Stockholder's Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2021	Dec. 31, 2022
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
Total unrecognized stock-based compensation expense related to unvested options, restricted stock and stock appreciation rights granted under the Plans		$ 132.8	$ 152.5	$ 132.8	$ 152.5
Unrecognized expense expected to be recognized period				3 years
Shares, granted	[1]			90,116	25,289
Restricted shares outstanding		2,786,961	2,064,197	2,786,961	2,064,197
Montrose 2017 Stock Incentive Plan
Class Of Stock [Line Items]
Shares, granted		90,384		343,971	0
Montrose 2017 Stock Incentive Plan | Restricted Stock
Class Of Stock [Line Items]
Shares RSAs granted		0	0	17,346	10,920
Montrose 2017 Stock Incentive Plan | Stock Options | Board of Directors
Class Of Stock [Line Items]
Shares of vested over period				4 years
Montrose 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 2017 Stock Incentive Plan | Stock Options | On the two-year anniversary of the grant | Board of Directors
Class Of Stock [Line Items]
Shares, vesting rights, percentage				50.00%
Montrose 2017 Stock Incentive Plan | Stock Options | Fourth Anniversary | Board of Directors
Class Of Stock [Line Items]
Shares, vesting rights, percentage				50.00%
Montrose Amended and Restated 2013 Stock Option Plan
Class Of Stock [Line Items]
Shares outstanding from exercised options		1,527,913	1,271,383	1,527,913	1,271,383
Two Thousand Seventeen Plan | Stock Appreciation Rights (SARs)
Class Of Stock [Line Items]
Shares, granted				3,000,000
Exercise price						$ 66.79
Shares of vested over period						5 years
Remaining contractual terms				10 years
Fair value vested in period				$ 46.0
Weighted average remaining contract life				8 years 2 months 15 days
Supplemental Incentive Plans
Class Of Stock [Line Items]
Shares, granted			0	370,349	95,404
Supplemental Incentive Plans | RSUs
Class Of Stock [Line Items]
Shares of vested over period				4 years
Supplemental Incentive Plans | RSUs | First Quater
Class Of Stock [Line Items]
Shares, vesting rights, percentage				33.33%
Supplemental Incentive Plans | RSUs | First Anniversary
Class Of Stock [Line Items]
Shares, vesting rights, percentage				33.33%
Supplemental Incentive Plans | RSUs | Second Anniversary
Class Of Stock [Line Items]
Shares, vesting rights, percentage				33.33%

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