Stockholders' Equity (Deficit) - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Exercised	(959,890)	(47,600)	(27,350)
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	1,840,229,000	617,852,000	257,762,000
Options to Purchase Common Stock Granted	300,620,000	1,243,027,000	380,820,000
Options to Purchase Common Stock Forfeited/cancelled	(33,875,000)	(17,225,000)	(20,730,000)
Options to Purchase Common Stock Exercised	(68,695,000)	(925,000)
Options to Purchase Common Stock Outstanding Ending Balance	2,036,729,000	1,840,229,000	617,852,000	257,762,000
Options to Purchase Common Stock Expired	(1,550,000)	(2,500,000)
Options to Purchase Common Stock, Exercisable	349,316,000
Options to Purchase Common Stock Options vested and expected to vest	2,036,729,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-Average Exercise Price per Share Beginning Balance	$ 23	$ 24	$ 20
Weighted-Average Exercise Price per Share Granted	44	22	27
Weighted-Average Exercise Price per Share Forfeited/cancelled	28	22	20
Weighted-Average Exercise Price per Share Exercised	22	14
Weighted-Average Exercise Price per Share Ending Balance	26	23	24	$ 20
Weighted-Average Exercise Price per Share Expired	19	18
Weighted-Average Exercise Price per Share, Exercisable	28
Weighted-Average Exercise Price per Share Options vested and expected to vest	26
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	12	10
Weighted Average Grant Date Fair Value per Share Granted	23	12	14
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 14	$ 12	$ 12	$ 10
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	9 years 1 month 2 days	7 years 9 months 25 days
Weighted Average Remaining Contract Life, Exercisable	7 years 3 months 29 days
Weighted Average Remaining Contract Life (in Years) Outstanding	8 years 3 months 18 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 15,598	$ 4,696	$ 1,151
Aggregate Intrinsic Value of In-The-Money Options Exercised	2,169		8
Aggregate Intrinsic Value of In-The-Money Options Outstanding	91,030	$ 15,598	$ 4,696	$ 1,151
Aggregate Intrinsic Value of In-The-Money Options Exercisable	15,022
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 91,030
Weighted-Average Exercise Price per Share Options vested and expected to vest	$ 26
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	1,787,869,000	1,855,469,000	1,900,404,000
Options to Purchase Common Stock Forfeited/cancelled		(7,500,000)	(10,635,000)
Options to Purchase Common Stock Exercised	(889,570,000)	(48,800,000)	(27,350,000)
Options to Purchase Common Stock Outstanding Ending Balance	897,674,000	1,787,869,000	1,855,469,000	1,900,404,000
Options to Purchase Common Stock Expired	(625,000)	(11,300,000)	(6,950,000)
Options to Purchase Common Stock, Exercisable	897,674,000
Options to Purchase Common Stock Options vested and expected to vest	897,674,000
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
Weighted-Average Exercise Price per Share Forfeited/cancelled		10	8
Weighted-Average Exercise Price per Share Exercised	6	8	6
Weighted-Average Exercise Price per Share Ending Balance	6	6	6	$ 6
Weighted-Average Exercise Price per Share Expired	6	6	8
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 Beginning Balance	1	1	1
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 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 4 months 13 days	5 years 4 months 24 days	6 years 4 months 24 days	7 years 3 months 18 days
Weighted Average Remaining Contract Life, Exercisable	4 years 4 months 13 days
Weighted Average Remaining Contract Life (in Years) Outstanding	4 years 4 months 13 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 43,867	$ 46,617	$ 33,290
Aggregate Intrinsic Value of In-The-Money Options Exercised	45,161	908
Aggregate Intrinsic Value of In-The-Money Options Outstanding	57,529	$ 43,867	$ 46,617	$ 33,290
Aggregate Intrinsic Value of In-The-Money Options Exercisable	57,529
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 57,529
Weighted-Average Exercise Price per Share Options vested and expected to vest	$ 6