Defined Contribution Plan	9 Months Ended
Sep. 30, 2020
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	21. DEFINED CONTRIBUTION PLAN On January 1, 2014, the Company established the Montrose Environmental Group 401(k) Savings Plan. As of September 30, 2020 and December 31, 2019, plan participants may defer up to 85.0% of their eligible wages for the year, up to the Internal Revenue Service dollar limit and catch up contribution allowed by law. Up to May 22, 2020, the Company provided employer matching contributions equal to 100.0% of the first 3.0% of the participant’s compensation and 50.0% of the participant’s elective deferrals that exceed 3.0% but do not exceed 4.0% of the participant’s compensation. Subsequent to May 22, 2020, the Company temporarily ceased making employer contributions. Employer contributions for the three and nine months ended September 30, 2020 were zero and $1.2 million, respectively, and $0.5 million and $1.9 million for the three and nine months ended September 30, 2019, respectively, and are included within selling, general, and administrative expenses on the condensed consolidated statements of operations.

Defined Contribution Plan

9 Months Ended

Sep. 30, 2020

Compensation And Retirement Disclosure [Abstract]

21. DEFINED CONTRIBUTION PLAN

On January 1, 2014, the Company established the Montrose Environmental Group 401(k) Savings Plan. As of September 30, 2020 and December 31, 2019, plan participants may defer up to 85.0% of their eligible wages for the year, up to the Internal Revenue Service dollar limit and catch up contribution allowed by law. Up to May 22, 2020, the Company provided employer matching contributions equal to 100.0% of the first 3.0% of the participant’s compensation and 50.0% of the participant’s elective deferrals that exceed 3.0% but do not exceed 4.0% of the participant’s compensation. Subsequent to May 22, 2020, the Company temporarily ceased making employer contributions. Employer contributions for the three and nine months ended September 30, 2020 were zero and $1.2 million, respectively, and $0.5 million and $1.9 million for the three and nine months ended September 30, 2019, respectively, and are included within selling, general, and administrative expenses on the condensed consolidated statements of operations.