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Stock-Based Plans and Compensation - Schedule of Restricted Stock Activity (Details) - Montrose 2017 Stock Incentive Plan - Restricted stock units awards - $ / shares	9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, Shares, Beginning Balance	2,617,059	2,468,722
Granted, Shares	1,350,599	354,060
Forfeited/cancelled, Shares	(104,996)	(43,115)
Vested, Shares	(617,957)	(150,641)
Outstanding, Shares, Ending Balance	3,244,705	2,629,026
Weighted Average Grant Date Fair Value per Share Granted	$ 18.02	$ 40.12
Weighted Average Grant Date Fair Value per Share Forfeited/ cancelled	28.44	36.7
Weighted Average Grant Date Fair Value per Share Vested	$ 27.21	$ 37.46

X

- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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