Stockholder's Equity - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	Jan. 31, 2021	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Exercise of options, Shares		(30,607)	(330,060)
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	2,036,729	1,840,229	1,840,229
Options to Purchase Common Stock Granted		547,884	224,270
Options to Purchase Common Stock Forfeited/cancelled		(23,082)	(10,000)
Options to Purchase Common Stock Expired			(1,250)
Exercise of options, Shares		(11,547)	(17,188)
Options to Purchase Common Stock Outstanding Ending Balance		2,549,984	2,036,061	2,036,729	1,840,229
Options to Purchase Common Stock Exercisable		515,943
Options to Purchase Common Stock Options vested and expected to vest		2,549,984
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	$ 26	$ 23	$ 23
Weighted-Average Exercise Price per Share Granted		44	38
Weighted-Average Exercise Price per Share Forfeited/cancelled		36	26
Weighted-Average Exercise Price per Share Expired			18
Weighted-Average Exercise Price per Share Exercised		27	16
Weighted-Average Exercise Price per Share Outstanding Ending Balance		30	24	26	$ 23
Weighted-Average Exercise Price per Share Exercisable		30
Weighted-Average Exercise Price per Share Options vested and expected to vest		30
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	14	12	12
Weighted Average Grant Date Fair Value per Share Granted		16	20
Weighted Average Grant Date Fair Value per Share Ending Balance		$ 14	$ 13	$ 14	$ 12
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life (in Years) Outstanding	8 years 3 months 18 days	8 years 5 months 15 days	8 years 11 months 19 days		9 years 1 month 2 days
Weighted Average Remaining Contract Life (in Years) Exercisable		7 years 5 months 4 days
Weighted Average Remaining Contract Life (in Years) Options vested and expected to vest		8 years 5 months 15 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding		$ 59,955	$ 52,317	$ 91,030	$ 15,598
Aggregate Intrinsic Value of In-The-Money Options Exercised		313	$ 519
Aggregate Intrinsic Value of In-The-Money Options Exercisable		12,083
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest		$ 59,955
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	897,674	1,787,869	1,787,869
Exercise of options, Shares		(19,060)	(311,872)
Options to Purchase Common Stock Outstanding Ending Balance		878,614	1,475,997	897,674	1,787,869
Options to Purchase Common Stock Exercisable		878,614
Options to Purchase Common Stock Options vested and expected to vest		878,614
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	$ 6
Weighted-Average Exercise Price per Share Exercised		6	6
Weighted-Average Exercise Price per Share Outstanding Ending Balance		6	$ 6	6	$ 6
Weighted-Average Exercise Price per Share Exercisable		6
Weighted-Average Exercise Price per Share Options vested and expected to vest		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		2
Weighted Average Grant Date Fair Value per Share Ending Balance		$ 2		$ 2
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life (in Years) Outstanding		4 years 1 month 17 days	5 years 1 month 20 days	4 years 4 months 13 days	5 years 4 months 24 days
Weighted Average Remaining Contract Life (in Years) Exercisable		4 years 1 month 17 days
Weighted Average Remaining Contract Life (in Years) Options vested and expected to vest		4 years 1 month 17 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding		$ 40,860	$ 64,492	$ 57,529	$ 43,867
Aggregate Intrinsic Value of In-The-Money Options Exercised		821	$ 12,321
Aggregate Intrinsic Value of In-The-Money Options Exercisable		40,860
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest		$ 40,860