Stockholder's Equity (Deficit) - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Exercised	(47,600)	(27,350)	(5,000)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life, Exercisable	7 years	7 years	7 years
Montrose Amended & Restated 2017 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-Average Exercise Price per Share Beginning Balance	$ 24	$ 20	$ 14
Weighted-Average Exercise Price per Share Granted	22	27	21
Weighted-Average Exercise Price per Share Forfeited/cancelled	22	20	18
Weighted-Average Exercise Price per Share Expired			14
Weighted-Average Exercise Price per Share Exercised	14
Weighted-Average Exercise Price per Share Ending Balance		24	20	$ 14
Weighted-Average Exercise Price per Share, Exercisable	21
Weighted-Average Exercise Price per Share Options vested and expected to vest	23
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	10	7
Weighted Average Grant Date Fair Value per Share Granted	12	14	11
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 12	$ 12	$ 10	$ 7
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life (in Years) Outstanding	9 years 1 month 2 days	7 years 9 months 25 days
Weighted Average Remaining Contract Life, Exercisable	7 years 6 months 25 days
Weighted Average Remaining Contract Life (in Years) Outstanding	9 years 1 month 2 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 4,696	$ 1,151	$ 40
Aggregate Intrinsic Value of In-The-Money Options Exercised	7,918
Aggregate Intrinsic Value of In-The-Money Options Outstanding	15,598	$ 4,696	$ 1,151	$ 40
Aggregate Intrinsic Value of In-The-Money Options Exercisable	1,538
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 15,598
Montrose Amended & Restated 2017 Stock Incentive Plan | Common Stock
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Outstanding Beginning Balance	617,852,000	257,762,000	49,875,000
Options to Purchase Common Stock Granted	1,243,027,000	380,820,000	220,900,000
Options to Purchase Common Stock Forfeited/cancelled	(17,225,000)	(20,730,000)	(6,000,000)
Options to Purchase Common Stock Expired	(2,500,000)		(7,013,000)
Options to Purchase Common Stock Exercised	(925,000)
Options to Purchase Common Stock Outstanding Ending Balance	1,840,229,000	617,852,000	257,762,000	49,875,000
Options to Purchase Common Stock, Exercisable	147,833,000
Options to Purchase Common Stock Options vested and expected to vest	1,840,229,000
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, Exercisable	249,200,000	201,600,000
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	7
Weighted-Average Exercise Price per Share Expired	6	8	6
Weighted-Average Exercise Price per Share Exercised	8	6	6
Weighted-Average Exercise Price per Share 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 Beginning Balance	1	1	1
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 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	5 years 4 months 24 days	6 years 4 months 24 days	7 years 3 months 18 days	7 years 9 months 29 days
Weighted Average Remaining Contract Life, Exercisable	5 years 4 months 20 days
Weighted Average Remaining Contract Life (in Years) Outstanding	5 years 4 months 24 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 46,617	$ 33,290	$ 16,534
Aggregate Intrinsic Value of In-The-Money Options Exercised	908		60
Aggregate Intrinsic Value of In-The-Money Options Outstanding	43,867	$ 46,617	$ 33,290	$ 16,534
Aggregate Intrinsic Value of In-The-Money Options Exercisable	43,482
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 43,867
Montrose Amended and Restated 2013 Stock Option Plan | Common Stock
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Outstanding Beginning Balance	1,855,469,000	1,900,404,000	2,066,000,000
Options to Purchase Common Stock Forfeited/cancelled	(7,500,000)	(10,635,000)	(43,225,000)
Options to Purchase Common Stock Expired	(11,300,000)	(6,950,000)	(117,371,000)
Options to Purchase Common Stock Exercised	(48,800,000)	(27,350,000)	(5,000,000)
Options to Purchase Common Stock Outstanding Ending Balance	1,787,869,000	1,855,469,000	1,900,404,000	2,066,000,000
Options to Purchase Common Stock, Exercisable	1,768,794,000
Options to Purchase Common Stock Options vested and expected to vest	1,787,869,000