ) — (10,935 ) Total $ 84,705 $ 16,706 $ 57,623 $ 8,219 $ 219,502 $ 36,157 $ 165,978 $ 21,836 Presented below is a reconciliation of the Company’s segment measure to loss before benefit from income taxes for the three and nine months ended September 30, 2020 and September 30, 2019: For the Three Months Ended September 30, For the Nine Months Ended September 30, 2020 2019 2020 2019 Total $ 16,706 $ 8,219 $ 36,157 $ 21,836 Interest expense, net (3,043 ) (2,130 ) (10,896 ) (4,590 ) Income tax (expense) benefit (3,348 ) 412 1,563 1,308 Depreciation and amortization (9,740 ) (7,412 ) (27,084 ) (20,262 ) Stock-based compensation (1,149 ) (1,055 ) (3,439 ) (3,577 ) Start-up (602 ) (634 ) (1,283 ) (1,091 ) Acquisition costs (6 ) (1,278 ) (3,767 ) (2,350 ) Fair value changes in contingent put option 26,265 — 19,240 — Fair value changes in warrant options (9,311 ) (2,510 ) (9,312 ) (4,059 ) Fair value changes in compound embedded option (26,664 ) — (27,420 ) — Fair value changes in business acquisitions contingent consideration (13,404 ) (256 ) (17,387 ) 670 Short term purchase accounting fair value adjustment to deferred revenue — — (243 ) — IPO preparation costs and IPO related bonuses (6,378 ) (20 ) (6,908 ) (105 ) Discontinued services (i) (30 ) — (7,526 ) — Expenses related to financing transactions — — (277 ) — Other expenses (33 ) (19 ) (179 ) (27 ) Net loss $ (30,737 ) $ (6,683 ) $ (58,761 ) $ (12,247 ) (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	21. 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 sum of income (loss) from operations before income taxes, adjusted to exclude interest expense, depreciation and amortization, stock-based compensation, start-up 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 for the years ended December 31, 2019 and 2018 consisted of the following: 2019 Segment Segment Assessment, Permitting and Response $ 21,071 $ 7,572 Measurement and Analysis 135,531 27,828 Remediation and Reuse 77,252 9,736 Total Operating Segments 233,854 45,136 Corporate and Other (13,641 ) Total $ 233,854 $ 31,495 2018 Segment Segment Assessment, Permitting and Response $ 3,663 $ 1,339 Measurement and Analysis 117,373 20,779 Remediation and Reuse 67,769 11,400 Total Operating Segments 188,805 33,518 Corporate and Other (11,701 ) Total $ 188,805 $ 21,817 Presented below is a reconciliation of the Company’s segment measure to loss before benefit from income taxes for the years ended December 31, 2019 and 2018: 2019 2018 Total $ 31,495 $ 21,817 Interest expense, net (6,755 ) (11,085 ) Income tax benefit 3,121 4,968 Depreciation and amortization (27,705 ) (23,915 ) Stock-based compensation (4,345 ) (5,794 ) Start-up losses and investment in new services (1,874 ) (999 ) Acquisition costs (3,474 ) (1,595 ) Fair value changes in financial instruments (11,160 ) 352 Expenses related to financing transactions (398 ) Fair value changes in contingent liabilities (1,392 ) 158 Short term purchase accounting fair value adjustments (858 ) IPO preparation (610 ) Net loss $ (23,557 ) $ (16,491 )

Related-Party Transactions

Related-Party Transactions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Related Party Transactions [Abstract]
Related-Party Transactions	20. RELATED-PARTY TRANSACTIONS The Company engages a related party to provide Quality of Earnings reports on acquisition targets. The Company paid this related party approximately zero and $0.1 million during the three and nine months ended September 30, 2020 and $0.1 million and $0.4 million during the three and nine months ended September 30, 2019, respectively for its services. This expense is included within selling, general and administrative on the condensed consolidated statements of operations. As of September 30, 2020 and December 31, 2019, the Company had no significant unpaid invoices to this related party, which would be/are included in accounts payable and other accrued liabilities on the condensed consolidated statements of financial position. The related party used by the Company is partially owned through investment vehicles controlled by certain members of the Company’s Board of Directors. During the three months ended September 30, 2020, the holder of the Redeemable Series A-1 A-2 A-1	22. RELATED-PARTY TRANSACTIONS On July 5, 2013, the Company entered into an agreement with an entity owned and controlled by stockholders and directors of the Company to provide for the provision of certain advisory services (the “Monitoring Fee Agreement”). On October 25, 2017, the Company entered into an agreement to amend the terms of the existing Monitoring Fee Agreement. The amendment provided for the payment of services under the original Monitoring Fee Agreement through September 30, 2018 (or earlier, upon the occurrence of certain triggering events) to be settled through the issuance of shares under the 2017 Stock Incentive Plan. As a result, during 2017 the Company issued 175,625 restricted shares at with a fair market value of $13.64 per share in order to settle all fees payable through September 30, 2018. These restricted shares became fully vested on September 30, 2018. Total amortization expense recorded for the years ended December 31, 2018 related to the management agreement was $1.8 million, and is included in related-party expense on the accompanying consolidated statements of operations. Furthermore, the Company engages a related party to provide Quality of Earnings reports on acquisition targets. During 2019 and 2018, the Company paid this related party approximately $0.5 million and $0.4 million, respectively, for its services. This expense is included within related-party expense on the consolidated statements of operations. As of December 31, 2018, the Company had $0.01 million, in unpaid invoices to this related party which are included in accounts payable and other accrued liabilities on the consolidated statements of financial position. As of December 31, 2019, there were no unpaid invoices. The related party used by the Company is partially owned through investment vehicles controlled by certain members of the Company’s board of directors. During 2019, the Company received payment for the notes receivable outstanding from stockholders amounting to $0.1 million.

Defined Contribution Plan

Defined Contribution Plan	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Retirement Benefits [Abstract]
Defined Contribution Plan	21. DEFINED CONTRIBUTION PLAN On January 1, 2014, the Company established the Montrose Environmental Group 401(k) Savings Plan. As of September 30, 2020 and December 31, 2019, 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. Up 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. Subsequent to May 22, 2020, the Company temporarily ceased making employer contributions. Employer contributions for the three and nine months ended September 30, 2020 were zero and $1.2 million, respectively, and $0.5 million and $1.9 million for the three and nine months ended September 30, 2019, respectively, and are included within selling, general, and administrative expenses on the condensed consolidated statements of operations.	23. DEFINED CONTRIBUTION PLAN On January 1, 2014, the Company established the Montrose Environmental Group 401(k) Savings Plan. As of December 31, 2019 and 2018, plan participants may defer up to 85% and 100%, respectively, of their eligible wages for the year, up to the Internal Revenue Service dollar limit and catch up contribution allowed by law. The Company provides employer matching contributions equal to 100% of the first 3% of the participant’s compensation and 50% of the participant’s elective deferrals that exceed 3% but do not exceed 4% of the participant’s compensation. Employer contributions for the years ended December 31, 2019 and 2018 were $2.6 million $2.1 million, respectively, and are included within selling, general, and administrative expenses on the consolidated statements of operations.

Subsequent Events

Subsequent Events	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Subsequent Events [Abstract]
Subsequent Events	22. SUBSEQUENT EVENTS Effective October 6, 2020, the Company amended the New 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.	24. SUBSEQUENT EVENTS COVID-19 Series A-2 A-2 10-year A-2 A-2 A-2 The Company may, at its option on any one or more dates, redeem all or a minimum portion (the lesser of (i) $50.0 million in aggregate stated value of the Series A-2 A-2 A-2 On the occurrence of (i)(x) before a private offering which results in a redemption in full of the Redeemable Series A-1 four-and-a-half-year A-1 non-qualifying A-1 A-2 “ A-2 A-2 A-2 Upon a qualifying IPO, following which the Redeemable Series A-1 A-2 60-day A-2 A-2 A-1 With respect to any redemption of any share of the Series A-2 A-2 Following a private offering, following which the Redeemable Series A-1 A-2 A-1 Following a partial redemption of outstanding Redeemable Series A-1 A-2 A-1 Warrant Option A-2 pre-defined Business Acquisition Cash Common Other Contingent (1) Total CTEH $ 175,000 $ 25,000 $ (1,746 ) $ $ 198,254 (1) The contingent consideration element of the purchase price of CTEH’s acquisition is 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 first year earnout is to be 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 earn-out earn-out The cash portion of the CTEH purchase price was funded via the proceeds from the issuance of the Series A-2 The common stock component was paid through the issuance of 791,139 shares of common stock valued at $31.60 per share. The other purchase price component of the CTEH purchase price consists of a target working capital amount. CTEH’s resulting working capital at closing did not meet the target amount, therefore resulting in a working capital deficit due to the Company. The Company has not yet completed the initial purchase price allocation for this acquisition, 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. Supplemental Unaudited Pro-Forma A-2 As Acquisitions Consolidated Revenues $ 233,854 $ 110,119 $ 343,973 Net (loss) income (23,557 ) 34,928 11,371 Redeemable Series A-1 A-1 A-1 A-2 A-2 A-1 A-1 A-1 A-1 A-1 UniTranche Credit Facility A-1 A-1 The Company has evaluated subsequent events through June 3, 2020, which is the date the consolidated financial statements were available to be issued. There have been no additional subsequent events, other than those described above, that management believes would have a material impact on the Company or its consolidated financial statements.

Significant Accounting Policies

Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Significant Accounting Policies	2. SIGNIFICANT ACCOUNTING POLICIES Use of Estimates Cash Cash and cash equivalents and long-term debt financial instruments subject the Company to concentrations of credit risk. To minimize the risk of credit loss, these financial instruments are primarily held with large, reputable financial institutions. The Company has not experienced losses in such accounts and believes it is not exposed to any significant credit risk associated with these accounts. Restricted Cash Financial Instruments Level 1 Level 2 Level 3 The Company considers the carrying values of cash, restricted cash, accounts receivable, accounts payable, and accrued expenses to approximate fair value for these financial instruments due to the short maturities of these instruments. The Company’s contingent put option, warrant option and any acquisition’s contingent consideration are carried at fair value and determined according to the fair value hierarchy above. The Company’s variable rate borrowings under its Credit Facility (Note 14) is tied to market indices and, thus, approximate fair value. The estimated fair value of the long-term debt under the credit facility is based on borrowing rates currently available to the Company for loans with similar terms and remaining maturities. Impairment of Long-Lived Assets Acquisitions Goodwill The annual evaluation for impairment of goodwill does not include a qualitative assessment and proceeds directly to a two-step Management determined that no impairment of goodwill existed as of the testing dates (October 1, 2019 and October 1, 2018). Also, no triggering events or changes in circumstances occurred during the period October 1, 2019 through December 31, 2019 that would warrant retesting goodwill for impairment. Contingent Consideration Embedded Derivatives de-recognized A-1 These embedded derivatives were bifurcated, accounted for at its fair value and presented separately on the consolidated statements of financial position. Changes in fair value of the contingent put option was recognized as a component of other income (expense) on the Company’s consolidated statements of operations. Foreign Currency non-U.S. Accumulated Other Comprehensive Loss A-1 Revenue Recognition The Company’s services are performed under two general types of contracts (i) fixed-price and (ii) time-and-materials. time-and-materials time-and-materials The Company accounts for individual promises in contracts as separate performance obligations if the promises are distinct. The assessment requires judgment. The majority of our contracts have a single performance obligation as the promise to transfer the individual goods or services is not separately identifiable from other promises in the contracts and is, therefore, not distinct. Certain contracts in our Measurement and Analysis have multiple performance obligations, most commonly due to the contracts providing for multiple laboratory tests which are individual performance obligations. For the Measurement and Analysis contracts with multiple performance obligations, the Company allocates the transaction price to each performance obligation based on the relative standalone selling price of each performance obligation. The standalone selling price of each performance obligation is generally determined by the observable price of a service when sold separately. Fixed fee contracts There are inherent uncertainties in the estimation process for cost to cost contracts, as the estimation of total contract costs is complex, subject to many variables and requires judgment. It is possible that estimates of costs to complete a performance obligation will be revised in the near-term based on actual progress and costs incurred. These uncertainties primarily impact the Company’s contracts in the Remediation and Reuse segment including those contracts associated with Emerging Compounds Treatments Technologies, Inc. which was acquired in August 2019. Time-and-materials —Time-and-materials Segment Reporting Cost of Revenues Selling, General and Administrative Expenses Offering Costs Stock-Based Compensation non-employees Non-Employees. non-employee Fair Value of Common Stock Following an IPO by the Company, valuation models, including estimates and assumptions used in such models, will not be necessary to estimate the fair value of the Company’s common stock, as shares of the Company’s common stock will be traded in the public market and the fair value will be determined based on the closing price of the Company’s common stock. Income Taxes A valuation allowance is recorded when it is more-likely-than-not For acquired business entities, if the Company identifies changes to acquired deferred tax asset valuation allowances or liabilities related to uncertain tax positions during the measurement period, and they relate to new information obtained about facts and circumstances existing as of the acquisition date, those changes are considered a measurement period adjustment and the offset is recorded to goodwill. The Company records uncertain tax positions on the basis of the two-step more-likely-than-not more-likely-than-not

Contingent Put Option

Contingent Put Option	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Contingent Put Option	13. CONTINGENT PUT OPTION As of December 31, 2018, the Company determined that the fair value of the contingent put option related to the Redeemable Series A-1

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Convertible Preferred Stock	18. CONVERTIBLE PREFERRED STOCK Between May and June 2015, the Company issued 69,817 shares of Series A Convertible Preferred Stock. Each preferred share was issued as part of a unit, which consisted of one share of Series A Convertible Preferred Stock of $429.70 per share and a commitment to purchase $143.23 per share of convertible subordinated debt (see Note 14). On March 16, 2017 the holders of the Series A Convertible Preferred Stock agreed to certain changes to the terms of the Series A Convertible Preferred Stock. The Company concluded that such modifications resulted in an extinguishment of the Series A Convertible Preferred Stock and a new issuance for accounting purposes. In making this assessment, the Company applied the qualitative value method. As such, the Company derecognized the carrying value of the Series A Convertible Preferred Stock within equity, recognized the modified Series A Convertible Preferred Stock at its fair value, and recorded the difference as a deemed dividend paid on preferred stock and reduced the income available to common shareholders. The Company classified the Series A Preferred Stock as mezzanine equity as the Series A Convertible Preferred Stock contained a redemption feature (Deemed Liquidation Event) which was contingent upon certain change of control events, the occurrence of which was not solely within the control of the Company. These contingent events were not considered probable of occurring and as such the Company did not accrete the mezzanine equity to its redemption value each period. Furthermore, the Company determined that none of the features embedded in the Convertible Series A Preferred Stock were required to be accounted for separately as a derivative. The Series A Preferred Stock was entitled to receive a 12% cumulative dividend, compounded annually, and payable only upon a liquidation event of the Company. On October 19, 2018, the Company redeemed 48,074 shares of the Series A Convertible Preferred Stock for cash at a price of $644.55 per share and converted the remaining outstanding preferred shares into common stock at a 1:31 rate resulting in a total of 674,033 shares of common stock issued at a price of $22.20 per share. Total consideration transferred in cash and common stock amounted to $45.9 million. The Series A Convertible Preferred Stock was remeasured before the redemption transaction resulting in a mark to market adjustment of $3.2 million and a total carrying value of $48.1 million. Because the carrying value of the Series A Preferred Stock exceeded the fair value of the consideration transferred by $2.2 million, such excess was treated as a return from the holders. Both, the remeasurement and the carrying value excess adjustments, are presented net within additional paid in capital in the Company’s statements of convertible preferred stock, redeemable series A-1 A-1

Description of the Business a_2

Description of the Business and Basis of Presentation (Policies)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation	Basis of Presentation
Recently Adopted Accounting Pronouncements /Recently Issued Accounting Pronouncements Not Yet Adopted	Recently Adopted Accounting Pronouncements In June 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standard Update (“ASU”) 2018-07, Compensation—Stock to non-employees will In January 2017, the FASB issued ASU 2017-04, Intangibles—Goodwill Recently Issued Accounting Pronouncements Not Yet Adopted 2020-06, 470-20) 815-40)—Accounting In March 2020, the FASB issued ASU 2020-04, Reference ASU 2020-04 provides ASU 2020-04 was In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. ASU 2019-12 removes certain exceptions to the general principles in ASC 740 and clarifies and amends certain guidance to promote consistent application. ASU 2019-12 is effective for the Company’s annual and interim periods beginning on January 1, 2021, with early adoption permitted. Depending on the amendment, adoption may be applied on a retrospective, modified retrospective or prospective basis. The Company is currently evaluating the impact of the adoption of the standard on the condensed consolidated financial statements and does not anticipate the standard to have a significant impact. In June 2016, the FASB issued ASU 2016-13, Financial In February 2016, the FASB issued ASU 2016-2, Leases
Use of Estimates		Use of Estimates
Cash		Cash Cash and cash equivalents and long-term debt financial instruments subject the Company to concentrations of credit risk. To minimize the risk of credit loss, these financial instruments are primarily held with large, reputable financial institutions. The Company has not experienced losses in such accounts and believes it is not exposed to any significant credit risk associated with these accounts.
Restricted Cash		Restricted Cash
Financial Instruments		Financial Instruments Level 1 Level 2 Level 3 The Company considers the carrying values of cash, restricted cash, accounts receivable, accounts payable, and accrued expenses to approximate fair value for these financial instruments due to the short maturities of these instruments. The Company’s contingent put option, warrant option and any acquisition’s contingent consideration are carried at fair value and determined according to the fair value hierarchy above. The Company’s variable rate borrowings under its Credit Facility (Note 14) is tied to market indices and, thus, approximate fair value. The estimated fair value of the long-term debt under the credit facility is based on borrowing rates currently available to the Company for loans with similar terms and remaining maturities.
Impairment of Long-Lived Assets		Impairment of Long-Lived Assets
Acquisitions		Acquisitions
Goodwill		Goodwill The annual evaluation for impairment of goodwill does not include a qualitative assessment and proceeds directly to a two-step Management determined that no impairment of goodwill existed as of the testing dates (October 1, 2019 and October 1, 2018). Also, no triggering events or changes in circumstances occurred during the period October 1, 2019 through December 31, 2019 that would warrant retesting goodwill for impairment.
Contingent Consideration		Contingent Consideration
Embedded Derivatives		Embedded Derivatives de-recognized A-1 These embedded derivatives were bifurcated, accounted for at its fair value and presented separately on the consolidated statements of financial position. Changes in fair value of the contingent put option was recognized as a component of other income (expense) on the Company’s consolidated statements of operations.
Foreign Currency		Foreign Currency non-U.S.
Accumulated Other Comprehensive Loss		Accumulated Other Comprehensive Loss A-1
Revenue Recognition		Revenue Recognition The Company’s services are performed under two general types of contracts (i) fixed-price and (ii) time-and-materials. time-and-materials time-and-materials The Company accounts for individual promises in contracts as separate performance obligations if the promises are distinct. The assessment requires judgment. The majority of our contracts have a single performance obligation as the promise to transfer the individual goods or services is not separately identifiable from other promises in the contracts and is, therefore, not distinct. Certain contracts in our Measurement and Analysis have multiple performance obligations, most commonly due to the contracts providing for multiple laboratory tests which are individual performance obligations. For the Measurement and Analysis contracts with multiple performance obligations, the Company allocates the transaction price to each performance obligation based on the relative standalone selling price of each performance obligation. The standalone selling price of each performance obligation is generally determined by the observable price of a service when sold separately. Fixed fee contracts There are inherent uncertainties in the estimation process for cost to cost contracts, as the estimation of total contract costs is complex, subject to many variables and requires judgment. It is possible that estimates of costs to complete a performance obligation will be revised in the near-term based on actual progress and costs incurred. These uncertainties primarily impact the Company’s contracts in the Remediation and Reuse segment including those contracts associated with Emerging Compounds Treatments Technologies, Inc. which was acquired in August 2019. Time-and-materials —Time-and-materials
Segment Reporting		Segment Reporting
Cost of Revenues		Cost of Revenues
Selling, General and Administrative Expenses		Selling, General and Administrative Expenses
Offering Costs		Offering Costs
Stock-Based Compensation		Stock-Based Compensation non-employees Non-Employees. non-employee
Fair Value of Common Stock		Fair Value of Common Stock Following an IPO by the Company, valuation models, including estimates and assumptions used in such models, will not be necessary to estimate the fair value of the Company’s common stock, as shares of the Company’s common stock will be traded in the public market and the fair value will be determined based on the closing price of the Company’s common stock.
Income Taxes		Income Taxes A valuation allowance is recorded when it is more-likely-than-not For acquired business entities, if the Company identifies changes to acquired deferred tax asset valuation allowances or liabilities related to uncertain tax positions during the measurement period, and they relate to new information obtained about facts and circumstances existing as of the acquisition date, those changes are considered a measurement period adjustment and the offset is recorded to goodwill. The Company records uncertain tax positions on the basis of the two-step more-likely-than-not more-likely-than-not

Revenues and Accounts Receiva_2

Revenues and Accounts Receivable (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Revenues And Accounts Receivable [Abstract]
Schedule of Contract Balances	The following table presents the Company’s contract balances: September 30, 2020 December 31, 2019 Contract assets $ 35,441 $ 13,605 Contract liabilities 9,335 3,314	The following table presents the Company’s contract balances as of December 31, 2019 and 2018: 2019 2018 Contract assets $ 13,605 $ 10,112 Contract liabilities 3,314 2,395
Schedule of Accounts Receivable, Net	Accounts Receivable, Net September 30, December 31, Accounts receivable, invoiced $ 52,372 $ 46,643 Accounts receivable, other 929 611 Allowance for doubtful accounts (8,516 ) (1,327 ) Accounts receivable—net $ 44,785 $ 45,927	Accounts Receivable, Net 2019 2018 Accounts receivable, invoiced $ 46,643 $ 38,786 Accounts receivable, other 611 135 Allowance for doubtful accounts (1,327 ) (453 ) Accounts receivable—net $ 45,927 $ 38,468
Schedule of Allowance for Doubtful Accounts	The allowance for doubtful accounts as of September 30, 2020 and December 31, 2019 consisted of the following: Beginning Balance Bad Debt Expense Charged to Allowance Other (1) Ending Balance Nine months ended September 30, 2020 $ 1,327 $ 6,445 $ (294 ) $ 1,038 $ 8,516 Year ended December 31, 2019 453 1,246 (556 ) 184 1,327 (1) This amount consists of additions to the allowance due to business acquisitions.	The allowance for doubtful accounts as of December 31, 2019 and 2018 consisted of the following: Beginning Bad Debt Charged to Other (1) Ending Year ended December 31, 2018 $ 573 $ (624 ) $ 504 $ 453 Year ended December 31, 2019 453 1,246 (556 ) 184 1,327 (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	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Schedule of Prepaid and Other Current Assets	Prepaid and other current assets as of September 30, 2020 and December 31, 2019 consisted of the following: September 30, December 31, Deposits $ 733 $ 605 Prepaid expenses 1,665 1,235 Prepaid insurance 2,577 170 Supplies 2,428 2,368 Offering costs — 1,240 Income tax receivable 1,182 1,205 Prepaid and other current assets $ 8,585 $ 6,823	Prepaid and other current assets as of December 31, 2019 and 2018 consisted of the following: 2019 2018 Deposits $ 605 $ 598 Prepaid expenses 1,235 1,016 Prepaid insurance 170 85 Supplies 2,368 745 Offering costs 1,240 Other 154 Prepaid and other current assets $ 5,618 $ 2,598

Property and Equipment (Tables)

Property and Equipment (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	Property and equipment, net, as of September 30, 2020 and December 31, 2019 consisted of the following: Estimated September 30, December 31, Lab and test equipment 7 years $ 17,755 $ 14,810 Vehicles 5 years 12,869 11,073 Equipment 3–7 years 31,539 29,922 Furniture and fixtures 7 years 2,854 1,119 Leasehold improvements 7 years 6,758 5,954 Aircraft 10 years 834 — Building 39 years 2,975 — 75,584 62,878 Land 725 — Construction in progress 156 796 Less accumulated depreciation (41,906 ) (36,638 ) Total property and equipment—net $ 34,559 $ 27,036	Property and equipment, net, as of December 31, 2019 and 2018 consisted of the following: Estimated 2019 2018 Lab and test equipment 7 years $ 14,810 $ 10,275 Vehicles 5 years 11,073 8,122 Equipment 3-7 years 29,922 25,468 Furniture and fixtures 7 years 1,119 1,004 Leasehold improvements 7 years 5,954 5,137 62,878 50,006 Construction in progress 796 Less accumulated depreciation and amortization (36,638 ) (29,214 ) Total property and equipment—net $ 27,036 $ 20,792

Business Acquisitions (Tables)

Business Acquisitions (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Summary of Purchase Price Attributable to Acquisitions	The purchase price attributable to the acquisitions was allocated as follows: ECT2 All Other Acquisitions Total Cash $ 3,149 $ 2,137 $ 5,286 Restricted cash 629 — 629 Accounts receivable 1,707 3,751 5,458 Other current assets 498 61 559 Current assets 5,983 5,949 11,932 Property and equipment 776 3,288 4,064 Customer relationships 13,840 12,748 26,588 Trade names 1,008 659 1,667 Covenants not to compete 3,360 2,083 5,443 Proprietary software — 2,560 2,560 Patent 17,479 — 17,479 Goodwill 16,395 20,227 36,622 Total assets 58,841 47,514 106,355 Current liabilities 5,024 977 6,001 Non- — 2,171 2,171 Total liabilities 5,024 3,148 8,172 Purchase price $ 53,817 $ 44,366 $ 98,183	The purchase price attributable to each acquisition was allocated as follows: ECT2 All Other Total Cash $ 3,149 $ 2,137 $ 5,286 Restricted cash 629 629 Accounts receivable 1,707 3,751 5,458 Other current assets 498 61 559 Current assets 5,983 5,949 11,932 Property and equipment 776 3,288 4,064 Customer relationships 13,840 12,748 26,588 Trade names 1,008 659 1,667 Covenants not to compete 3,360 2,083 5,443 Proprietary software 2,560 2,560 Patent 17,479 17,479 Goodwill 16,395 20,227 36,622 Total assets 58,841 47,514 106,355 Current liabilities 5,024 977 6,001 Non- 2,171 2,171 Total liabilities 5,024 3,148 8,172 Purchase price $ 53,817 $ 44,366 $ 98,183
Summary of Elements of Purchase Price of Acquisitions	The following table summarizes the elements of purchase price of the acquisitions completed during the nine months ended September 30, 2020: Cash Common Stock Other Purchase Price Component Current Other Purchase Price Component Long Term Contingent Consideration Current Contingent Consideration Long Term Total Purchase Price CTEH $ 175,000 $ 25,000 $ (2,297 ) $ — $ 34,451 $ 10,543 $ 242,697 All other acquisitions 450 — 50 100 210 — 810 $ 175,450 $ 25,000 $ (2,247 ) $ 100 $ 34,661 $ 10,543 $ 243,507 The following table summarizes the elements of purchase price of the acquisitions completed during the nine months ended September 30, 2019: Cash Common Stock Other Purchase Price Components Contingent Consideration Total Purchase Price Golden $ 1,500 — — $ 477 $ 1,977 TES 2,359 322 25 4,911 7,617 TESUS 18,683 3,041 1,495 — 23,219 AWS 6,020 — 150 — 6,170 AEC 808 — — — 808 LEHDER 3,878 684 — 13 4,575 ECT2 54,037 — (220 ) — 53,817 $ 87,285 $ 4,047 $ 1,450 $ 5,401 $ 98,183	The following table summarizes the elements of purchase price of the 2019 acquisitions: Cash Common Other Contingent Total Golden $ 1,500 $ $ $ 477 $ 1,977 TES 2,359 322 25 4,911 7,617 TESUS 18,683 3,041 1,495 23,219 AWS 6,020 150 6,170 AEC 808 808 LEHDER 3,878 684 13 4,575 ECT2 54,037 (220 ) 53,817 $ 87,285 $ 4,047 $ 1,450 $ 5,401 $ 98,183 The following table summarizes the elements of purchase price of the 2018 acquisitions: Cash Common Other Contingent Total SES $ 450 $ $ 50 $ $ 500 FA 1,096 360 372 1,828 AGC 7,400 925 95 871 9,291 Streamline 5,678 631 180 6,489 Leymaster 2,465 435 250 434 3,584 AES 21,877 2,028 (16 ) 1,274 25,163 EPS 9,995 1,615 (71 ) 11,539 $ 48,961 $ 5,634 $ 848 $ 2,951 $ 58,394
Summary of Supplemental Unaudited Pro Forma Information	The unaudited condensed consolidated financial information summarized in the following table gives effect to the 2020 and the 2019 acquisitions discussed above assuming they occurred on January 1, 2019. 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, 2019, nor does the information purport to reflect results for any future period. For the Three Months Ended September 30, 2020 2019 As Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) As Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) Revenues $ 84,705 $ — $ 84,705 $ 57,623 $ 18,551 $ 76,174 Net (loss) income (30,737 ) — (30,737 ) (6,683 ) 4,042 (2,641 ) For the Nine Months Ended September 30, 2020 2019 As Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) As Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) Revenues $ 219,502 $ 31,253 $ 250,755 $ 165,978 $ 107,459 $ 273,437 Net (loss) income (58,761 ) 10,288 (48,473 ) (12,247 ) 30,002 17,755	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, 2018, nor does the information project results for any future period. As reported Acquisitions Pro-Forma Consolidated Pro-Forma 2019 Revenues $ 233,854 $ 25,446 $ 259,300 Net income (loss) (23,557 ) 5,199 (18,358 ) 2018 Revenues $ 188,805 $ 58,526 $ 247,331 Net income (loss) (16,491 ) 14,580 (1,911 )
AES
Summary of Purchase Price Attributable to Acquisitions		The purchase price attributable to each acquisition was allocated as follows: AES All Other Total Cash $ 2,746 $ 380 $ 3,126 Accounts receivable 1,404 5,045 6,449 Other current assets 32 288 320 Current assets 4,182 5,713 9,895 Property and equipment 532 1,002 1,534 Customer relationships 11,296 15,543 26,839 Trade names 241 812 1,053 Covenants not to compete 961 1,813 2,774 Goodwill 11,019 13,661 24,680 Total assets 28,231 38,544 66,775 Current liabilities 3,068 2,141 5,209 Non- 3,172 3,172 Total liabilities 3,068 5,313 8,381 Purchase price $ 25,163 $ 33,231 $ 58,394
CTEH
Summary of Purchase Price Attributable to Acquisitions	The preliminary purchase price attributable to the acquisitions was allocated as follows: CTEH All Other 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 146,625 626 147,251 Total assets 252,418 810 253,228 Current liabilities 9,721 — 9,721 Total liabilities 9,721 — 9,721 Purchase price $ 242,697 $ 810 $ 243,507
Subsequent Event
Summary of Elements of Purchase Price of Acquisitions		The transaction 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 CTEH: Cash Common Other Contingent (1) Total CTEH $ 175,000 $ 25,000 $ (1,746 ) $ $ 198,254 (1) The contingent consideration element of the purchase price of CTEH’s acquisition is 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 first year earnout is to be 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 earn-out earn-out
Summary of Supplemental Unaudited Pro Forma Information		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, nor does the information purport to reflect results for any future period. As Acquisitions Consolidated Revenues $ 233,854 $ 110,119 $ 343,973 Net (loss) income (23,557 ) 34,928 11,371

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Amounts Related to Goodwil	Amounts related to goodwill as of September 30, 2020 and December 31, 2019 are as follows: Assessment, Permitting and Measurement and Analysis Remediation and Reuse Total Balance as of December 31, 2019 $ 15,173 $ 68,628 $ 43,257 $ 127,058 Goodwill acquired during the period 146,625 426 200 147,251 Balance as of September 30, 2020 $ 161,798 $ 69,054 $ 43,457 $ 274,309	Amounts related to goodwill as of December 31, 2019 and 2018 are as follows: Assessment, Measurements Remediation Total Balance as of January 1, 2018 $ — $ 44,123 $ 21,695 $ 65,818 Goodwill acquired during the year 15,173 4,340 5,167 24,680 Balance as of December 31, 2018 15,173 48,463 26,862 90,498 Goodwill acquired during the year 20,165 16,395 36,560 Balance as of December 31, 2019 $ 15,173 $ 68,628 $ 43,257 $ 127,058
Schedule of Amounts Related to Finite-Lived Intangible Assets	Amounts related to finite-lived intangible assets as of September 30, 2020 and December 31, 2019 are as follows: September 30, 2020 Estimated Useful Life Gross Balance Accumulated Amortization Total Intangible Assets—Net Finite lived intangible assets Customer relationships 7–15 years $ 164,782 $ 49,068 $ 115,714 Covenants not to compete 4–5 years 29,942 20,444 9,498 Trade names 1–5 years 16,938 12,196 4,742 Proprietary software 3-5 20,808 4,915 15,893 Patent 16 years 17,479 1,125 16,354 Total other intangible assets—net $ 249,949 $ 87,748 $ 162,201 December 31, 2019 Estimated Useful Life Gross Balance Accumulated Amortization Total Intangible Assets—Net Finite lived intangible assets Customer relationships 7–10 years $ 108,782 $ 36,700 $ 72,082 Covenants not to compete 4–5 years 25,832 17,572 8,260 Trade names 1–5 years 12,738 10,230 2,508 Proprietary software 3 years 3,885 1,359 2,526 Patent 16 years 17,479 306 17,173 Total other intangible assets—net $ 168,716 $ 66,167 $ 102,549	Amounts related to finite-lived intangible assets as of December 31, 2019 and 2018 are as follows: 2019 Estimated Gross Accumulated Total Finite lived intangible assets Customer relationships 7–10 years $ 108,782 $ 36,700 $ 72,082 Covenants not to compete 4–5 years 25,832 17,572 8,260 Trade names 1–5 years 12,738 10,230 2,508 Proprietary software 3 years 3,885 1,359 2,526 Patent 16 years 17,479 306 17,173 Total other intangible assets—net $ 168,716 $ 66,167 $ 102,549 2018 Estimated Gross Accumulated Total Finite lived intangible assets Customer relationships 7–10 years $ 82,176 $ 23,932 $ 58,244 Covenants not to compete 4–5 years 20,390 13,785 6,605 Trade names 1–5 years 11,072 8,282 2,790 Proprietary software 3 years 1,303 379 924 Total other intangible assets—net $ 114,941 $ 46,378 $ 68,563
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, 2020 (remaining) $ 7,543 2021 28,140 2022 24,053 2023 20,073 2024 16,951 Thereafter 65,441 $ 162,201	Future amortization expense is estimated to be as follows for each of the five following years and thereafter ending December 31, 2019: 2020 $ 21,473 2021 18,895 2022 14,810 2023 12,025 2024 9,357 Thereafter 25,989 $ 102,549

Accounts Payable and Other Ac_2

Accounts Payable and Other Accrued Liabilities (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Summary of Accounts Payable and Other Accrued Liabilities	Accounts payable and other accrued liabilities consisted of the following as of September 30, 2020 and December 31, 2019: September 30, December 31, Accounts payable $ 13,360 $ 15,034 Accrued expenses 10,395 10,733 Other purchase obligations 572 — Contract liabilities 9,335 3,314 Other current liabilities 235 504 Total accounts payable and other accrued liabilities $ 33,897 $ 29,585	Accounts payable and other accrued liabilities consisted of the following as of December 31, 2019 and 2018: 2019 2018 Accounts payable $ 15,034 $ 8,679 Accrued expenses 10,733 4,414 Business acquisitions contingent consideration 8,614 2,754 Other business acquisitions purchase price obligations 525 Contract liabilities 3,314 2,395 Other current liabilities 504 241 Total accounts payable and other accrued liabilities $ 38,199 $ 19,008

Accrued Payroll and Benefits (T

Accrued Payroll and Benefits (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Schedule of Accrued Payroll and Benefits	Accrued payroll and benefits consisted of the following as of September 30, 2020 and December 31, 2019: September 30, December 31, Accrued bonuses $ 7,854 $ 3,449 Accrued paid time off 2,157 2,154 Accrued payroll 5,536 4,470 Accrued other 2,410 959 Total accrued payroll and benefits $ 17,957 $ 11,032	Accrued payroll and benefits consisted of the following as of December 31, 2019 and 2018: 2019 2018 Accrued bonuses $ 3,449 $ 1,585 Accrued paid time off 2,154 2,289 Accrued payroll 4,470 3,496 Accrued other 959 837 Total accrued payroll and benefits $ 11,032 $ 8,207

Debt (Tables)

Debt (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Debt Disclosure [Abstract]
Schedule of Debt	Debt as of September 30, 2020 and December 31, 2019 consisted of the following: September 30, December 31, Term Loan Facility $ 174,453 $ 48,750 Revolving Line of Credit — 97,590 Capital leases 3,127 3,765 Other leases 2 12 Equipment line of credit 3,214 3,124 Less deferred debt issuance costs (4,345 ) (1,052 ) Total debt 176,451 152,189 Less current portion of long term debt (5,034 ) (7,143 ) Long-term debt, less current portion $ 171,417 $ 145,046	Debt as of December 31, 2019 and 2018 consists of the following: 2019 2018 Term Loan Facility $ 48,750 $ 50,000 Revolving Line of Credit 97,590 20,000 Capital leases 3,765 2,640 Other leases 12 51 Equipment line of credit 3,124 1,712 Less deferred debt issuance costs (1,052 ) (1,176 ) Total debt 152,189 73,227 Less current portion of long term debt (7,143 ) (2,262 ) Long- term debt, less current portion $ 145,046 $ 70,965
Schedule of Applicable Percentages	The applicable margin means a percentage per annum determined in accordance with the following table as of December 31, 2019: Pricing Consolidated Leverage Ratio Commitment Fee Eurodollar Rate Loans and LIBOR Letter of Credit Fee Daily Floating Rate Rate Loans 1 > 3.75 to 1.0 0.50 % 4.00 % 4.00 % 3.00 % 2 £ 0.50 3.50 3.50 2.50 3 £ 0.40 3.00 3.00 2.00 4 < 2.25 to 1.0 0.30 2.50 2.50 1.50	The applicable margin means a percentage per annum determined in accordance with the following table for December 31, 2019 and 2018: Pricing Consolidated Leverage Ratio Commitment Eurodollar Daily Rate 1 > 3.75 to 1.0 0.50 % 4.00 % 4.00 % 3.00 % 2 £ 0.50 3.50 3.50 2.50 3 £ 0.40 3.00 3.00 2.00 4 £ 0.30 2.50 2.50 1.50
Schedule of Capital Leases By Maturity	All capital leases (including those purchased through the Company’s equipment line of credit) mature by 2025 as follows: September 30, Payments Interest Principal 2021 $ 2,670 $ 368 $ 2,302 2022 2,178 265 1,913 2023 1,582 139 1,443 2024 667 32 635 2025 49 1 48 $ 7,146 $ 805 $ 6,341	All capital leases (including those purchased through the Company’s equipment line of credit) mature by 2024 as follows: Payments Interest Principal 2020 $ 2,531 $ 372 $ 2,159 2021 2,230 298 1,932 2022 1,750 199 1,551 2023 1,041 73 968 2024 286 7 279 $ 7,838 $ 949 $ 6,889
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 condensed consolidated statement of financial position, based on the terms of the credit facility, capital lease obligations and equipment line of credit as of September 30, 2020: September 30, 2021 $ 5,034 2022 6,839 2023 8,006 2024 7,198 2025 153,719 Total $ 180,796	The following is a schedule of the aggregate annual maturities of long-term debt presented on the consolidated statement of financial position, based on the terms of the Credit Facility, operating and capital lease obligations as of December 31, 2019: 2020 $ 7,143 2021 143,300 2022 1,551 2023 968 2024 279 Total $ 153,241

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Summary of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis	As of September 30, 2020 and December 31, 2019, the following financial liabilities are measured at fair value on a recurring basis using significant unobservable inputs (Level 3). September 30, December 31, Business acquisitions contingent consideration, current $ 49,170 $ 8,614 Business acquisitions contingent consideration, long-term 9,742 379 Contingent put option — 7,100 Warrant option — 16,878 Conversion option 18,059 — Total $ 76,971 $ 32,971	As of December 31, 2019 and 2018, the following financial liabilities are measured at fair value on a recurring basis using significant unobservable inputs (Level 3). Liabilities at Fair Value 2019 Level 1 Level 2 Level 3 Total Contingent consideration payable (Note 7) $ $ $ 8,993 $ 8,993 Warrant option (Note 12) 16,878 16,878 Contingent put option (Note 13) 7,100 7,100 Total $ $ $ 32,971 $ 32,971 Liabilities at Fair Value 2018 Level 1 Level 2 Level 3 Total Contingent consideration payable (Note 7) $ $ $ 2,754 $ 2,754 Warrant option (Note 12) 12,818 12,818 Total $ $ $ 15,572 $ 15,572
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 for the nine months ended September 30, 2020 and September 30, 2019: Level 3 Embedded Option Business Consideration, (1) Business Long-term (2) Contingent Put Option Warrant Options Total Balance—at December 31, 2018 $ — $ 2,754 $ — $ — $ 12,818 $ 15,572 Acquisitions 31 — — — 31 Changes in fair value included in earnings — (670 ) — — 4,059 3,389 Payment of contingent consideration payable — (554 ) — — — (554 ) Balance—at September 30, 2019 $ — $ 1,561 $ — $ — $ 16,877 $ 18,438 Balance—at December 31, 2019 $ — $ 8,614 $ 379 $ 7,100 $ 16,878 $ 32,971 Foreign currency translation of contingent consideration payment — (208 ) — — — (208 ) Payment of contingent consideration payable — (12,464 ) — — — (12,464 ) Acquisitions 34,661 10,543 45,204 Series A-2 (9,361 ) — — — — (9,361 ) Issuance of warrant option — — — — 30,097 30,097 Reclass of long term to short term contingent consideration — 180 (180 ) — — — Changes in fair value included in earnings 27,420 18,387 (1,000 ) (19,240 ) 9,312 34,879 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 (1) Current portion of the contingent consideration is recorded in business acquisitions contingent consideration, current. (2) Long term portion of the contingent consideration is recorded in business acquisitions contingent consideration, long-term.	The following table sets forth the Company’s financial instruments that were measured at fair value on a recurring basis at December 31, 2019 and 2018: Level 3 Contingent Contingent Consideration Derivative Warrant Total Balance—at January 1, 2018 $ 1,345 $ $ 2,647 $ $ 3,992 Acquisitions 2,951 2,951 Issuance of financial instruments 12,818 12,818 Change in fair value included in earnings (158 ) (352 ) (510 ) Payment of contingent consideration payable (1,384 ) (1,384 ) Write off of financial instruments (2,295 ) (2,295 ) Balance—at December 31, 2018 2,754 12,818 15,572 Acquisitions 5,022 379 5,401 Issuance of financial instruments Change in fair value included in earnings 1,392 7,100 4,060 12,552 Payment of contingent consideration payable (554 ) (554 ) Balance—at December 31, 2019 $ 8,614 $ 379 $ 7,100 $ 16,878 $ 32,971
Summary of Fair Value of Warrant Options Associated with Issuance of Redeemable Series A-1 Preferred Stock	The fair value of the warrant options associated with the issuance of the Redeemable Series A-1 Preferred A-2 Series A-1 July 30, September 30, Common stock value (per share) $ 22.22 $ 31.60 Expected volatility 44.35 % 47.63 % Risk-free interest rate 0.55 % 1.68 % Expected life (years) 10 10 Series A-2 July 30, September 30, Common stock value (per share) $ 22.22 n/a Expected volatility 44.35 % n/a Risk-free interest rate 0.55 % n/a Expected life (years) 10 n/a	The fair value of the warrant option associated with the issuance of the Redeemable Series A-1 Common stock value (per share) $ 31.60 Expected volatility 47.51 % Risk-free interest rate 1.92 % Expected life (years) 10.00

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments under Leases	The following is a schedule of the future minimum lease payments by year under the leases as of September 30, 2020: September 30, Rent Office Equipment Total 2021 $ 5,863 $ 332 $ 6,195 2022 4,865 327 5,192 2023 2,961 129 3,090 2024 1,410 39 1,449 2025 and thereafter 1,385 1 1,386 $ 16,484 $ 828 $ 17,312	The following is a schedule of the future minimum lease payments by year under the leases as of December 31, 2019: Rent Office Total 2020 $ 6,123 $ 299 $ 6,422 2021 4,983 252 5,235 2022 3,957 232 4,189 2023 2,567 96 2,663 2024 and thereafter 2,281 28 2,309 $ 19,911 $ 907 $ 20,818

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit) (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Summary of Issued and Cancelled Shares of Common Stock	Common Stock Issuances and Cancellations Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Shares Average Shares Average Shares Average Shares Average Common stock issued in connection with initial public offering 11,500,000 $ 15.00 — $ — 11,500,000 $ 15.00 — $ — Acquisitions — — 25,408 26.90 791,139 31.60 150,439 22.41 Redemption of series A-1 1,786,739 15.00 — — 1,786,739 15.00 — — Exercise of warrant options 2,534,239 0.01 — — 2,534,239 0.01 — — Exercise of options 16,850 8.92 7,150 6.03 20,350 9.27 22,675 18.01 Restricted shares, net 8,333 24.00 — — 8,333 24.00 — — Cancellation of shares (117,785 ) 17.15 — — (117,785 ) 17.15 — — 15,728,376 $ 12.82 32,558 $ 22.32 16,523,015 $ 13.72 173,114 $ 21.83	Common Stock Issuances and Repurchases 2019 Number of Average Price Acquisitions 150,439 $ 26.90 Exercise of options 27,350 6.16 Shares sold 42,415 31.60 Release of restricted stock 12,132 18.82 232,336 2018 Number of Average Price Acquisitions 264,582 $ 20.26 Exercise of options 5,000 6.03 Release of restricted stock 184,400 13.64 453,982
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 as of September 30, 2020 and September 30, 2019: September 30, 2020 2017 Plan 2013 Plan Shares authorized to be issued 2,945,443 2,047,269 Shares available for grant 1,233,266 — September 30, 2019 2017 Plan 2013 Plan Shares authorized to be issued 981,800 2,074,604 Shares available for grant 159,428 —
Schedule of Stock Compensation Expense	Total stock compensation expense for the Plans was as follows: Three Months Ended September 30, 2020 2019 2017 plan 2013 plan 2017 plan 2013 plan Options Restricted Stock Options Total Options Restricted Stock Options Total Cost of revenue $ 276 $ — $ (5 ) $ 271 $ 278 $ — $ 97 $ 375 Selling, general and administrative expense 592 233 53 878 185 111 384 680 $ 868 $ 233 $ 48 $ 1,149 $ 463 $ 111 $ 481 $ 1,055 Nine Months Ended September 30, 2020 2019 2017 plan 2013 plan 2017 plan 2013 plan Options Restricted Stock Options Total Options Restricted Stock Options Total Cost of revenue $ 973 $ — $ 126 $ 1,099 $ 623 $ — $ 390 $ 1,013 Selling, general and administrative expense 1,178 973 189 2,340 457 323 1,784 2,564 $ 2,151 $ 973 $ 315 $ 3,439 $ 1,080 $ 323 $ 2,174 $ 3,577
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 for the nine months ended September 30, 2020 and September 30, 2019: September 30, September 30, Common stock value (per share) $ 16.36 $ 27.30 Expected volatility 46.14 47.94 Risk-free interest rate 0.41 2.15 Expected life (years) 7 7 Forfeiture rate None None Dividend rate None None	The following weighted-average assumptions were used in the Black-Sholes option-pricing model calculation for the years ended December 31, 2019 and 2018: 2019 2018 Common stock value (per share) $24.00–$31.60 $18.00–$24.00 Expected volatility 48.13% 48.69% Risk- free interest rate 1.43%–2.63% 2.86%–2.89% Expected life (years) 7.00 7.00 Forfeiture rate None None Dividend rate None None
Schedule of Common Stock Reserved for Future Issuance	Common Stock Reserved for Future Issuances September 30, December 31, Warrants 116,350 650,590 Montrose 2013 Stock Incentive Plan 2,047,269 2,058,619 Montrose 2017 Stock Incentive Plan 2,945,443 997,785 Common stock reserved for future issuance 5,109,062 3,706,994	Common Stock Reserved for Future Issuances 2019 2018 Warrants 650,590 650,590 Montrose 2013 Stock Incentive Plan 2,058,619 2,154,600 Montrose 2017 Stock Incentive Plan 997,785 600,900 Common stock reserved for future issuance 3,706,994 3,406,090
Montrose Amended & Restated 2017 Stock Incentive Plan
Summary of Stock Option Activity	The following summarizes the options activity of the 2017 Plan for the nine months ended September 30, 2020 and September 30, 2019: 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 December 31, 2018 257,762 $ 20 $ 10 — $ 1,151 Granted 356,470 27 14 — — Forfeited/ cancelled (17,700 ) 19 — — — Outstanding at September 30, 2019 596,532 24 12 8.07 4,693 Outstanding at December 31, 2019 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 Exercised at September 30, 2020 925 14 — 7.33 7,649 Options vested and expected to vest 1,454,429 21 9.04 7,649	The following summarizes the options activity of the 2017 Stock Incentive Plan for the years ended December 31, 2019 and 2018: Weighted- Weighted Weighted Aggregate Options to Average Average Average Intrinsic Value Purchase Exercise Grant Date Remaining of In-The-Money Common Price per Fair Value Contract Life Options Stock Share per Share (in Years) (in Thousands) Outstanding at January 1, 2018 49,875 $ 14 $ 7 $ 40 Granted 220,900 21 11 Forfeitured/cancelled (6,000 ) 18 Expired (7,013 ) 14 Outstanding at December 31, 2018 257,762 20 10 1,151 Granted 380,820 27 14 Forfeitured/cancelled (20,730 ) 20 Outstanding at December 31, 2019 617,852 24 12 7.82 4,696 Options vested and expected to vest 617,852 24 8.79 4,696
Montrose Amended and Restated 2013 Stock Option Plan
Summary of Stock Option Activity	Montrose Amended & Restated 2013 Stock Option Plan Options to Purchase Common Stock Weighted- Average Exercise Price per Share Weighted Average Grant Date Fair Value per Share Weighted Average Remaining Contract (in Years) Aggregate Intrinsic Value of In-The- Money Options (in Thousands) Outstanding at December 31, 2018 1,900,404 $ 6 $ 1 7.30 33,290 Forfeited/ cancelled (14,500 ) 8 — — — Expired (5,400 ) 8 — — — Exercised (19,010 ) 6 — — 466 Outstanding at September 30, 2019 1,861,494 6 1 6.65 47,381 Outstanding at December 31, 2019 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 Exercised at September 30, 2020 221,025 6 — 5.14 33,772 Options vested and expected to vest 1,817,494 6 — 5.17 33,772	The following summarizes the activity of the Montrose Amended & Restated 2013 Stock Option Plan for the years ended December 31, 2019 and 2018: Weighted- Weighted Weighted Aggregate Options to Average Average Average Intrinsic Value Purchase Exercise Grant Date Remaining of In-The-Money Common Price per Fair Value Contract Life Options Stock Share per Share (in Years) (in Thousands) Outstanding at January 1, 2018 2,066,000 $ 6 $ 1 7.83 $ 16,534 Forfeitured/cancelled (43,225 ) 7 Expired (117,371 ) 6 Exercised (5,000 ) 6 60 Outstanding at December 31, 2018 1,900,404 6 1 7.30 33,290 Forfeitured/cancelled (10,635 ) 8 Expired (6,950 ) 8 Exercised (27,350 ) 6 Outstanding at December 31, 2019 1,855,469 6 1 6.40 46,617 Exercised at December 31, 2019 201,600 6 5.90 46,617 Options vested and expected to vest 1,855,494 6 5.86 46,617

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders	The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company for the three and nine months ended September 30, 2020 and September 30, 2019: Three Months Ended Nine Months Ended In thousands, except for net loss per share 2020 2019 2020 2019 Net loss $ (30,737 ) $ (6,683 ) $ (58,761 ) $ (12,247 ) Accretion of redeemable series A-1 (6,542 ) (5,030 ) (17,601 ) (14,341 ) Redeemable series A-1 (24,341 ) — (24,341 ) — Convertible and redeemable series A-2 preferred stock dividend (2,870 ) — (2,870 ) — Net loss attributable to common stockholders—basic and diluted (64,490 ) (11,713 ) (103,573 ) (26,588 ) Weighted-average common shares outstanding—basic and diluted 21,554 8,718 13,669 8,640 Net loss per share attributable to common stockholders—basic and diluted $ (2.99 ) $ (1.34 ) $ (7.58 ) $ (3.08 )	The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company for the years ended December 31, 2019 and 2018: In thousands, except for net loss per share 2019 2018 Net loss $ (23,557 ) $ (16,491 ) Convertible preferred stock deemed dividend, net of return from holders (932 ) Accretion of redeemable series A- (19,616 ) (3,605 ) Net loss attributable to common stockholders—basic and diluted (43,173 ) (21,028 ) Weighted- average common shares outstanding—basic and diluted 8,789 7,533 Net loss per share attributable to common stockholders—basic and diluted $ (4.91 ) $ (2.79 )
Equity Shares Excluded from Calculation of Diluted Net Loss per Share Attributable to Common Stockholders	The following equity shares were excluded from the calculation of diluted net loss per share attributable to common stockholders because their effect would have been anti-dilutive for the three and nine months ended September 30, 2020 and September 30, 2019: September 30, 2020 2019 Stock options 2,423,900 2,062,825 Restricted stock 176,384 158,426 Warrants 116,350 116,350	The following equity shares were excluded from the calculation of diluted net loss per share attributable to common stockholders because their effect would have been anti-dilutive for the years ended December 31, 2019 and 2018: 2019 2018 Stock options 2,048,738 1,321,890 Restricted stock 153,352 112,637 Warrants 116,350 108,267

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Segment Reporting [Abstract]
Components of Segment Revenues and Adjusted EBITDA	Segment revenues and Adjusted EBITDA for the three and nine months ended September 30, 2020 and September 30, 2019 were as follows: Three Months Ended September 30, Nine Months Ended September 30, 2020 2019 2020 2019 Segment Segment Segment Segment Segment Segment Segment Segment Assessment, Permitting and Response $ 26,593 $ 8,230 $ 5,413 $ 1,695 $ 49,768 $ 14,665 $ 15,017 $ 5,693 Measurement and Analysis 39,770 11,226 35,672 7,691 113,130 30,422 98,521 19,761 Remediation and Reuse 18,342 2,012 16,538 2,400 56,604 6,495 52,440 7,317 Total Operating Segments 84,705 21,468 57,623 11,786 219,502 51,582 165,978 32,771 Corporate and Other — (4,762 ) — (3,567 ) — (15,425 ) — (10,935 ) Total $ 84,705 $ 16,706 $ 57,623 $ 8,219 $ 219,502 $ 36,157 $ 165,978 $ 21,836	Segment revenues and Adjusted EBITDA for the years ended December 31, 2019 and 2018 consisted of the following: 2019 Segment Segment Assessment, Permitting and Response $ 21,071 $ 7,572 Measurement and Analysis 135,531 27,828 Remediation and Reuse 77,252 9,736 Total Operating Segments 233,854 45,136 Corporate and Other (13,641 ) Total $ 233,854 $ 31,495 2018 Segment Segment Assessment, Permitting and Response $ 3,663 $ 1,339 Measurement and Analysis 117,373 20,779 Remediation and Reuse 67,769 11,400 Total Operating Segments 188,805 33,518 Corporate and Other (11,701 ) Total $ 188,805 $ 21,817
Reconciliation of Segment Measure to Income (Loss) Before Benefit from Income Taxes	Presented below is a reconciliation of the Company’s segment measure to loss before benefit from income taxes for the three and nine months ended September 30, 2020 and September 30, 2019: For the Three Months Ended September 30, For the Nine Months Ended September 30, 2020 2019 2020 2019 Total $ 16,706 $ 8,219 $ 36,157 $ 21,836 Interest expense, net (3,043 ) (2,130 ) (10,896 ) (4,590 ) Income tax (expense) benefit (3,348 ) 412 1,563 1,308 Depreciation and amortization (9,740 ) (7,412 ) (27,084 ) (20,262 ) Stock-based compensation (1,149 ) (1,055 ) (3,439 ) (3,577 ) Start-up (602 ) (634 ) (1,283 ) (1,091 ) Acquisition costs (6 ) (1,278 ) (3,767 ) (2,350 ) Fair value changes in contingent put option 26,265 — 19,240 — Fair value changes in warrant options (9,311 ) (2,510 ) (9,312 ) (4,059 ) Fair value changes in compound embedded option (26,664 ) — (27,420 ) — Fair value changes in business acquisitions contingent consideration (13,404 ) (256 ) (17,387 ) 670 Short term purchase accounting fair value adjustment to deferred revenue — — (243 ) — IPO preparation costs and IPO related bonuses (6,378 ) (20 ) (6,908 ) (105 ) Discontinued services (i) (30 ) — (7,526 ) — Expenses related to financing transactions — — (277 ) — Other expenses (33 ) (19 ) (179 ) (27 ) Net loss $ (30,737 ) $ (6,683 ) $ (58,761 ) $ (12,247 ) (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	Presented below is a reconciliation of the Company’s segment measure to loss before benefit from income taxes for the years ended December 31, 2019 and 2018: 2019 2018 Total $ 31,495 $ 21,817 Interest expense, net (6,755 ) (11,085 ) Income tax benefit 3,121 4,968 Depreciation and amortization (27,705 ) (23,915 ) Stock-based compensation (4,345 ) (5,794 ) Start-up losses and investment in new services (1,874 ) (999 ) Acquisition costs (3,474 ) (1,595 ) Fair value changes in financial instruments (11,160 ) 352 Expenses related to financing transactions (398 ) Fair value changes in contingent liabilities (1,392 ) 158 Short term purchase accounting fair value adjustments (858 ) IPO preparation (610 ) Net loss $ (23,557 ) $ (16,491 )

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Summary of Income Tax Benefit	Income tax benefit for the years ended December 31, 2019 and 2018 is comprised of the following: 2019 2018 Current: Federal $ 4 $ 7 State 289 245 Foreign 41 Total 334 252 Deferred: Federal (2,323 ) (3,639 ) State (1,132 ) (1,581 ) Total (3,455 ) (5,220 ) Income tax benefit $ (3,121 ) $ (4,968 )
Schedule of Significant Components of Deferred Tax Assets and Liabilities	Significant components of the Company’s deferred tax assets and liabilities as of December 31, 2019 and 2018 are as follows: 2019 2018 Deferred tax assets: Net operating losses $ 3,850 $ 2,570 Allowance for bad debts 331 122 Employee related 1,348 1,834 Interest expense 588 1,472 Other 318 Total deferred tax asset 6,435 5,998 Deferred tax liabilities: Intangible assets (4,825 ) (7,032 ) Property and equipment (4,589 ) (3,172 ) Other (551 ) (662 ) Total deferred tax liability (9,965 ) (10,866 ) Net deferred tax liability $ (3,530 ) $ (4,868 )
Reconciliation of Federal Statutory Income Tax Rate to Effective Income Tax Rate	A reconciliation of the federal statutory income tax rate to the Company’s effective income tax rate for the years ended December 31, 2019 and 2018 is as follows: 2019 2018 Tax completed at federal statutory rate 21.00 % 21.00 % State tax net of federal benefit 2.51 4.96 Non- (1.38 ) (0.91 ) Equity compensation (2.41 ) (0.83 ) Contingent put option (5.59 ) 0.00 Foreign taxes (0.15 ) 0.00 Warrant option (3.20 ) Other 0.92 (1.03 ) Effective income tax rate 11.70 % 23.19 %

Description of the Business a_3

Description of the Business and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands	Jul. 27, 2020$ / sharesshares	Jul. 27, 2020USD ($)$ / sharesshares	Sep. 30, 2020Employee$ / sharesshares	Sep. 30, 2020USD ($)EmployeeAcquisitionsOffice$ / sharesshares	Sep. 30, 2019USD ($)$ / shares	Dec. 31, 2019USD ($)EmployeeAcquisitionsOfficeshares	Dec. 31, 2018USD ($)
Description Of Business And Basis Of Presentation [Line Items]
Number of offices in which entity operates | Office				70		60
Entity number of employees | Employee			1,700	1,700		1,300
Entity formation, month and year				2013-11		2013-11
Number of acquisitions completed | Acquisitions				56		53
Common stock shares sold under IPO | shares	1,786,739					42,415
Share price | $ / shares			$ 16.36	$ 16.36	$ 27.30
Net proceeds from sale of common stock				$ 171	$ 139	$ 1,509	$ 30
IPO
Description Of Business And Basis Of Presentation [Line Items]
Common stock shares sold under IPO | shares		11,500,000	11,500,000	11,500,000
Share price | $ / shares	$ 15	$ 15
Net proceeds from sale of common stock		$ 161,300
Deferred offset		$ 4,400
Underwriters
Description Of Business And Basis Of Presentation [Line Items]
Common stock shares sold under IPO | shares		1,500,000
Net proceeds from sale of common stock		$ 11,200

Summary of New Accounting Pro_2

Summary of New Accounting Pronouncements - Additional Information (Detail)	Sep. 30, 2020	Dec. 31, 2019
Accounting Standards Update 2018-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted	true	true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2020	Jan. 1, 2020
Change in accounting principle, accounting standards update, immaterial effect	true	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	true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2020	Jan. 1, 2020
Change in accounting principle, accounting standards update, immaterial effect	true	true

Revenues and Accounts Receiva_3

Revenues and Accounts Receivable - Schedule of Contract Balances (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Contract assets	$ 35,441	$ 13,605	$ 10,112
Contract liabilities	$ 9,335	$ 3,314	$ 2,395

Revenues and Accounts Receiva_4

Revenues and Accounts Receivable - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Revenues And Accounts Receivable [Abstract]
Contracts assets acquired through business acquisitions		$ 6,500		$ 700	$ 700
Contract liabilities acquired through business acquisitions		0		2,200	$ 0
Contract with customer, liability, revenue recognized		2,100		500
Revenue remaining performance obligations		25,200		13,000
Provision for bad debt	$ 6,300	$ (6,445)	$ (590)	$ (1,246)
Customer Concentration Risk | Accounts Receivable | Minimum
Revenues And Accounts Receivable [Abstract]
Concentration Risk Percentage		10.00%		10.00%
Previously Reported
Revenues And Accounts Receivable [Abstract]
Contracts assets acquired through business acquisitions				$ 700
Contract liabilities acquired through business acquisitions				2,200
Contract with customer, liability, revenue recognized				600
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenues And Accounts Receivable [Abstract]
Revenue remaining performance obligations				$ 8,700
Revenue remaining performance obligations, satisfaction period				1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Revenues And Accounts Receivable [Abstract]
Revenue remaining performance obligations		$ 24,000		$ 4,200
Revenue remaining performance obligations, satisfaction period		1 year		2 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenues And Accounts Receivable [Abstract]
Revenue remaining performance obligations		$ 1,200		$ 100
Revenue remaining performance obligations, satisfaction period				3 years
Remediation and Reuse Segment
Revenues And Accounts Receivable [Abstract]
Provision for bad debt		$ 5,500

Revenues and Accounts Receiva_5

Revenues and Accounts Receivable - Schedule of Accounts Receivable, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Accounts receivable, invoiced	$ 52,372	$ 46,643	$ 38,786
Accounts receivable, other	929	611	135
Allowance for doubtful accounts	(8,516)	(1,327)	(453)	$ (573)
Accounts receivable-net	$ 44,785	$ 45,927	$ 38,468

Revenues and Accounts Receiva_6

Revenues and Accounts Receivable - Schedule of Allowance for Doubtful Accounts (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Receivables [Abstract]
Beginning Balance	$ 1,327	$ 1,327	$ 453	$ 453	$ 573
Bad Debt Expense	$ (6,300)	6,445	$ 590	1,246
Charged to Allowance		(294)		(556)	(624)
Other		1,038		184	504
Ending Balance		$ 8,516		$ 1,327	$ 453

Prepaid and Other Current Ass_3

Prepaid and Other Current Assets - Schedule of Prepaid and Other Current Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Deposits	$ 733	$ 605	$ 598
Prepaid expenses	1,665	1,235	1,016
Prepaid insurance	2,577	170	85
Supplies	2,428	2,368	745
Offering costs		1,240
Income tax receivable	1,182	1,205	332
Other			154
Prepaid and other current assets	$ 8,585	$ 6,823	$ 2,598

Property and Equipment - Proper

Property and Equipment - Property and Equipment, Net (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 75,584	$ 62,878	$ 50,006
Less accumulated depreciation	(41,906)	(36,638)	(29,214)
Total property and equipment-net	$ 34,559	$ 27,036	20,792
Lab and Test Equipment
Property, Plant and Equipment [Line Items]
Estimated Useful Life	7 years	7 years
Property and equipment, gross	$ 17,755	$ 14,810	10,275
Vehicles
Property, Plant and Equipment [Line Items]
Estimated Useful Life	5 years	5 years
Property and equipment, gross	$ 12,869	$ 11,073	8,122
Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 31,539	$ 29,922	25,468
Equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	3 years	3 years
Equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated Useful Life	7 years	7 years
Furniture and Fixtures
Property, Plant and Equipment [Line Items]
Estimated Useful Life	7 years	7 years
Property and equipment, gross	$ 2,854	$ 1,119	1,004
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Estimated Useful Life	7 years	7 years
Property and equipment, gross	$ 6,758	$ 5,954	$ 5,137
Aircraft
Property, Plant and Equipment [Line Items]
Estimated Useful Life	10 years
Property and equipment, gross	$ 834
Rent
Property, Plant and Equipment [Line Items]
Estimated Useful Life	39 years
Property and equipment, gross	$ 2,975
Land
Property, Plant and Equipment [Line Items]
Property and equipment, gross	725
Construction in Progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 156	$ 796

Property and Equipment - Additi

Property and Equipment - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 2.1	$ 2.1	$ 6.1	$ 5.9	$ 7.7	$ 7.6

Business Acquisitions - Additio

Business Acquisitions - Additional Information (Detail) - USD ($) $ in Thousands

1 Months Ended

3 Months Ended

9 Months Ended

12 Months Ended

Apr. 30, 2020

Sep. 30, 2020

Sep. 30, 2019

Sep. 30, 2020

Sep. 30, 2019

Dec. 31, 2019

Dec. 31, 2018

Aug. 31, 2019

Jul. 31, 2019

Jun. 30, 2019

Apr. 30, 2019

Mar. 31, 2019

Mar. 15, 2019

Nov. 30, 2018

Oct. 31, 2018

Mar. 31, 2018

Feb. 01, 2018

Jan. 31, 2018

Pci Loans Acquired [Line Items]

External transaction costs related to business combinations

$ (6)

$ (1,278)

$ (3,767)

$ (2,350)

$ 2,400

$ 1,200

Number of common stock shares issued in connection with business acquisition

25,408

791,139

150,439

150,439

264,582

Revenue

$ 37,200

$ 7,700

$ 23,200

$ 17,900

Pre-tax income (loss)

$ 5,700

$ 1,500

$ 2,400

$ 300

Customer Relationships

Pci Loans Acquired [Line Items]

Weighted average useful lives for acquired intangible assets

15 years

9 years 6 months

9 years 6 months

7 years

Trade Names

Pci Loans Acquired [Line Items]

Weighted average useful lives for acquired intangible assets

4 years

1 year 6 months

1 year 6 months

1 year 6 months

Covenants Not to Compete

Pci Loans Acquired [Line Items]

Weighted average useful lives for acquired intangible assets

5 years

4 years

4 years

4 years

Proprietary Software [Member]

Pci Loans Acquired [Line Items]

Weighted average useful lives for acquired intangible assets

3 years

3 years

Patent

Pci Loans Acquired [Line Items]

Weighted average useful lives for acquired intangible assets

16 years

Proprietary Software

Pci Loans Acquired [Line Items]

Weighted average useful lives for acquired intangible assets

3 years

Patent [Member]

Pci Loans Acquired [Line Items]

Weighted average useful lives for acquired intangible assets

16 years

CTEH

Pci Loans Acquired [Line Items]

Percentage of interests acquired

100.00%

CTEH | Earn Out Payment 2021 [Member]

Pci Loans Acquired [Line Items]

Business combination, contingent consideration arrangements, basis for amount

$ 18,300

Business combination, earn-out payment, maximum

$ 50,000

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 2020 earn out was initially payable 100% in common stock, but as a result of the completion of the Company's IPO (Note 1), 50% of any 2020 earnout payment will be payable in cash and 50% will be payable, at the Company's election, in cash or shares of common stock. The 2021 earn out, if any, is payable 100% in cash.

CTEH | Earn Out Payment 2021 [Member] | Minimum

Pci Loans Acquired [Line Items]

Business combination, actual EBITDA

$ 18,300

CTEH | Earn Out Payment 2021 [Member] | Maximum

Pci Loans Acquired [Line Items]

Business combination, actual EBITDA

$ 22,500

CTEH | 2020 Earn Out

Pci Loans Acquired [Line Items]

Percentage of earn out payable in cash

50.00%

Percentage of earn out payable in common stock

100.00%

Percentage of earn out payable in cash, as result of completing of IPO

100.00%

Percentage of earn out payable in cash or shares, as a result of completion of IPO, at company's election

50.00%

CTEH | 2020 Earn Out | Minimum

Pci Loans Acquired [Line Items]

Business combination, actual EBITDA

$ 18,300

CTEH | 2020 Earn Out | Maximum

Pci Loans Acquired [Line Items]

Business combination, actual EBITDA

$ 22,500

CTEH | Convertible And Redeemable Series A-2 Preferred Stock

Pci Loans Acquired [Line Items]

Number of common stock shares issued in connection with business acquisition

791,139

Emission Services Inc [Member]

Pci Loans Acquired [Line Items]

Percentage of interests acquired

100.00%

Emission Services USA [Member]

Pci Loans Acquired [Line Items]

Percentage of interests acquired

100.00%

Air Water & Soil Laboratories, Inc [Member]

Pci Loans Acquired [Line Items]

Percentage of interests acquired

100.00%

LEHDER

Pci Loans Acquired [Line Items]

Percentage of interests acquired

100.00%

ECT2

Pci Loans Acquired [Line Items]

Percentage of interests acquired

100.00%

AES

Pci Loans Acquired [Line Items]

Percentage of interests acquired

100.00%

Revenue

$ 11,300

$ 1,000

Pre-tax income (loss)

$ 1,200

$ 100

Streamline Environmental Inc

Pci Loans Acquired [Line Items]

Percentage of interests acquired

100.00%

Leymaster Environmental Consulting, LLC

Pci Loans Acquired [Line Items]

Percentage of interests acquired

100.00%

Environmental Planning Specialists, Inc

Pci Loans Acquired [Line Items]

Percentage of interests acquired

100.00%

Golden

Pci Loans Acquired [Line Items]

Percentage of interests acquired

100.00%

100.00%

Advanced GeoServices Corp [Member]

Pci Loans Acquired [Line Items]

Percentage of interests acquired

100.00%

Business Acquisitions - Summary

Business Acquisitions - Summary of Elements of Purchase Price of Acquisitions (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Cash	$ 175,450	$ 87,285	$ 87,285	$ 48,961
Common Stock	25,000	4,047	4,047	5,634
Other Purchase Price Component Current	(2,247)
Other Purchase Price Components		1,450	1,450	848
Other Purchase Price Component Long Term	100
Contingent Consideration		5,401	5,401	2,951
Contingent Consideration Current	34,661
Contingent Consideration Long Term	10,543
Total Purchase Price	243,507	98,183	98,183	58,394
CTEH
Business Acquisition [Line Items]
Cash	175,000
Common Stock	25,000
Other Purchase Price Component Current	(2,297)
Contingent Consideration Current	34,451
Contingent Consideration Long Term	10,543
Total Purchase Price	242,697
Other Acquisition
Business Acquisition [Line Items]
Cash	450
Other Purchase Price Component Current	50
Other Purchase Price Component Long Term	100
Contingent Consideration Current	210
Total Purchase Price	$ 810
Golden
Business Acquisition [Line Items]
Cash		1,500	1,500
Contingent Consideration		477	477
Total Purchase Price		1,977	1,977
TES
Business Acquisition [Line Items]
Cash		2,359	2,359
Common Stock		322	322
Other Purchase Price Components		25	25
Contingent Consideration		4,911	4,911
Total Purchase Price		7,617	7,617
TESUS
Business Acquisition [Line Items]
Cash		18,683	18,683
Common Stock		3,041	3,041
Other Purchase Price Components		1,495	1,495
Total Purchase Price		23,219	23,219
AWS
Business Acquisition [Line Items]
Cash		6,020	6,020
Other Purchase Price Components		150	150
Total Purchase Price		6,170	6,170
AEC
Business Acquisition [Line Items]
Cash		808	808
Total Purchase Price		808	808
LEHDER
Business Acquisition [Line Items]
Cash		3,878	3,878
Common Stock		684	684
Contingent Consideration		13	13
Total Purchase Price		4,575	4,575
ECT2
Business Acquisition [Line Items]
Cash		54,037	54,037
Other Purchase Price Components		(220)	(220)
Total Purchase Price		$ 53,817	$ 53,817
SES [Member]
Business Acquisition [Line Items]
Cash				450
Other Purchase Price Components				50
Total Purchase Price				500
Fa [Member]
Business Acquisition [Line Items]
Cash				1,096
Other Purchase Price Components				360
Contingent Consideration				372
Total Purchase Price				1,828
AGC [Member]
Business Acquisition [Line Items]
Cash				7,400
Common Stock				925
Other Purchase Price Components				95
Contingent Consideration				871
Total Purchase Price				9,291
Streamline [Member]
Business Acquisition [Line Items]
Cash				5,678
Common Stock				631
Other Purchase Price Components				180
Total Purchase Price				6,489
Leymaster [Member]
Business Acquisition [Line Items]
Cash				2,465
Common Stock				435
Other Purchase Price Components				250
Contingent Consideration				434
Total Purchase Price				3,584
AES
Business Acquisition [Line Items]
Cash				21,877
Common Stock				2,028
Other Purchase Price Components				(16)
Contingent Consideration				1,274
Total Purchase Price				25,163
EPS [Member]
Business Acquisition [Line Items]
Cash				9,995
Common Stock				1,615
Other Purchase Price Components				(71)
Total Purchase Price				$ 11,539

Business Acquisitions - Summa_2

Business Acquisitions - Summary of Purchase Price Attributable to Acquisitions (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Apr. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Cash		$ 1,527	$ 5,286	$ 5,286	$ 3,126
Restricted cash			629	629
Accounts receivable		17,059	5,458	5,458	6,449
Other current assets		1,265	559	559	320
Current assets		19,851	11,932	11,932	9,895
Property and equipment		7,117	4,064	4,064	1,534
Goodwill	$ 274,309	147,251	127,058	36,622	90,498	$ 65,818
Total assets		253,228	106,355	106,355	66,775
Current liabilities		9,721	6,001	6,001	5,209
Non- current liabilities			2,171	2,171	3,172
Total liabilities		9,721	8,172	8,172	8,381
Purchase price		243,507	98,183	98,183	58,394
Customer Relationships
Business Acquisition [Line Items]
Intangible assets		56,000	26,588	26,588	26,839
Trade Names
Business Acquisition [Line Items]
Intangible assets		4,200	1,667	1,667	1,053
Covenants Not to Compete
Business Acquisition [Line Items]
Intangible assets		4,109	5,443	5,443	2,774
Proprietary Software
Business Acquisition [Line Items]
Intangible assets		14,700	2,560	2,560
Patent
Business Acquisition [Line Items]
Intangible assets			17,479	17,479
Goodwill [Member]
Business Acquisition [Line Items]
Intangible assets			36,622		24,680
CTEH
Business Acquisition [Line Items]
Cash		1,527
Accounts receivable		17,059
Other current assets		1,265
Current assets		19,851
Property and equipment		7,042
Goodwill		146,625
Total assets		252,418
Current liabilities		9,721
Total liabilities		9,721
Purchase price		242,697
CTEH | Customer Relationships
Business Acquisition [Line Items]
Intangible assets		56,000
CTEH | Trade Names
Business Acquisition [Line Items]
Intangible assets		4,200
CTEH | Covenants Not to Compete
Business Acquisition [Line Items]
Intangible assets		4,000
CTEH | Proprietary Software
Business Acquisition [Line Items]
Intangible assets		14,700
Other Acquisition
Business Acquisition [Line Items]
Cash			2,137	2,137	380
Accounts receivable			3,751	3,751	5,045
Other current assets			61	61	288
Current assets			5,949	5,949	5,713
Property and equipment		75	3,288	3,288	1,002
Goodwill		626		20,227
Total assets		810	47,514	47,514	38,544
Current liabilities			977	977	2,141
Non- current liabilities			2,171	2,171	3,172
Total liabilities			3,148	3,148	5,313
Purchase price		810	44,366	44,366	33,231
Other Acquisition | Customer Relationships
Business Acquisition [Line Items]
Intangible assets			12,748	12,748	15,543
Other Acquisition | Trade Names
Business Acquisition [Line Items]
Intangible assets			659	659	812
Other Acquisition | Covenants Not to Compete
Business Acquisition [Line Items]
Intangible assets		$ 109	2,083	2,083	1,813
Other Acquisition | Proprietary Software
Business Acquisition [Line Items]
Intangible assets			2,560	2,560
Other Acquisition | Goodwill [Member]
Business Acquisition [Line Items]
Intangible assets			20,227		13,661
ECT2
Business Acquisition [Line Items]
Cash			3,149	3,149	2,746
Restricted cash			629	629
Accounts receivable			1,707	1,707	1,404
Other current assets			498	498	32
Current assets			5,983	5,983	4,182
Property and equipment			776	776	532
Goodwill				16,395
Total assets			58,841	58,841	28,231
Current liabilities			5,024	5,024	3,068
Total liabilities			5,024	5,024	3,068
Purchase price			53,817	53,817	25,163
ECT2 | Customer Relationships
Business Acquisition [Line Items]
Intangible assets			13,840	13,840	11,296
ECT2 | Trade Names
Business Acquisition [Line Items]
Intangible assets			1,008	1,008	241
ECT2 | Covenants Not to Compete
Business Acquisition [Line Items]
Intangible assets			3,360	3,360	961
ECT2 | Patent
Business Acquisition [Line Items]
Intangible assets			17,479	$ 17,479
ECT2 | Goodwill [Member]
Business Acquisition [Line Items]
Intangible assets			$ 16,395		$ 11,019

Business Acquisitions - Summa_3

Business Acquisitions - Summary of Supplemental Unaudited Pro-Forma Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Revenues	$ 84,705	$ 57,623	$ 219,502	$ 165,978	$ 233,854	$ 188,805
Net (loss) income	(30,737)	(6,683)	(58,761)	(12,247)	(23,557)	(16,491)
Acquisition Proforma
Revenues		18,551	31,253	107,459	25,446	58,526
Net (loss) income		4,042	10,288	30,002	5,199	14,580
Consolidated Proforma
Revenues	84,705	76,174	250,755	273,437	259,300	247,331
Net (loss) income	$ (30,737)	$ (2,641)	$ (48,473)	$ 17,755	$ (18,358)	$ (1,911)

Goodwill and Intangible Asset_2

Goodwill and Intangible Assets - Schedule of Amounts Related to Goodwill (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Beginning balance	$ 127,058	$ 90,498	$ 65,818
Goodwill acquired during the period	147,251	36,560	24,680
Ending balance	274,309	127,058	90,498
Assessment, Permitting and Response
Goodwill [Line Items]
Beginning balance	15,173	15,173
Goodwill acquired during the period	146,625		15,173
Ending balance	161,798	15,173	15,173
Measurement and Analysis
Goodwill [Line Items]
Beginning balance	68,628	48,463	44,123
Goodwill acquired during the period	426	20,165	4,340
Ending balance	69,054	68,628	48,463
Remediation and Reuse Segment
Goodwill [Line Items]
Beginning balance	43,257	26,862	21,695
Goodwill acquired during the period	200	16,395	5,167
Ending balance	$ 43,457	$ 43,257	$ 26,862

Goodwill and Intangible Asset_3

Goodwill and Intangible Assets - Schedule of Amounts Related to Finite-Lived Intangible Assets (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross Balance	$ 249,949	$ 168,716	$ 114,941
Accumulated Amortization	87,748	66,167	46,378
Finite-lived intangible assets, net	162,201	102,549	68,563
Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Gross Balance	164,782	108,782	82,176
Accumulated Amortization	49,068	36,700	23,932
Finite-lived intangible assets, net	115,714	72,082	58,244
Covenants Not to Compete
Finite-Lived Intangible Assets [Line Items]
Gross Balance	29,942	25,832	20,390
Accumulated Amortization	20,444	17,572	13,785
Finite-lived intangible assets, net	9,498	8,260	6,605
Trade Names
Finite-Lived Intangible Assets [Line Items]
Gross Balance	16,938	12,738	11,072
Accumulated Amortization	12,196	10,230	8,282
Finite-lived intangible assets, net	4,742	$ 2,508	$ 2,790
Proprietary Software
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life		3 years	3 years
Gross Balance	20,808	$ 3,885	$ 1,303
Accumulated Amortization	4,915	1,359	379
Finite-lived intangible assets, net	$ 15,893	$ 2,526	$ 924
Patent
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	16 years	16 years
Gross Balance	$ 17,479	$ 17,479
Accumulated Amortization	1,125	306
Finite-lived intangible assets, net	$ 16,354	$ 17,173
Minimum | Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	7 years	7 years	7 years
Minimum | Covenants Not to Compete
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	4 years	4 years	4 years
Minimum | Trade Names
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	1 year	1 year	1 year
Minimum | Proprietary Software
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	3 years
Maximum | Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	15 years	10 years	10 years
Maximum | Covenants Not to Compete
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	5 years	5 years	5 years
Maximum | Trade Names
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	5 years	5 years	5 years
Maximum | Proprietary Software
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	5 years

Goodwill and Intangible Asset_4

Goodwill and Intangible Assets - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization expense	$ 7,600	$ 5,300	$ 21,000	$ 14,400	$ 20,000	$ 16,300
Goodwill foreign currency translation adjustment					$ 60	$ 0

Goodwill and Intangible Asset_5

Goodwill and Intangible Assets - Schedule of Future Amortization Expense (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
2020 (remaining)	$ 7,543
Year 1	28,140	$ 21,473
Year 2	24,053	18,895
Year 3	20,073	14,810
Year 4	16,951	12,025
Thereafter	65,441
Year 5		9,357
Thereafter		25,989
Finite-lived intangible assets, net	$ 162,201	$ 102,549	$ 68,563

Accounts Payable and Other Ac_3

Accounts Payable and Other Accrued Liabilities - Summary of Accounts Payable and Other Accrued Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Accounts payable	$ 13,360	$ 15,034	$ 8,679
Accrued expenses	10,395	10,733	4,414
Business acquisitions contingent consideration		8,614	2,754
Other purchase obligations	572
Other business acquisitions purchase price obligations			525
Contract liabilities	9,335	3,314	2,395
Other current liabilities	235	504	241
Total accounts payable and other accrued liabilities	$ 33,897	$ 29,585	$ 19,008

Accrued Payroll and Benefits -

Accrued Payroll and Benefits - Schedule of Accrued Payroll and Benefits (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Accrued bonuses	$ 7,854	$ 3,449	$ 1,585
Accrued paid time off	2,157	2,154	2,289
Accrued payroll	5,536	4,470	3,496
Accrued other	2,410	959	837
Total accrued payroll and benefits	$ 17,957	$ 11,032	$ 8,207

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Annual effective tax rate ("ETR") from continuing operations	(12.20%)	5.80%	2.60%	9.60%	11.70%	23.19%
Federal statutory rate			21.00%		21.00%	21.00%
Income tax benefits	$ 3,348	$ (412)	$ (1,563)	$ (1,308)	$ (3,121)	$ (4,968)
Net deferred tax assets, valuation allowance	$ 7,400		$ 7,400		$ 331	$ 122
Percentage Of Annual Taxable Income On Use Of Net Operating Loss Carryforwards					80.00%
Income Tax Examination, Description					The Company's federal income tax returns remain subject to examination for the 2016 through 2019 tax years. The Company files in multiple state jurisdictions which remain subject to examination for the 2015 through 2019 tax years.
Domestic Tax Authority [Member]
Operating Loss Carryforwards					$ 15,700
Loss Carryforwards Not Subject To Expiration					7,000
State and Local Jurisdiction [Member]
Operating Loss Carryforwards					$ 10,400

Warrant Options - Additional In

Warrant Options - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands

Jul. 27, 2020

Sep. 30, 2020

Sep. 30, 2019

Sep. 30, 2020

Sep. 30, 2019

Dec. 31, 2019

Jul. 30, 2020

Apr. 30, 2020

Apr. 13, 2020

Dec. 31, 2018

Oct. 31, 2018

Oct. 19, 2018

Class of Warrant or Right [Line Items]

Period between issuance and expiration of outstanding warrant

10 years

Fair value adjustment of warrants

$ 9,312

$ 4,059

Share price

$ 16.36

$ 27.30

$ 16.36

$ 27.30

Redeemable 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

Fair value of warrants

$ 16,900

$ 12,800

Redeemable Preferred Stock | Other Expense

Class of Warrant or Right [Line Items]

Fair value adjustment of warrants

4,100

Redeemable Series A-1 Preferred Stock

Class of Warrant or Right [Line Items]

Warrants to purchase common stock

534,240

534,240

534,240

Warrant exercise price per share

$ 0.01

$ 0.01

Period between issuance and expiration of outstanding warrant

10 years

Fair value of warrants

$ 16,900

$ 11,900

Redeemable Series A-1 Preferred Stock | Other Expense

Class of Warrant or Right [Line Items]

Fair value adjustment of warrants

$ 5,000

$ (2,500)

$ 5,000

$ (4,100)

Redeemable Series A-2 Preferred Stock

Class of Warrant or Right [Line Items]

Warrants to purchase common stock

1,999,999

1,351,960

Warrant exercise price per share

$ 0.01

$ 0.01

Period between issuance and expiration of outstanding warrant

10 years

Fair value of warrants

$ 44,400

Aggregate value to determine number of shares underlying the warrant and issuable upon exercise of warrants

30,000

30,000

Redeemable Series A-2 Preferred Stock | Other Expense

Class of Warrant or Right [Line Items]

Fair value adjustment of warrants

$ 14,300

$ 14,300

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

Debt - Schedule of Debt (Detail

Debt - Schedule of Debt (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Apr. 13, 2020	Dec. 31, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Total debt		$ 225,000
Capital leases	$ 3,127		$ 3,765	$ 2,640
Other leases	2		12	51
Less deferred debt issuance costs	(4,345)		(1,052)	(1,176)
Total debt	176,451		152,189	73,227
Less current portion of long term debt	(5,034)		(7,143)	(2,262)
Long- term debt, less current portion	171,417		145,046	70,965
Term Loan Facility
Line of Credit Facility [Line Items]
Total debt	174,453	175,000	48,750	50,000
Revolving Line of Credit
Line of Credit Facility [Line Items]
Total debt		$ 50,000	97,590	20,000
Equipment Line Of Credit
Line of Credit Facility [Line Items]
Total debt	$ 3,214		$ 3,124	$ 1,712

Debt - Additional Information (

Debt - Additional Information (Detail)

Sep. 30, 2022USD ($)

Sep. 30, 2021USD ($)

Sep. 30, 2020USD ($)

Apr. 13, 2020USD ($)

Oct. 19, 2018USD ($)

Mar. 31, 2017USD ($)

Jul. 31, 2019USD ($)

Sep. 30, 2020USD ($)

Sep. 30, 2019USD ($)

Sep. 30, 2020USD ($)

Sep. 30, 2019USD ($)

Sep. 30, 2010

Dec. 31, 2019USD ($)

Dec. 31, 2018USD ($)

Dec. 31, 2022

Dec. 31, 2021

Oct. 31, 2019USD ($)

May 16, 2019USD ($)

Mar. 12, 2019USD ($)

Oct. 31, 2018USD ($)

Sep. 29, 2017USD ($)

Line of Credit Facility [Line Items]

Line of credit

$ 225,000,000

Line of credit facility maturity date

Apr. 30, 2025

Debt instrument, repayments term

Quarterly

Debt instrument, repayments

$ 500,000

Maximum consolidated leverage ratio

425.00%

425.00%

425.00%

400.00%

375.00%

Minimum consolidated fixed charge coverage ratio

1.25

1.25

Percentage of prepayment on excess cash flow less voluntary prepayments of term loan

50.00%

Consolidated fixed leverage ratio

3.00%

Weighted average interest rate

6.00%

6.00%

6.00%

5.41%

6.31%

Gain on extingushment of debt

$ 356,000

Mandatory prepayment percentage of cash proceeds

100.00%

Mandatory prepayment period

$ 0

$ 0

Mandatory prepayment on proceeds from sale of assets

1,000,000

Amended covenant ratio, description

(i) remove the Consolidated Senior Leverage ratio; (ii) increase the Consolidated Fixed Charge Coverage ratio to greater than 1.25 from 1.125; and (iii) decrease the Consolidated Total Leverage ratio to below 3.75 times beginning with the first fiscal quarter ending September 30, 2018 and lower than 4.00 beginning with the first fiscal quarter during which a pro forma compliance certificate is delivered in accordance with the agreement’s terms, from a 4.25 rate in 2017.

Debt issuance cost write-off

$ 0

200,000

Loan modification and amendment fees

500,000

1,000,000

Forecast

Line of Credit Facility [Line Items]

Debt instrument, repayments

$ 1,600,000

$ 1,100,000

$ 500,000

Maximum consolidated leverage ratio

375.00%

400.00%

Capital Lease Obligations

Line of Credit Facility [Line Items]

Capital leased assets, gross

6,300,000

$ 6,300,000

$ 6,300,000

6,900,000

5,500,000

Capital leases, income statement, amortization expense

600,000

$ 400,000

1,600,000

$ 1,000,000

$ 1,500,000

800,000

Capital Lease Obligations | Minimum

Line of Credit Facility [Line Items]

Capital lease asset estimated useful lives

4 years

4 years

Capital Lease Obligations | Maximum

Line of Credit Facility [Line Items]

Capital lease asset estimated useful lives

6 years

6 years

Debt Issuance Cost

Line of Credit Facility [Line Items]

Loan modification and amendment fees

$ 400,000

900,000

Selling, General and Administrative Expenses

Line of Credit Facility [Line Items]

Loan modification and amendment fees

$ 100,000

100,000

Prior Senior Secured Credit Facility

Line of Credit Facility [Line Items]

Gain on extingushment of debt

$ 1,400,000

$ 1,100,000

Debt fee paid

400,000

400,000

Unamortized debt issuance costs

1,000,000

700,000

Prior Senior Secured Credit Facility | Fed Funds Effective Rate Overnight Index Swap Rate

Line of Credit Facility [Line Items]

Interest rate

0.50%

Prior Senior Secured Credit Facility | LIBOR Plus

Line of Credit Facility [Line Items]

Interest rate

1.00%

Equipment Line Of Credit

Line of Credit Facility [Line Items]

Line of credit

$ 3,100,000

2,800,000

Line of credit facility, maximum borrowing capacity

$ 2,000,000

Canadian Equipment Line Of Credit Facility

Line of Credit Facility [Line Items]

Line of credit facility, maximum borrowing capacity

$ 1,000,000

Convertible Subordinated Debenture

Line of Credit Facility [Line Items]

Gain on extingushment of debt

700,000

Debt covenant, minimum EBITDA requirement

$ 30,000,000

Annual coupon payable in kind

12.00%

Repayment of debt

12,000,000

Interest amount of loan repayment

$ 2,000,000

Conversion price discount rate to the IPO share price

30.00%

Incremental Term Loans

Line of Credit Facility [Line Items]

Line of credit facility, maximum borrowing capacity

$ 100,000,000

Incremental Term Loans | Minimum

Line of Credit Facility [Line Items]

Percentage of pari passu debt of all lenders to be held by unitranche lenders

70.00%

Leverage Ratio Greater Than Or Equal To Three Point Two Five [Member ]

Line of Credit Facility [Line Items]

Percentage of prepayment on excess cash flow less voluntary prepayments of term loan

50.00%

Consolidated fixed leverage ratio

325.00%

Leverage Ratio Less Than Three [Member]

Line of Credit Facility [Line Items]

Percentage of prepayment on excess cash flow less voluntary prepayments of term loan

25.00%

Consolidated fixed leverage ratio

300.00%

Term Loan Facility

Line of Credit Facility [Line Items]

Line of credit

$ 174,453,000

$ 175,000,000

$ 174,453,000

$ 174,453,000

$ 48,750,000

50,000,000

Debt instrument, repayments term

Quarterly

Line of credit facility, maximum borrowing capacity

$ 50,000,000

Debt instrument, periodic repayments

$ 1,300,000

Amended quarterly term loan repayment rate

2.50%

Term Loan Facility | LIBOR Plus

Line of Credit Facility [Line Items]

Interest rate

5.00%

5.00%

Term Loan Facility | LIBOR Floor

Line of Credit Facility [Line Items]

Interest rate

4.50%

1.00%

Term Loan Facility | Base Rat

Line of Credit Facility [Line Items]

Interest rate

4.00%

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

$ 50,000,000

$ 97,590,000

20,000,000

Line of credit facility maturity date

Oct. 19, 2021

Line of credit facility unused commitment fee

0.35%

Line of credit facility, maximum borrowing capacity

$ 110,000,000

$ 130,000,000

Debt instrument, periodic repayments

$ 1,300,000

Revolving Line of Credit | LIBOR Plus

Line of Credit Facility [Line Items]

Interest rate

3.50%

Revolving Line of Credit | Base Rat

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

$ 70,000,000

Second Lien Term Loan Facility

Line of Credit Facility [Line Items]

Line of credit

$ 40,000,000

Fed Funds Effective Rate Overnight Index Swap Rate | Prior Senior Secured Credit Facility

Line of Credit Facility [Line Items]

Interest rate

0.50%

LIBOR Plus | Prior Senior Secured Credit Facility

Line of Credit Facility [Line Items]

Interest rate

1.00%

Debt - Schedule of Applicable P

Debt - Schedule of Applicable Percentages (Detail) - Prior Senior Secured Credit Facility	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Pricing Tier1
Line of Credit Facility [Line Items]
Commitment Fee	0.50%	0.50%
Pricing Tier1 | Minimum
Line of Credit Facility [Line Items]
Consolidated Leverage Ratio	375.00%	375.00%
Pricing Tier1 | Eurodollar Rate Loans And LIBOR Letter Of Credit Fee
Line of Credit Facility [Line Items]
Interest rate	4.00%	4.00%
Pricing Tier1 | Daily Floating Rate Loans
Line of Credit Facility [Line Items]
Interest rate	4.00%	4.00%
Pricing Tier1 | Rate Loans
Line of Credit Facility [Line Items]
Interest rate	3.00%	3.00%
Pricing Tier2
Line of Credit Facility [Line Items]
Commitment Fee	0.50%	0.50%
Pricing Tier2 | Minimum
Line of Credit Facility [Line Items]
Consolidated Leverage Ratio	300.00%	300.00%
Pricing Tier2 | Maximum
Line of Credit Facility [Line Items]
Consolidated Leverage Ratio	375.00%	375.00%
Pricing Tier2 | Eurodollar Rate Loans And LIBOR Letter Of Credit Fee
Line of Credit Facility [Line Items]
Interest rate	3.50%	3.50%
Pricing Tier2 | Daily Floating Rate Loans
Line of Credit Facility [Line Items]
Interest rate	3.50%	3.50%
Pricing Tier2 | Rate Loans
Line of Credit Facility [Line Items]
Interest rate	2.50%	2.50%
Pricing Tier3
Line of Credit Facility [Line Items]
Commitment Fee	0.40%	0.40%
Pricing Tier3 | Minimum
Line of Credit Facility [Line Items]
Consolidated Leverage Ratio	225.00%	225.00%
Pricing Tier3 | Maximum
Line of Credit Facility [Line Items]
Consolidated Leverage Ratio	300.00%	300.00%
Pricing Tier3 | Eurodollar Rate Loans And LIBOR Letter Of Credit Fee
Line of Credit Facility [Line Items]
Interest rate	3.00%	3.00%
Pricing Tier3 | Daily Floating Rate Loans
Line of Credit Facility [Line Items]
Interest rate	3.00%	3.00%
Pricing Tier3 | Rate Loans
Line of Credit Facility [Line Items]
Interest rate	2.00%	2.00%
Pricing Tier4
Line of Credit Facility [Line Items]
Commitment Fee	0.30%	0.30%
Pricing Tier4 | Maximum
Line of Credit Facility [Line Items]
Consolidated Leverage Ratio	225.00%	225.00%
Pricing Tier4 | Eurodollar Rate Loans And LIBOR Letter Of Credit Fee
Line of Credit Facility [Line Items]
Interest rate	2.50%	2.50%
Pricing Tier4 | Daily Floating Rate Loans
Line of Credit Facility [Line Items]
Interest rate	2.50%	2.50%
Pricing Tier4 | Rate Loans
Line of Credit Facility [Line Items]
Interest rate	1.50%	1.50%

Debt - Schedule of Capital Leas

Debt - Schedule of Capital Leases Mature by 2025 (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Line of Credit Facility [Line Items]
Payments	$ 7,146	$ 7,838
Interest	805	949
Principal	6,341	6,889
2020
Line of Credit Facility [Line Items]
Payments		2,531
Interest		372
Principal		2,159
2021
Line of Credit Facility [Line Items]
Payments	2,670	2,230
Interest	368	298
Principal	2,302	1,932
2022
Line of Credit Facility [Line Items]
Payments	2,178	1,750
Interest	265	199
Principal	1,913	1,551
2023
Line of Credit Facility [Line Items]
Payments	1,582	1,041
Interest	139	73
Principal	1,443	968
2024
Line of Credit Facility [Line Items]
Payments	667	286
Interest	32	7
Principal	635	$ 279
2025
Line of Credit Facility [Line Items]
Payments	49
Interest	1
Principal	$ 48

Debt - Schedule of Aggregate An

Debt - Schedule of Aggregate Annual Maturities of Long-Term Debt (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Disclosure [Abstract]
2020	$ 5,034	$ 7,143
2021	6,839	143,300
2022	8,006	1,551
2023	7,198	968
2024	153,719	279
Total	$ 180,796	$ 153,241

Fair Value of Financial Instr_3

Fair Value of Financial Instruments - Summary of Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Recurring - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Level 3 Liabilities		$ 32,971		$ 15,572
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Level 3 Liabilities	$ 76,971	32,971	$ 18,438	15,572	$ 3,992
Business Acquisitions Contingent Consideration Current [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Level 3 Liabilities	49,170	8,614
Business Acquisitions Contingent Consideration Long Term [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Level 3 Liabilities	9,742	379
Contingent Put Option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Level 3 Liabilities		7,100
Contingent Put Option | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Level 3 Liabilities		7,100
Warrants
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Level 3 Liabilities		16,878		12,818
Warrants | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Level 3 Liabilities		16,878		12,818
Conversion Option [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Level 3 Liabilities	$ 18,059
Business Acquisitions Contingent Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Level 3 Liabilities		8,993		2,754
Business Acquisitions Contingent Liabilities [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Level 3 Liabilities		$ 8,993		$ 2,754

Fair Value of Financial Instr_4

Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Recurring - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	$ 32,971	$ 15,572	$ 15,572
Ending balance			32,971	$ 15,572
Warrants
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	16,878	12,818	12,818
Ending balance			16,878	12,818
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	32,971	15,572	15,572	3,992
Foreign currency translation of contingent consideration payment	(208)
Payment of contingent consideration payable	(12,464)	(554)	(554)	(1,384)
Acquisitions	45,204	31	5,401	2,951
Series A-2 compound embedded option	(9,361)
Issuance of financial instruments	30,097			12,818
Change in fair value included in earnings	34,879	3,389	12,552	(510)
Write off of financial instruments	12,140			(2,295)
Exercise of warrant options	(56,287)
Ending balance	76,971	18,438	32,971	15,572
Level 3 | Contingent Put Option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	7,100			2,647
Change in fair value included in earnings			7,100	(352)
Write off of financial instruments				(2,295)
Ending balance			7,100
Level 3 | Business Acquisitions Contingent Consideration, Current
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	8,614	2,754	2,754	1,345
Payment of contingent consideration payable			(554)	(1,384)
Acquisitions			5,022	2,951
Change in fair value included in earnings			1,392	(158)
Ending balance			8,614	2,754
Level 3 | Business Acquisitions Contingent Consideration, Long- term
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	379			0
Acquisitions			379
Ending balance			379
Level 3 | Warrants
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	16,878	12,818	12,818
Issuance of financial instruments				12,818
Change in fair value included in earnings			4,060
Ending balance			16,878	12,818
Level 3 | Compound Embedded Option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Series A-2 compound embedded option	(9,361)
Change in fair value included in earnings	27,420
Ending balance	18,059
Level 3 | Business Acquisitions Contingent Consideration, Current
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	8,614	2,754	2,754
Foreign currency translation of contingent consideration payment	(208)
Payment of contingent consideration payable	(12,464)	(554)
Acquisitions	34,661	31
Reclass of long term to short term contingent consideration	180
Change in fair value included in earnings	18,387	(670)
Ending balance	49,170	1,561	8,614	2,754
Level 3 | Business Acquisitions Contingent Consideration, Long- term
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	379
Acquisitions	10,543
Reclass of long term to short term contingent consideration	(180)
Change in fair value included in earnings	(1,000)
Ending balance	9,742		379
Level 3 | Contingent Put Option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	7,100
Change in fair value included in earnings	(19,240)
Write off of financial instruments	12,140
Ending balance			7,100
Level 3 | Warrants
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	16,878	12,818	12,818
Issuance of financial instruments	30,097
Change in fair value included in earnings	9,312	4,059
Exercise of warrant options	$ (56,287)
Ending balance		$ 16,877	$ 16,878	$ 12,818

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 (Detail)	Sep. 30, 2020	Jul. 31, 2020	Jul. 30, 2020	Apr. 30, 2020	Apr. 13, 2020	Dec. 31, 2019	Oct. 31, 2018
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option					10 years
Common Stock Value (per Share)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option						31.60
Expected Volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option						0.4751
Risk-free Interest Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option						0.0192
Expected Life (Years)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option						10 years
Redeemable Series A-1 Preferred Stock
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option							10 years
Redeemable Series A-1 Preferred Stock | Common Stock Value (per Share)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option	31.60	22.22
Redeemable Series A-1 Preferred Stock | Expected Volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option	0.4763	0.4435
Redeemable Series A-1 Preferred Stock | Risk-free Interest Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option	0.0168	0.0055
Redeemable Series A-1 Preferred Stock | Expected Life (Years)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option	10 years	10 years
Redeemable Series A-2 Preferred Stock
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option				10 years
Redeemable Series A-2 Preferred Stock | Common Stock Value (per Share)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option			22.22
Redeemable Series A-2 Preferred Stock | Expected Volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option			0.4435
Redeemable Series A-2 Preferred Stock | Risk-free Interest Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option			0.0055
Redeemable Series A-2 Preferred Stock | Expected Life (Years)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option			10 years

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Commitments and Contingencies [Line Items]
Lessee operating lease, expiration year			2028		2027
Rent expense	$ 2.2	$ 2	$ 6.4	$ 5.6	$ 7.6	$ 6.7
Office Equipment
Commitments and Contingencies [Line Items]
Lessee operating lease, expiration year			2025		2024

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Lease Payments under Leases (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Commitments and Contingencies [Line Items]
2020	$ 6,195	$ 6,422
2021	5,192	5,235
2022	3,090	4,189
2023	1,449	2,663
2024 and thereafter	1,386	2,309
Total	17,312	20,818
Rent
Commitments and Contingencies [Line Items]
2020	5,863	6,123
2021	4,865	4,983
2022	2,961	3,957
2023	1,410	2,567
2024 and thereafter	1,385	2,281
Total	16,484	19,911
Office Equipment
Commitments and Contingencies [Line Items]
2020	332	299
2021	327	252
2022	129	232
2023	39	96
2024 and thereafter	1	28
Total	$ 828	$ 907

Redeemable Series A-1 Preferr_2

Redeemable Series A-1 Preferred Stock - Additional Information (Detail) - USD ($)	Jul. 27, 2020	Jul. 26, 2020	Apr. 13, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Jul. 30, 2020	Oct. 31, 2018	Oct. 19, 2018
Temporary Equity [Line Items]
Share price				$ 16.36			$ 27.30			$ 16.36	$ 27.30
Payment for redemption of outstanding shares	$ 131,800,000									$ 131,821,000
Common stock shares sold under IPO	1,786,739											42,415
Accretion of dividends													$ 932,000
Percentage of dividends accrued			14.20%
Fair value changes in the contingent put option				$ (26,265,000)						(19,240,000)		$ 7,100,000
Temporary equity, issuance cost										$ 2,925,000	$ 29,000
Maximum
Temporary Equity [Line Items]
Share price												$ 31.60	$ 24
Minimum
Temporary Equity [Line Items]
Share price												$ 24	$ 18
Redeemable Series A-1 Preferred Stock
Temporary Equity [Line Items]
Temporary equity, shares issued				0						0		12,000	12,000			12,000
Temporary equity, par value per share				$ 0.0001						$ 0.0001		$ 0.0001	$ 0.0001			$ 0.0001
Warrants to purchase common stock														534,240	534,240	534,240
Temporary equity, liquidation preference per share												$ 10,000				$ 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 dividend payable in cash			1.00%
Total accreted value		$ 40,800,000		$ 6,542,000	$ 5,644,000	$ 5,415,000	$ 5,030,000	$ 4,777,000	$ 4,534,000			$ 19,616,000	$ 3,605,000
Accretion of dividends		$ 24,300,000
Temporary equity, covenant description										The Redeemable Series A-1 Preferred Stock contained restrictive covenants. As of December 31, 2019, 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. The Company was in compliance with the covenants as of December 31, 2019.		The Redeemable Series A-1Preferred Stock contains restrictive covenants. As of December 31, 2019 and 2018, 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) to be lower than 10.0 times as of the end of any fiscal quarter ending until maturity. The Company was in compliance with the covenants as of December 31, 2019 and 2018.
Percentage of dividends accrued										15.00%		15.00%
Percentage of dividend payable in cash										14.20%		14.20%
Accrued and unpaid dividends												$ 21,900,000	3,400,000
Fair value changes in the contingent put option	$ 12,100,000											$ 7,100,000
Changes in fair value				$ 26,300,000						$ 19,200,000
Proceeds from issuance of temporary equity													120,000,000
Temporary equity, issuance cost													1,600,000
Warrants issued													$ 12,800,000
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 (Detail) - USD ($) $ / shares in Units, $ in Thousands	Apr. 13, 2020	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
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		$ 2,870	$ 2,870
Convertible And Redeemable Series A-2 Preferred Stock
Temporary Equity [Line Items]
Number of shares issued	17,500	17,500	17,500	0
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		$ 2,900	$ 2,900
Percentage of dividend rate increase per annum in the event of noncompliance			12.00%
Percentage of dividend rate increase per annum upon noncompliance occurred and thereafter			14.00%
Percentage of discount on common stock market price			15.00%
Percentage of dividend rate steps downs per year			9.00%
Number of times increase in debt incurrence test ratio			4.5
Minimum repayment amount			$ 50,000
Minimum repayment amount drop down			$ 25,000
Increase in dividend rate			12.00%
Dividend rate thereafter			14.00%
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. In the event the Company redeemed all or a portion of the Convertible and Redeemable Series A-2Preferred Stock prior to the three year anniversary of the issuance date, such redemption amount shall include principal, accrued but unpaid interest and a make whole payment such that the aggregate amount reflects three years of dividends.
Aggregate stated value of stock redeemed		25,000	$ 25,000
Aggregate liquidation preference		182,200	182,200	$ 0
Compound embedded derivative, fair value net		18,100	18,100
Convertible And Redeemable Series A-2 Preferred Stock | Other Expense
Temporary Equity [Line Items]
Compound embedded derivative, change in value		26,700	27,400
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)

Stockholder's Equity (Deficit) - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended
	Nov. 30, 2018	Oct. 25, 2017	May 31, 2015	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Common stock, shares authorized				190,000,000		190,000,000		25,000,000	25,000,000
Common stock, par value				$ 0.000004		$ 0.000004		$ 0.000004	$ 0.000004
Total unrecognized stock compensation expense related to unvested options and restricted stock granted under the Plans				$ 10,400	$ 6,700	$ 10,400	$ 6,700	$ 6,300
Unrecognized expense expected to be recognized period						2 years		2 years
Number of restricted stock shares issued								12,132	184,400
Treasury stock, purchase price			$ 9.80
Number of options repurchased	86,884
Options repurchased, fair value price per option	$ 22.20
Options repurchased, total purchase price	$ 1,400
Stock compensation expense				1,149	1,055	$ 3,439	3,577
Private Offering
Class Of Stock [Line Items]
Warrants to purchase common stock			116,350
Warrant exercise price per share			$ 17.19
Cost of Revenue
Class Of Stock [Line Items]
Stock compensation expense				271	375	1,099	1,013
Selling, General and Administrative Expenses
Class Of Stock [Line Items]
Stock compensation expense				$ 878	$ 680	$ 2,340	$ 2,564
Restricted Stock
Class Of Stock [Line Items]
Stock compensation expense								$ 400	$ 2,000
Restricted Stock | Board of Directors
Class Of Stock [Line Items]
Stock compensation expense									200
Restricted Stock | Advisors [Member]
Class Of Stock [Line Items]
Stock compensation expense								400	1,800
Stock Options
Class Of Stock [Line Items]
Stock compensation expense								4,300	5,800
Stock Options | Cost of Revenue
Class Of Stock [Line Items]
Stock compensation expense								1,400	800
Stock Options | Selling, General and Administrative Expenses
Class Of Stock [Line Items]
Stock compensation expense								$ 2,900	$ 3,100
Montrose Amended & Restated 2017 Stock Incentive Plan
Class Of Stock [Line Items]
Number of restricted stock shares issued		175,625				33,229	30,000	40,000	26,400
Montrose Amended & Restated 2017 Stock Incentive Plan | Restricted Stock
Class Of Stock [Line Items]
Restricted stock fair value market price per share		$ 13.64				$ 31.60	$ 24	$ 24	$ 13.64
Shares of vested over period						1 year	3 years	3 years
Restricted shares became fully vested and released as common stock						10,000	0	12,132	8,775
Restricted shares outstanding				267,173	242,025	267,173	242,025	239,893	212,025
Restricted shares forfeited						1,667	0	0	0
Montrose Amended & Restated 2017 Stock Incentive Plan | Stock Options
Class Of Stock [Line Items]
Stock compensation expense								$ 2,100	$ 2,300
Montrose Amended & Restated 2017 Stock Incentive Plan | Stock Options | Board of Directors
Class Of Stock [Line Items]
Shares of vested over period						4 years		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		3 years
Montrose Amended and Restated 2013 Stock Option Plan
Class Of Stock [Line Items]
Shares outstanding from exercised options				221,950		221,950		201,600
Stock compensation expense								$ 2,200	$ 3,500
2017 Plan
Class Of Stock [Line Items]
Share-based compensation arrangement by share-based payment award, number of shares authorized				2,945,443	981,800	2,945,443	981,800	997,785	600,900
Share-based compensation arrangement by share-based payment award, number of shares available for grant				1,233,266	159,428	1,233,266	159,428	137,908	131,113
2017 Plan | Restricted Stock
Class Of Stock [Line Items]
Stock compensation expense				$ 233	$ 111	$ 973	$ 323
2017 Plan | Restricted Stock | Selling, General and Administrative Expenses
Class Of Stock [Line Items]
Stock compensation expense				233	111	973	323
2017 Plan | Stock Options
Class Of Stock [Line Items]
Stock compensation expense				868	463	2,151	1,080
2017 Plan | Stock Options | Cost of Revenue
Class Of Stock [Line Items]
Stock compensation expense				276	278	973	623	$ 1,000	$ 200
2017 Plan | Stock Options | Selling, General and Administrative Expenses
Class Of Stock [Line Items]
Stock compensation expense				$ 592	$ 185	$ 1,178	$ 457	1,100	200
2017 Plan | Stock Options | Related Party Expense
Class Of Stock [Line Items]
Stock compensation expense								$ 0	$ 1,900
2013 Plan
Class Of Stock [Line Items]
Share-based compensation arrangement by share-based payment award, number of shares authorized				2,047,269	2,074,604	2,047,269	2,074,604	2,058,619	2,154,600
Share-based compensation arrangement by share-based payment award, number of shares available for grant								1,550	79,946
Option expiration date								10 years
2013 Plan | Stock Options
Class Of Stock [Line Items]
Stock compensation expense				$ 48	$ 481	$ 315	$ 2,174
2013 Plan | Stock Options | Cost of Revenue
Class Of Stock [Line Items]
Stock compensation expense				(5)	97	126	390	$ 400	$ 600
2013 Plan | Stock Options | Selling, General and Administrative Expenses
Class Of Stock [Line Items]
Stock compensation expense				$ 53	$ 384	$ 189	$ 1,784	$ 1,800	$ 2,900

Stockholder's Equity (Deficit_2

Stockholder's Equity (Deficit) - Summary of Issued and Cancelled Shares of Common Stock (Detail) - $ / shares	Jul. 27, 2020	Jul. 27, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common Stock Issued and Cancelled, Shares			15,728,376	32,558	16,523,015	173,114
Common Stock Issued and Cancelled, Average Price per share			$ 12.82	$ 22.32	$ 13.72	$ 21.83
Common stock issued, shares	1,786,739						42,415
Number of common stock shares issued in connection with business acquisition				25,408	791,139	150,439	150,439	264,582
Acquisitions, Average Price per Share				$ 26.90	$ 31.60	$ 22.41
Redemption of Series A-1, Shares			1,786,739		1,786,739
Redemption of Series A-1, Average Price per Share			$ 15		$ 15
Exercise of warrant options, Shares			2,534,239		2,534,239
Exercise of warrant options, Average Price per Share			$ 0.01		$ 0.01
Exercise of options, Shares			16,850	7,150	20,350	22,675	27,350	5,000
Exercise of options, Average Price per Share			$ 8.92	$ 6.03	$ 9.27	$ 18.01
Restricted shares, net			8,333		8,333
Restricted shares, net, Average Price per Share			$ 24		$ 24
Cancellation of shares			(117,785)		(117,785)
Cancellation of shares, Average Price per Share			$ 17.15		$ 17.15
IPO
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock issued, shares		11,500,000	11,500,000		11,500,000
Common stock issued in connection with initial public offering, Average Price per Share			$ 15		$ 15

Stockholder's Equity (Deficit_3

Stockholder's Equity (Deficit) - Summary of Number of Shares Authorized to be Issued and Available for Grant (Detail) - shares	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
2017 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized to be issued	2,945,443	997,785	981,800	600,900
Shares available for grant	1,233,266	137,908	159,428	131,113
2013 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized to be issued	2,047,269	2,058,619	2,074,604	2,154,600
Shares available for grant		1,550		79,946

Stockholder's Equity (Deficit_4

Stockholder's Equity (Deficit) - Schedule of Stock Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense	$ 1,149	$ 1,055	$ 3,439	$ 3,577
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense					$ 4,300	$ 5,800
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense					400	2,000
Cost of Revenue
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense	271	375	1,099	1,013
Cost of Revenue | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense					1,400	800
Selling, General and Administrative Expenses
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense	878	680	2,340	2,564
Selling, General and Administrative Expenses | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense					2,900	3,100
2017 Plan | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense	868	463	2,151	1,080
2017 Plan | Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense	233	111	973	323
2017 Plan | Cost of Revenue | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense	276	278	973	623	1,000	200
2017 Plan | Selling, General and Administrative Expenses | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense	592	185	1,178	457	1,100	200
2017 Plan | Selling, General and Administrative Expenses | Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense	233	111	973	323
2013 Plan | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense	48	481	315	2,174
2013 Plan | Cost of Revenue | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense	(5)	97	126	390	400	600
2013 Plan | Selling, General and Administrative Expenses | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense	$ 53	$ 384	$ 189	$ 1,784	$ 1,800	$ 2,900

Stockholder's Equity (Deficit_5

Stockholder's Equity (Deficit) - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Exercised	(16,850)	(7,150)	(20,350)	(22,675)	(27,350)	(5,000)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-Average Exercise Price per Share Exercised					$ 6.16	$ 6.03
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 Forfeited/ cancelled					$ 18.82	$ 13.64
Weighted Average Remaining Contract Life, Exercised			7 years	7 years	7 years	7 years
Montrose Amended & Restated 2017 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-Average Exercise Price per Share Beginning Balance			$ 24	$ 20	$ 20	$ 14
Weighted-Average Exercise Price per Share Granted			32	27	27	21
Weighted-Average Exercise Price per Share Forfeited/cancelled			22	19	20	18
Weighted-Average Exercise Price per Share Expired						14
Weighted-Average Exercise Price per Share Exercised			14
Weighted-Average Exercise Price per Share Ending Balance	$ 21	$ 24	21	24	24	20	$ 14
Weighted-Average Exercise Price per Share, Exercised	14		14
Weighted-Average Exercise Price per Share Options vested and expected to vest	21		21		24
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 Beginning Balance			12	10	10	7
Weighted Average Grant Date Fair Value per Share Granted			12	14	14	11
Weighted Average Grant Date Fair Value per Share Forfeited/ cancelled			0	0
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 12	$ 12	$ 12	$ 12	$ 12	$ 10	$ 7
Weighted Average Remaining Contract Life (in Years) Outstanding			7 years 7 months 2 days	8 years 25 days	7 years 9 months 25 days
Weighted Average Remaining Contract Life, Exercised			7 years 3 months 29 days
Weighted Average Remaining Contract Life (in Years) Options vested and expected to vest			9 years 14 days		8 years 9 months 14 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding			$ 4,696	$ 1,151	$ 1,151	$ 40
Aggregate Intrinsic Value of In-The-Money Options Exercised			7,918
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 7,649	$ 4,693	7,649	$ 4,693	4,696	$ 1,151	$ 40
Aggregate Intrinsic Value of In-The-Money Options Exercised	7,649		7,649
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 7,649		$ 7,649		$ 4,696
Montrose Amended & Restated 2017 Stock Incentive Plan | Common Stock
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Outstanding Beginning Balance			617,852	257,762	257,762	49,875
Options to Purchase Common Stock Granted			851,877	356,470	380,820	220,900
Options to Purchase Common Stock Forfeited/cancelled			(14,375)	(17,700)	(20,730)	(6,000)
Options to Purchase Common Stock Expired						(7,013)
Options to Purchase Common Stock Exercised			(925)
Options to Purchase Common Stock Outstanding Ending Balance	1,454,429	596,532	1,454,429	596,532	617,852	257,762	49,875
Options to Purchase Common Stock, Exercised	925		925
Options to Purchase Common Stock Options vested and expected to vest	1,454,429		1,454,429		617,852
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, Exercised	221,950		221,950		201,600
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-Average Exercise Price per Share Beginning Balance			$ 6	$ 6	$ 6	$ 6
Weighted-Average Exercise Price per Share Forfeited/cancelled			10	8	8	7
Weighted-Average Exercise Price per Share Expired			6	8	8	6
Weighted-Average Exercise Price per Share Exercised			8	6	6	6
Weighted-Average Exercise Price per Share Ending Balance	$ 6	$ 6	6	6	6	6	$ 6
Weighted-Average Exercise Price per Share, Exercised	6		6		6
Weighted-Average Exercise Price per Share Options vested and expected to vest	6		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 Beginning Balance			1	1	1	1
Weighted Average Grant Date Fair Value per Share Forfeited/ cancelled			0	0
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 1	$ 1	$ 1	$ 1	$ 1	$ 1	$ 1
Weighted Average Remaining Contract Life (in Years) Outstanding			5 years 7 months 17 days	6 years 7 months 24 days	6 years 4 months 24 days	7 years 3 months 19 days	7 years 9 months 29 days
Weighted Average Remaining Contract Life, Exercised			5 years 1 month 20 days		5 years 10 months 25 days
Weighted Average Remaining Contract Life (in Years) Options vested and expected to vest			5 years 2 months 1 day		5 years 10 months 10 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding			$ 46,617	$ 33,290	$ 33,290	$ 16,534
Aggregate Intrinsic Value of In-The-Money Options Exercised			354	466		60
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 33,772	$ 47,381	33,772	$ 47,381	46,617	$ 33,290	$ 16,534
Aggregate Intrinsic Value of In-The-Money Options Exercised	33,772		33,772		46,617
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 33,772		$ 33,772		$ 46,617
Montrose Amended and Restated 2013 Stock Option Plan | Common Stock
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Outstanding Beginning Balance			1,855,469	1,900,404	1,900,404	2,066,000
Options to Purchase Common Stock Forfeited/cancelled			(7,500)	(14,500)	(10,635)	(43,225)
Options to Purchase Common Stock Expired			(11,300)	(5,400)	(6,950)	(117,371)
Options to Purchase Common Stock Exercised			(19,175)	(19,010)	(27,350)	(5,000)
Options to Purchase Common Stock Outstanding Ending Balance	1,817,494	1,861,494	1,817,494	1,861,494	1,855,469	1,900,404	2,066,000
Options to Purchase Common Stock, Exercised	221,025		221,025		201,600
Options to Purchase Common Stock Options vested and expected to vest	1,817,494		1,817,494		1,855,494

Stockholder's Equity (Deficit_6

Stockholder's Equity (Deficit) - Summary of Weighted Average Assumptions Used in Black-Sholes Option-pricing Model (Detail) - $ / shares	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Common stock value (per share)	$ 16.36	$ 27.30
Expected volatility	46.14%	47.94%	48.13%	48.69%
Risk- free interest rate	0.41%	2.15%
Expected life (years)	7 years	7 years	7 years	7 years
Forfeiture rate	0.00%	0.00%
Dividend rate	0.00%	0.00%
Minimum
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Common stock value (per share)			$ 24	$ 18
Risk- free interest rate			1.43%	2.86%
Forfeiture rate			0.00%	0.00%
Dividend rate			0.00%	0.00%
Maximum
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Common stock value (per share)			$ 31.60	$ 24
Risk- free interest rate			2.63%	2.89%

Stockholder's Equity (Deficit_7

Stockholder's Equity (Deficit) - Schedule of Common Stock Reserved for Future Issuance (Detail) - shares	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	5,109,062	3,706,994	3,406,090
Warrants
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	116,350	650,590	650,590
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	2,058,619	2,154,600
Montrose Amended & Restated 2017 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	2,945,443	997,785	600,900

Net Income (Loss) Per Share - C

Net Income (Loss) Per Share - Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net loss	$ (30,737)	$ 13,224	$ (41,248)	$ (6,683)	$ (322)	$ (5,242)	$ (58,761)	$ (12,247)	$ (23,557)	$ (16,491)
ACCRETION OF REDEEMABLE SERIES A- 1 PREFERRED STOCK	(6,542)			(5,030)			(17,601)	(14,341)	(19,616)	(3,605)
Convertible preferred stock deemed dividend, net of return from holders	(24,341)						(24,341)			(932)
CONVERTIBLE AND REDEEMABLE SERIES A-2 PREFERRED STOCK DIVIDEND	(2,870)						(2,870)
Net loss attributable to common stockholders-basic and diluted	$ (64,490)			$ (11,713)			$ (103,573)	$ (26,588)	$ (43,173)	$ (21,028)
Weighted- average common shares outstanding-basic and diluted	21,554			8,718			13,669	8,640	8,789	7,533
Net loss per share attributable to common stockholders-basic and diluted	$ (2.99)			$ (1.34)			$ (7.58)	$ (3.08)	$ (4.91)	$ (2.79)

Net Income (Loss) Per Share - E

Net Income (Loss) Per Share - Equity Shares Excluded from Calculation of Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - shares	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Stock Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of loss per share amount	2,423,900	2,062,825	2,423,900	2,062,825	2,048,738	1,321,890
Restricted Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of loss per share amount	176,384	158,426	176,384	158,426	153,352	112,637
Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of loss per share amount	116,350	116,350	116,350	116,350	116,350	108,267

Segment Information - Additiona

Segment Information - Additional Information (Detail) - Segment	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Segment Reporting [Abstract]
Number of operating segments	3	3
Number of reportable segments	3	3

Segment Information - Component

Segment Information - Components of Segment Revenues and Adjusted EBITDA (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Segment Revenues	$ 84,705	$ 57,623	$ 219,502	$ 165,978	$ 233,854	$ 188,805
Segment Adjusted EBITDA	16,706	8,219	36,157	21,836	31,495	21,817
Operating Segments
Segment Reporting Information [Line Items]
Segment Revenues	84,705	57,623	219,502	165,978	233,854	188,805
Segment Adjusted EBITDA	21,468	11,786	51,582	32,771	45,136	33,518
Assessment, Permitting and Response | Operating Segments
Segment Reporting Information [Line Items]
Segment Revenues	26,593	5,413	49,768	15,017	21,071	3,663
Segment Adjusted EBITDA	8,230	1,695	14,665	5,693	7,572	1,339
Measurement and Analysis | Operating Segments
Segment Reporting Information [Line Items]
Segment Revenues	39,770	35,672	113,130	98,521	135,531	117,373
Segment Adjusted EBITDA	11,226	7,691	30,422	19,761	27,828	20,779
Remediation and Reuse | Operating Segments
Segment Reporting Information [Line Items]
Segment Revenues	18,342	16,538	56,604	52,440	77,252	67,769
Segment Adjusted EBITDA	2,012	2,400	6,495	7,317	9,736	11,400
Corporate and Other
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	$ (4,762)	$ (3,567)	$ (15,425)	$ (10,935)	$ (13,641)	$ (11,701)

Segment Information - Reconcili

Segment Information - Reconciliation of Segment Measure to Income (Loss) Before Benefit from Income Taxes (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting [Abstract]
Total	$ 16,706	$ 8,219	$ 36,157	$ 21,836	$ 31,495	$ 21,817
Interest expense, net	(3,043)	(2,130)	(10,896)	(4,590)	(6,755)	(11,085)
Income tax (expense) benefit	(3,348)	412	1,563	1,308	3,121	4,968
Depreciation and amortization	(9,740)	(7,412)	(27,084)	(20,262)	(27,705)	(23,915)
Stock-based compensation	(1,149)	(1,055)	(3,439)	(3,577)	(4,345)	(5,794)
Start-up losses and investment in new services	(602)	(634)	(1,283)	(1,091)	(1,874)	(999)
Acquisition costs	(6)	(1,278)	(3,767)	(2,350)	2,400	1,200
Acquisition costs					(3,474)	(1,595)
Fair value changes in contingent put option	26,265		19,240		(7,100)
Fair value changes in financial instruments					(11,160)	352
Fair value changes in warrant options	(9,311)	(2,510)	(9,312)	(4,059)
Fair value changes in compound embedded option	(26,664)		(27,420)
Fair value changes in business acquisitions contingent consideration	(13,404)	(256)	(17,387)	670	(1,392)	158
Short term purchase accounting fair value adjustment to deferred revenue			(243)		(858)
IPO preparation	(6,378)	(20)	(6,908)	(105)	(610)
Discontinued services	(30)		(7,526)
Expenses related to financing transactions			(277)			(398)
Other expenses	(33)	(19)	(179)	(27)
Net loss	$ (30,737)	$ (6,683)	$ (58,761)	$ (12,247)	$ (23,557)	$ (16,491)

Related-Party Transactions - Ad

Related-Party Transactions - Additional Information (Detail) - USD ($)	Jul. 27, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Payment to related party						$ 448,000	$ 2,180,000
Amount due to related party		$ 0		$ 0		$ 0	10,000
Number of shares issued as dividend payment		1,786,739
Number of additional common stock outstanding owned by related party		2,534,239		2,534,239
Percentage of common stock shares outstanding owned by related party		17.30%		17.30%
Shares issued	1,786,739					42,415
Share price		$ 16.36	$ 27.30	$ 16.36	$ 27.30
Proceeds from notes receivable						$ 100,000
Restricted Stock
Related Party Transaction [Line Items]
Shares issued								175,625
Share price								$ 13.64
Acquisition Targets | Selling, General and Administrative Expenses
Related Party Transaction [Line Items]
Payment to related party		$ 0	$ 100,000	$ 100,000	$ 400,000	$ 500,000	400,000
Management Agreement [Member] | Selling, General and Administrative Expenses
Related Party Transaction [Line Items]
Payment to related party							$ 1,800,000

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, 2020	Sep. 30, 2019	May 22, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, description				401(k) Savings Plan		401(k) Savings Plan
Defined contribution plan, maximum annual contributions per employee, percent				85.00%		85.00%	100.00%
Defined contribution plan, employer matching contribution, percent of match			100.00%			100.00%
Defined contribution plan participant's compensation percent			3.00%			3.00%
Defined contribution plan percentage of participant's elective deferrals			50.00%			50.00%
Selling, General and Administrative Expenses
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, employer discretionary contribution amount	$ 0	$ 0.5		$ 1.2	$ 1.9	$ 2.6	$ 2.1
Minimum
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan participant's compensation percent				3.00%		3.00%
Maximum
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan participant's compensation percent				4.00%		4.00%

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands

Sep. 30, 2022

Sep. 30, 2021

Oct. 06, 2020

Sep. 30, 2020

Apr. 13, 2020

Apr. 30, 2020

Sep. 30, 2020

Sep. 30, 2019

Sep. 30, 2020

Sep. 30, 2019

Dec. 31, 2019

Dec. 31, 2018

Subsequent Event [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%

Exercise of warrant options, Shares

2,534,239

2,534,239

Exercise of warrant options, Average Price per Share

$ 0.01

$ 0.01

Number of common stock shares issued in connection with business acquisition

25,408

791,139

150,439

150,439

264,582

Line of credit

$ 225,000

Line of credit facility maturity date

Apr. 30, 2025

Debt instrument, repayments

$ 500

Forecast

Subsequent Event [Line Items]

Debt instrument, repayments

$ 1,600

$ 1,100

$ 500

CTEH

Subsequent Event [Line Items]

Percentage of interests acquired

100.00%

Convertible And Redeemable Series A-2 Preferred Stock

Subsequent Event [Line Items]

Temporary equity, shares issued

17,500

17,500

17,500

17,500

0

Temporary equity, par value per share

$ 0.0001

$ 0.0001

$ 0.0001

$ 0.0001

$ 0.0001

Percentage of dividends accrued

15.00%

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. In the event the Company redeemed all or a portion of the Convertible and Redeemable Series A-2Preferred Stock prior to the three year anniversary of the issuance date, such redemption amount shall include principal, accrued but unpaid interest and a make whole payment such that the aggregate amount reflects three years of dividends.

Percentage of discount on common stock market price

15.00%

Percentage of dividend rate steps downs per year

9.00%

Number of times increase in debt incurrence test ratio

4.5

Convertible And Redeemable Series A-2 Preferred Stock | CTEH

Subsequent Event [Line Items]

Number of common stock shares issued in connection with business acquisition

791,139

Convertible And Redeemable Series A-2 Preferred Stock | 60-Day Period Prior to Seventh Anniversary

Subsequent Event [Line Items]

Temporary equity convertible into common stock

$ 60,000

$ 60,000

$ 60,000

Convertible And Redeemable Series A-2 Preferred Stock | Year 5

Subsequent Event [Line Items]

Temporary equity convertible into common stock

60,000

60,000

60,000

Convertible And Redeemable Series A-2 Preferred Stock | Year 6

Subsequent Event [Line Items]

Temporary equity convertible into common stock

120,000

120,000

120,000

Previously Reported | Forecast

Subsequent Event [Line Items]

Debt instrument, repayments

$ 1,200

Term Loan Facility

Subsequent Event [Line Items]

Line of credit

$ 174,453

$ 175,000

$ 174,453

$ 174,453

$ 48,750

$ 50,000

Term Loan Facility | LIBOR Plus

Subsequent Event [Line Items]

Interest rate

5.00%

5.00%

Term Loan Facility | LIBOR Floor

Subsequent Event [Line Items]

Interest rate

4.50%

1.00%

Revolving Line of Credit

Subsequent Event [Line Items]

Line of credit

$ 50,000

$ 97,590

$ 20,000

Line of credit facility maturity date

Oct. 19, 2021

Revolving Line of Credit | LIBOR Plus

Subsequent Event [Line Items]

Interest rate

3.50%

Subsequent Event

Subsequent Event [Line Items]

Percentage of dividends accrued

14.20%

Line of credit

$ 225,000

Line of credit facility maturity date

Apr. 30, 2025

Subsequent Event | CTEH

Subsequent Event [Line Items]

Percentage of interests acquired

100.00%

Subsequent Event | Convertible And Redeemable Series A-2 Preferred Stock

Subsequent Event [Line Items]

Temporary equity, shares issued

17,500

Temporary equity, par value per share

$ 0.0001

Exercise of warrant options, Shares

$ 30,000

Period between issuance and expiration of outstanding warrant

10 years

Proceeds from the Series A-2 and Warrant

$ 175,000

Percentage of dividends accrued

15.00%

Temporary equity, covenant description

The Series A-2Preferred Stock contains restrictive covenants that include (i) maximum 4.0 times debt incurrence test and (ii) 10.0 times total leverage cap (inclusive of the outstanding balance on the Series A-2 Preferred Stock).

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) $50.0 million in aggregate stated value of the Series A-2 Preferred Stock and (ii) all of the Series A-2 Preferred Stock then outstanding) of the outstanding Series A-2 Preferred Stock in cash.

Percentage of discount on common stock market price

15.00%

Percentage of dividend rate steps downs per year

9.00%

Number of times increase in debt incurrence test ratio

4.5

Exercise of warrant options, Shares

1,351,960

Exercise of warrant options, Average Price per Share

$ 0.01

Warrant options, exrecisable

949,367

Warrant options, exrecisable period

10 years

Subsequent Event | Convertible And Redeemable Series A-2 Preferred Stock | CTEH

Subsequent Event [Line Items]

Number of common stock shares issued in connection with business acquisition

791,139

Common stock shares issued in connection with business acquisition, price per share

$ 31.60

Subsequent Event | Convertible And Redeemable Series A-2 Preferred Stock | 60-Day Period Prior to Seventh Anniversary

Subsequent Event [Line Items]

Temporary equity convertible into common stock

$ 60,000

Subsequent Event | Convertible And Redeemable Series A-2 Preferred Stock | Year 5

Subsequent Event [Line Items]

Temporary equity convertible into common stock

60,000

Subsequent Event | Convertible And Redeemable Series A-2 Preferred Stock | Year 6

Subsequent Event [Line Items]

Temporary equity convertible into common stock

120,000

Subsequent Event | Term Loan Facility

Subsequent Event [Line Items]

Line of credit

$ 175,000

Subsequent Event | Term Loan Facility | LIBOR Plus

Subsequent Event [Line Items]

Interest rate

1.00%

5.00%

Subsequent Event | Term Loan Facility | LIBOR Floor

Subsequent Event [Line Items]

Interest rate

1.00%

1.00%

Subsequent Event | Revolving Line of Credit

Subsequent Event [Line Items]

Line of credit

$ 50,000

Subsequent Event | Revolving Line of Credit | LIBOR Plus

Subsequent Event [Line Items]

Interest rate

3.50%

Significant Accounting Polici_2

Significant Accounting Policies - Additional Information (Detail) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020Segment	Dec. 31, 2019USD ($)Segment
Significant Accounting Policies [Line Items]
Number of operating segments | Segment	3	3
IPO
Significant Accounting Policies [Line Items]
Accrued stock issuance costs | $		$ 1.2

Income Taxes - Summary of Incom

Income Taxes - Summary of Income Tax Benefit (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Current:
Federal	$ 4	$ 7
State	289	245
Foreign	41
Total	334	252
Deferred:
Federal	(2,323)	(3,639)
State	(1,132)	(1,581)
Total	(3,455)	(5,220)
Income tax benefit	$ (3,121)	$ (4,968)

Income Taxes - Schedule of Sign

Income Taxes - Schedule of Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating losses		$ 3,850	$ 2,570
Allowance for bad debts	$ 7,400	331	122
Employee related		1,348	1,834
Interest expense		588	1,472
Other		318
Total deferred tax asset		6,435	5,998
Deferred tax liabilities:
Intangible assets		(4,825)	(7,032)
Property and equipment		(4,589)	(3,172)
Other		(551)	(662)
Total deferred tax liability		(9,965)	(10,866)
Net deferred tax liability	$ (1,936)	$ (3,530)	$ (4,868)

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Federal Statutory Income Tax Rate to Effective Income Tax Rate (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Tax completed at federal statutory rate			21.00%		21.00%	21.00%
State tax net of federal benefit					2.51%	4.96%
Non- deductible expenses					(1.38%)	(0.91%)
Equity compensation					(2.41%)	(0.83%)
Contingent put option					(5.59%)	0.00%
Foreign taxes					(0.15%)	(0.00%)
Warrant option					(3.20%)
Other					0.92%	(1.03%)
Effective income tax rate	(12.20%)	5.80%	2.60%	9.60%	11.70%	23.19%

Contingent Put Option - Additio

Contingent Put Option - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Option Indexed to Issuer's Equity [Line Items]
Fair value changes in the contingent put option	$ (26,265)	$ (19,240)	$ 7,100
Redeemable Preferred Stock
Option Indexed to Issuer's Equity [Line Items]
Fair value changes in the contingent put option			7,100
Changes in fair value			$ 7,100

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Apr. 13, 2020	Oct. 19, 2018	Oct. 18, 2018	Jun. 30, 2015	Sep. 30, 2020	Sep. 30, 2019
Temporary Equity [Line Items]
Temporary equity, share price					$ 16.36	$ 27.30
Temporary equity, cumulative dividend rate	14.20%
Convertible Preferred Stock
Temporary Equity [Line Items]
Temporary equity, shares issued		48,074		69,817
Temporary equity, share price		$ 644.55		$ 429.70
Convertible debt price per share				$ 143.23
Temporary equity, cumulative dividend rate				12.00%
Common stock issued conversion of shares		674,033
Conversion price		$ 22.20
Common stock issued conversion of shares		$ 45.9	$ 48.1
Mark to market adjustment amount			3.2
Mark to market adjustment, excess amounts return from holders			$ 2.2

Stockholder's Equity (Deficit_8

Stockholder's Equity (Deficit) - Common Stock Issuances (Detail) - $ / shares	Jul. 27, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Share-based Payment Arrangement [Abstract]
Number of Shares issued, acquistion			25,408	791,139	150,439	150,439	264,582
Number of Shares issued, exercise of options		16,850	7,150	20,350	22,675	27,350	5,000
Number of Shares issued, shares sold	1,786,739					42,415
Number of Shares issued, release of restricted stock						12,132	184,400
Number of Shares issued						232,336	453,982
Average Price per Share, acquistion						$ 26.90	$ 20.26
Average Price per Share, exercise of options						6.16	6.03
Average Price per Share, shares sold						31.60
Average Price per Share, release of restricted stock						$ 18.82	$ 13.64

Subsequent Events - Summary of

Subsequent Events - Summary of Elements of Purchase Price of Acquisitions (Detail) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended		12 Months Ended
	Apr. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Subsequent Event [Line Items]
Cash		$ 175,450	$ 87,285	$ 87,285	$ 48,961
Common Stock		25,000	4,047	4,047	5,634
Other Purchase Price Component			1,450	1,450	848
Contingent Consideration		34,661
Total Purchase Price		243,507	$ 98,183	$ 98,183	$ 58,394
CTEH
Subsequent Event [Line Items]
Cash		175,000
Common Stock		25,000
Contingent Consideration		34,451
Total Purchase Price		$ 242,697
CTEH | Subsequent Event
Subsequent Event [Line Items]
Cash	$ 175,000
Common Stock	25,000
Other Purchase Price Component	(1,746)
Contingent Consideration	0
Total Purchase Price	$ 198,254

Subsequent Events - Summary o_2

Subsequent Events - Summary of Elements of Purchase Price of Acquisitions (Parenthetical) (Detail) - Subsequent Event - CTEH $ in Millions	1 Months Ended
	Apr. 30, 2020USD ($)
Two Thousand And Twenty Earn Out [Member]
Subsequent Event [Line Items]
Business combination, contingent consideration arrangements, basis for amount	The first year earnout is to be 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-outpayment of $50.0 million.
Business combination, minimum EBITDA for contingent consideration arrangements	$ 18.3
Business combination, earn-out payment, maximum	$ 50
Business combination, contingent consideration arrangements, description	The 2020 earn out is payable 100% in common stock unless the Company has consummated an IPO or a private placement of common stock where proceeds are no less than $75.0 million, in which event 50% of the 2020 earn out is payable in common stock and 50% in cash.
Percentage of earn out payable in common stock	100.00%
Percentage of earn out payable in cash, as result of completing of IPO	50.00%
Percentage of earn out payable in cash or shares, as a result of completion of IPO, at company's election	50.00%
Two Thousand And Twenty One Earn Out [Member]
Subsequent Event [Line Items]
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 a maximum second year earn-out payment of $30.0 million.
Business combination, earn-out payment, maximum	$ 30
Business combination, contingent consideration arrangements, description	The 2021 earn out, if any, is payable in cash.

Subsequent Events - Schedule of

Subsequent Events - Schedule of Supplemental Unaudited Pro Forma Information (Detail) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Subsequent Event [Line Items]
REVENUES	$ 84,705			$ 57,623			$ 219,502	$ 165,978	$ 233,854	$ 188,805
Net loss	$ (30,737)	$ 13,224	$ (41,248)	$ (6,683)	$ (322)	$ (5,242)	$ (58,761)	$ (12,247)	(23,557)	$ (16,491)
Acquisition Proforma
Subsequent Event [Line Items]
REVENUES									110,119
Net loss									34,928
Pro Forma
Subsequent Event [Line Items]
REVENUES									343,973
Net loss									$ 11,371