Stockholder's Equity (Deficit) - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Exercised	(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	617,852	617,852
Options to Purchase Common Stock Granted	224,270	158,062
Options to Purchase Common Stock Forfeited/cancelled	(10,000)	(2,000)
Options to Purchase Common Stock Expired	(1,250)
Options to Purchase Common Stock Exercised	(17,188)
Options to Purchase Common Stock Outstanding Ending Balance	2,036,061	773,914	1,840,229	617,852
Options to Purchase Common Stock Exercisable	218,379
Options to Purchase Common Stock Options vested and expected to vest	2,036,061
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	26	32
Weighted-Average Exercise Price per Share Expired	18
Weighted-Average Exercise Price per Share Exercised	16
Weighted-Average Exercise Price per Share Outstanding Ending Balance	24	26	23	$ 24
Weighted-Average Exercise Price per Share Exercisable	24
Weighted-Average Exercise Price per Share Options vested and expected to vest	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 Outstanding Beginning Balance	12	12	12
Weighted Average Grant Date Fair Value per Share Granted	20	12
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 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 11 months 19 days	8 years 6 months	9 years 1 month 2 days	7 years 9 months 25 days
Weighted Average Remaining Contract Life (in Years) Exercisable	7 years 9 months 14 days
Weighted Average Remaining Contract Life (in Years) Options vested and expected to vest	8 years 11 months 19 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 52,317	$ 4,693	$ 15,598	$ 4,696
Aggregate Intrinsic Value of In-The-Money, Options Exercised	519
Aggregate Intrinsic Value of In-The-Money Options Exercisable	5,702
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 52,317
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 Expired		(725)
Options to Purchase Common Stock Exercised	(311,872)
Options to Purchase Common Stock Outstanding Ending Balance	1,475,997	1,854,744	1,787,869	1,855,469
Options to Purchase Common Stock Exercisable	1,456,922
Options to Purchase Common Stock Options vested and expected to vest	1,475,997
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 Expired		6
Weighted-Average Exercise Price per Share Exercised	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		1	$ 1
Weighted Average Grant Date Fair Value per Share Ending Balance		$ 1		$ 1
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life (in Years) Outstanding	5 years 1 month 20 days	6 years 1 month 20 days	5 years 4 months 24 days	6 years 1 month 20 days
Weighted Average Remaining Contract Life (in Years) Exercisable	5 years 1 month 17 days
Weighted Average Remaining Contract Life (in Years) Options vested and expected to vest	5 years 1 month 20 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 64,492	$ 46,521	$ 43,867	$ 46,617
Aggregate Intrinsic Value of In-The-Money, Options Exercised	12,321
Aggregate Intrinsic Value of In-The-Money Options Exercisable	63,740
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 64,492