Stockholder's Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended			12 Months Ended
		Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021		Dec. 31, 2021	Jan. 31, 2021	May 31, 2015
Class Of Stock [Line Items]
Common stock, shares authorized		190,000,000		190,000,000			190,000,000
Common stock, par value		$ 0.000004		$ 0.000004			$ 0.000004
Stock issued on exercise of warrants				67,713	67,713	[1]
Total unrecognized stock compensation expense related to unvested options and restricted stock granted under the Plans		$ 152,500	$ 15,000	$ 152,500	$ 15,000
Unrecognized expense expected to be recognized period				4 years
Restricted shares became fully vested and released as common stock		0	0	25,289	38,929
Shares, granted	[1]			25,289	38,929
Stock compensation expense		$ 11,018	$ 2,365	$ 32,375	$ 6,587
Shares reserved for future issuance		8,619,509	5,992,019	8,619,509	5,992,019
Restricted shares outstanding		2,064,197	286,239	2,064,197	286,239
Shares of vested over period		1 year	1 year	1 year	1 year
Shares RSAs granted		0	0
Supplemental Incentive Plans
Class Of Stock [Line Items]
Shares, granted				95,404	0
Montrose Amended & Restated 2017 Stock Incentive Plan
Class Of Stock [Line Items]
Shares, granted							1,671,391
Share based, description							These RSUs represent the right to receive one share of the Company’s common stock upon vesting. These incentives were designed by the Board, in coordination with the Company’s compensation advisors, to (i) retain selected employees of the Company for a minimum of 5 years, (ii) reward selected employees for the Company’s significant outperformance and stockholder value creation in 2021, and (iii) provide incentives to selected employees of the Company to accelerate value creation for stockholders and other stakeholders over the next five-year period
Stock compensation expense							$ 90,000
Shares reserved for future issuance		7,757,995	3,944,750	7,757,995	3,944,750		135,517	1,185,112
Restricted shares forfeited		0	0	0	0
Shares of vested over period							5 years
Montrose Amended & Restated 2017 Stock Incentive Plan | Restricted Stock
Class Of Stock [Line Items]
Shares RSAs granted		0	0	10,920	19,066
Montrose Amended & Restated 2017 Stock Incentive Plan | Stock Options | Board of Directors
Class Of Stock [Line Items]
Shares of vested over period				4 years
Montrose Amended & Restated 2017 Stock Incentive Plan | Stock Options | Executive Officers
Class Of Stock [Line Items]
Shares of vested over period				3 years
Vesting frequency of period				1 year
Montrose Amended & Restated 2017 Stock Incentive Plan | Stock Options | Second Anniversary | Board of Directors
Class Of Stock [Line Items]
Shares, vesting rights, percentage				50.00%
Montrose Amended & Restated 2017 Stock Incentive Plan | Stock Options | Fourth Anniversary | Board of Directors
Class Of Stock [Line Items]
Shares, vesting rights, percentage				50.00%
Montrose Amended & Restated 2017 Stock Incentive Plan | Performance-Vested RSUs
Class Of Stock [Line Items]
Shares, granted							316,209
Shares, vesting rights, percentage							50.00%
Montrose Amended & Restated 2017 Stock Incentive Plan | RSUs
Class Of Stock [Line Items]
Shares, granted							1,355,182
Shares, vesting rights, percentage							50.00%
Montrose Amended and Restated 2013 Stock Option Plan
Class Of Stock [Line Items]
Shares outstanding from exercised options		1,271,383	1,028,268	1,271,383	1,028,268
Two Thousand Seventeen Plan | Restricted Stock
Class Of Stock [Line Items]
Stock compensation expense			$ 237	$ 17,892	$ 749
Two Thousand Seventeen Plan | Stock Options
Class Of Stock [Line Items]
Stock compensation expense		$ 2,620	$ 2,126	7,518	$ 5,816
Two Thousand Seventeen Plan | Stock Appreciation Rights (SARs)
Class Of Stock [Line Items]
Shares, granted							3,000,000
Stock compensation expense		$ 2,315		$ 6,965
Exercise price							$ 66.79
Shares of vested over period							5 years
Remaining contractual terms				10 years
Weighted average remaining contract life				9 years 2 months 15 days
Fair value vested in period				$ 46,000
Private Offering
Class Of Stock [Line Items]
Warrants to purchase common stock									116,350
Warrant exercise price per share									$ 17.19

[1] Represents the non-cash release of common shares due to the exercise of warrants and the vesting of restricted stock.