Stockholder's Equity - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Exercised	(49,490)	(28,111)	(217,483)	(62,293)
Montrose 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			2,579,566	2,036,729	2,036,729
Options to Purchase Common Stock Granted			253,980	664,784
Options to Purchase Common Stock Forfeited/cancelled			(119,330)	(60,167)
Options to Purchase Common Stock Expired			(2,950)
Options to Purchase Common Stock Exercised			(173,554)	(26,258)
Options to Purchase Common Stock Outstanding Ending Balance	2,537,712	2,615,088	2,537,712	2,615,088	2,579,566	2,036,729
Options to Purchase Common Stock Exercisable	1,351,428		1,351,428
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			$ 31	$ 26	$ 26
Weighted-Average Exercise Price per Share Granted			32	44
Weighted-Average Exercise Price per Share Forfeited/cancelled			36	36
Weighted-Average Exercise Price per Share Expired			29
Weighted-Average Exercise Price per Share Exercised			24	22
Weighted-Average Exercise Price per Share Outstanding Ending Balance	$ 31	$ 30	31	30	31	$ 26
Weighted-Average Exercise Price per Share Exercisable	29		29
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			15	14	14
Weighted Average Grant Date Fair Value per Share Granted			14	16
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 16	$ 15	$ 16	$ 15	$ 15	$ 14
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life (in Years) Outstanding			7 years 3 months 25 days	8 years 14 days	7 years 9 months 3 days	8 years 3 months 18 days
Weighted Average Remaining Contract Life (in Years) Exercisable			6 years 8 months 26 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 10,427	$ 18,784	$ 10,427	$ 18,784	$ 37,295	$ 91,030
Aggregate Intrinsic Value of In-The-Money Options Exercised			3,701	$ 671
Aggregate Intrinsic Value of In-The-Money Options Exercisable	$ 6,037		$ 6,037
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			855,695	897,674	897,674
Options to Purchase Common Stock Expired			(800)	(125)
Options to Purchase Common Stock Exercised			(43,929)	(36,035)
Options to Purchase Common Stock Outstanding Ending Balance	810,966	861,514	810,966	861,514	855,695	897,674
Options to Purchase Common Stock Exercisable	810,966		810,966
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	6
Weighted-Average Exercise Price per Share Exercised			7	6
Weighted-Average Exercise Price per Share Outstanding Ending Balance	$ 6	$ 6	6	6	6	$ 6
Weighted-Average Exercise Price per Share Exercisable	6		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			2	2	2
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 2	$ 2	$ 2	$ 2	$ 2	$ 2
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life (in Years) Outstanding			2 years 6 months 29 days	3 years 7 months 9 days	3 years 3 months 21 days	4 years 4 months 13 days
Weighted Average Remaining Contract Life (in Years) Exercisable			2 years 6 months 29 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 18,554	$ 23,448	$ 18,554	$ 23,448	$ 32,478	$ 57,529
Aggregate Intrinsic Value of In-The-Money Options Exercised			1,557	$ 1,425
Aggregate Intrinsic Value of In-The-Money Options Exercisable	$ 18,554		$ 18,554