Stockholder's Equity - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Exercised	(27,396)	(124,455)
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	2,516,272	2,579,566	2,579,566
Options to Purchase Common Stock Granted		233,955
Options to Purchase Common Stock Forfeited/cancelled	(36,105)	(34,619)
Options to Purchase Common Stock Expired	(13,010)
Options to Purchase Common Stock Exercised	(18,746)	(101,279)
Options to Purchase Common Stock Outstanding Ending Balance	2,448,411	2,677,623	2,516,272	2,579,566
Options to Purchase Common Stock Exercisable	1,513,438
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	$ 30.92	$ 30.58	$ 30.58
Weighted-Average Exercise Price per Share Granted		32.16
Weighted-Average Exercise Price per Share Forfeited/cancelled	38.78	35.16
Weighted-Average Exercise Price per Share Expired	44.16
Weighted-Average Exercise Price per Share Exercised	23.26	24.55
Weighted-Average Exercise Price per Share Outstanding Ending Balance	30.8	30.89	30.92	$ 30.58
Weighted-Average Exercise Price per Share Exercisable	31.13
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	15.95	14.96	14.96
Weighted Average Grant Date Fair Value per Share Granted		13.9
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 16	$ 15.47	$ 15.95	$ 14.96
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life (in Years) Outstanding	6 years 9 months 18 days	7 years 9 months 18 days	7 years	7 years 9 months 18 days
Weighted Average Remaining Contract Life (in Years) Exercisable	6 years 4 months 24 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 25,468	$ 21,127	$ 13,825	$ 37,292
Aggregate Intrinsic Value of In-The-Money Options Exercised	200	$ 2,467
Aggregate Intrinsic Value of In-The-Money Options Exercisable	$ 15,380
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	792,191	855,695	855,695
Options to Purchase Common Stock Exercised	(8,650)	(23,176)
Options to Purchase Common Stock Outstanding Ending Balance	783,541	832,519	792,191	855,695
Options to Purchase Common Stock Exercisable	783,541
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	$ 6.4	$ 6.43	$ 6.43
Weighted-Average Exercise Price per Share Exercised	5.95	8.78
Weighted-Average Exercise Price per Share Outstanding Ending Balance	6.41	6.37	6.4	$ 6.43
Weighted-Average Exercise Price per Share Exercisable	6.41
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	2.16	2.1	2.1
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 2.18	$ 2.06	$ 2.16	$ 2.1
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life (in Years) Outstanding	2 years 2 months 12 days	3 years	2 years 4 months 24 days	3 years 3 months 18 days
Weighted Average Remaining Contract Life (in Years) Exercisable	2 years 2 months 12 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 25,669	$ 24,393	$ 20,380	$ 32,478
Aggregate Intrinsic Value of In-The-Money Options Exercised	289	$ 963
Aggregate Intrinsic Value of In-The-Money Options Exercisable	$ 25,669