Stockholder's Equity (Deficit) - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 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	(94,090)	(3,500)	(424,150)	(3,500)
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			232,470	160,712
Options to Purchase Common Stock Forfeited/cancelled			(13,425)	(5,500)
Options to Purchase Common Stock Expired			(1,250)
Exercise of options, Shares			(33,918)
Options to Purchase Common Stock Outstanding Ending Balance	2,024,106	773,064	2,024,106	773,064	1,840,229	617,852
Options to Purchase Common Stock Exercisable	293,299		293,299
Options to Purchase Common Stock Options vested and expected to vest	2,024,106		2,024,106
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			38	32
Weighted-Average Exercise Price per Share Forfeited/cancelled			27	20
Weighted-Average Exercise Price per Share Expired			18
Weighted-Average Exercise Price per Share Exercised			20
Weighted-Average Exercise Price per Share Outstanding Ending Balance	$ 25	$ 26	25	26	23	$ 24
Weighted-Average Exercise Price per Share Exercisable	25		25
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			21	12
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 13	$ 12	$ 13	$ 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 8 months 26 days	8 years 10 days	9 years 1 month 2 days	7 years 9 months 25 days
Weighted Average Remaining Contract Life (in Years) Exercisable			7 years 7 months 24 days
Weighted Average Remaining Contract Life (in Years) Options vested and expected to vest			8 years 8 months 26 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 58,785	$ 4,651	$ 58,785	$ 4,651	$ 15,598	$ 4,696
Aggregate Intrinsic Value of In-The-Money, Options Exercised			1,014
Aggregate Intrinsic Value of In-The-Money Options Exercisable	8,396		8,396
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 58,785		$ 58,785
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				(1,250)
Options to Purchase Common Stock Expired				(8,550)
Exercise of options, Shares			(389,232)	(3,500)
Options to Purchase Common Stock Outstanding Ending Balance	1,398,637	1,842,169	1,398,637	1,842,169	1,787,869	1,855,469
Options to Purchase Common Stock Exercisable	1,390,812		1,390,812
Options to Purchase Common Stock Options vested and expected to vest	1,398,637		1,398,637
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				6
Weighted-Average Exercise Price per Share Expired				6
Weighted-Average Exercise Price per Share Exercised			6	6
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	$ 1	$ 1	$ 1	$ 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 10 months 17 days	5 years 10 months 28 days	5 years 4 months 24 days	6 years 4 months 24 days
Weighted Average Remaining Contract Life (in Years) Exercisable			4 years 10 months 17 days
Weighted Average Remaining Contract Life (in Years) Options vested and expected to vest			4 years 10 months 17 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 65,982	$ 46,276	$ 65,982	$ 46,276	$ 43,867	$ 46,617
Aggregate Intrinsic Value of In-The-Money, Options Exercised			15,922
Aggregate Intrinsic Value of In-The-Money Options Exercisable	65,651		65,651
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 65,982		$ 65,982