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