Stockholder's Equity (Deficit) - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Exercised	(16,850)	(7,150)	(20,350)	(22,675)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life, Exercised at September 30, 2020			7 years	7 years
Montrose Amended & Restated 2017 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-Average Exercise Price per Share Beginning Balance			$ 24	$ 20	$ 20
Weighted-Average Exercise Price per Share Granted			32	27
Weighted-Average Exercise Price per Share Forfeited/cancelled			22	19
Weighted-Average Exercise Price per Share Exercised			14
Weighted-Average Exercise Price per Share Ending Balance	$ 21	$ 24	21	24	24	$ 20
Weighted-Average Exercise Price per Share, Exercised at September 30, 2020	14		14
Weighted-Average Exercise Price per Share Options vested and expected to vest	21		21
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 Beginning Balance			12	10	10
Weighted Average Grant Date Fair Value per Share Granted			12	14
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 12	$ 12	$ 12	$ 12	$ 12	$ 10
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life (in Years) Outstanding			7 years 7 months 2 days	8 years 25 days	7 years 9 months 25 days
Weighted Average Remaining Contract Life, Exercised at September 30, 2020			7 years 3 months 29 days
Weighted Average Remaining Contract Life (in Years) Options vested and expected to vest			9 years 14 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding			$ 4,693	$ 1,151	$ 1,151
Aggregate Intrinsic Value of In-The-Money Options Exercised			7,918
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 7,649	$ 4,693	7,649	$ 4,693	$ 4,693	$ 1,151
Aggregate Intrinsic Value of In-The-Money Options Exercised at September 30, 2020	7,649		7,649
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 7,649		$ 7,649
Montrose Amended & Restated 2017 Stock Incentive Plan | Common Stock
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Outstanding Beginning Balance			617,852	257,762	257,762
Options to Purchase Common Stock Granted			851,877	356,470
Options to Purchase Common Stock Forfeited/cancelled			(14,375)	(17,700)
Options to Purchase Common Stock Exercised			(925)
Options to Purchase Common Stock Outstanding Ending Balance	1,454,429	596,532	1,454,429	596,532	617,852	257,762
Options to Purchase Common Stock, Exercised at September 30, 2020	925		925
Options to Purchase Common Stock Options vested and expected to vest	1,454,429		1,454,429
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, Exercised at September 30, 2020	221,950		221,950		201,600
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-Average Exercise Price per Share Beginning Balance			$ 6	$ 6	$ 6
Weighted-Average Exercise Price per Share Forfeited/cancelled			10	8
Weighted-Average Exercise Price per Share Expired			6	8
Weighted-Average Exercise Price per Share Exercised			8	6
Weighted-Average Exercise Price per Share Ending Balance	$ 6	$ 6	6	6	6	$ 6
Weighted-Average Exercise Price per Share, Exercised at September 30, 2020	6		6
Weighted-Average Exercise Price per Share Options vested and expected to vest	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 Beginning Balance			1	1	1
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 1	$ 1	$ 1	$ 1	$ 1	$ 1
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life (in Years) Outstanding			5 years 7 months 17 days	6 years 7 months 24 days	6 years 4 months 24 days	7 years 3 months 18 days
Weighted Average Remaining Contract Life, Exercised at September 30, 2020			5 years 1 month 20 days
Weighted Average Remaining Contract Life (in Years) Options vested and expected to vest			5 years 2 months 1 day
Aggregate Intrinsic Value of In-The-Money Options Outstanding			$ 46,617	$ 33,290	$ 33,290
Aggregate Intrinsic Value of In-The-Money Options Exercised			354	466
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 33,772	$ 47,381	33,772	$ 47,381	$ 46,617	$ 33,290
Aggregate Intrinsic Value of In-The-Money Options Exercised at September 30, 2020	33,772		33,772
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 33,772		$ 33,772
Montrose Amended and Restated 2013 Stock Option Plan | Common Stock
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Outstanding Beginning Balance			1,855,469	1,900,404	1,900,404
Options to Purchase Common Stock Forfeited/cancelled			(7,500)	(14,500)
Options to Purchase Common Stock Expired			(11,300)	(5,400)
Options to Purchase Common Stock Exercised			(19,175)	(19,010)
Options to Purchase Common Stock Outstanding Ending Balance	1,817,494	1,861,494	1,817,494	1,861,494	1,855,469	1,900,404
Options to Purchase Common Stock, Exercised at September 30, 2020	221,025		221,025
Options to Purchase Common Stock Options vested and expected to vest	1,817,494		1,817,494