Stockholders Deficit - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
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	24
Weighted-Average Exercise Price per Share Forfeitured/cancelled	20	24
Weighted-Average Exercise Price per Share Ending Balance	26	23	24	$ 20
Weighted-Average Exercise Price per Share Options vested and expected to vest	26
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	13
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 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	8 years 10 days		7 years 9 months 25 days
Weighted Average Remaining Contract Life (in Years) Options vested and expected to vest	8 years 6 months 25 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 4,696	$ 1,151	$ 1,151
Aggregate Intrinsic Value of In-The-Money Options Outstanding	4,651	$ 2,136	$ 4,696	$ 1,151
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 4,651
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	160,712	308,990
Options to Purchase Common Stock Forfeitured/cancelled	(5,500)	(14,800)
Options to Purchase Common Stock Outstanding Ending Balance	773,064	551,952	617,852	257,762
Options to Purchase Common Stock Options vested and expected to vest	773,064
Montrose Amended and Restated 2013 Stock Option 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	$ 6	$ 6	$ 6
Weighted-Average Exercise Price per Share Forfeitured/cancelled	6	14
Weighted-Average Exercise Price per Share Expired	6	8
Weighted-Average Exercise Price per Share Exercised	6	5
Weighted-Average Exercise Price per Share Ending Balance	6	6	6	$ 6
Weighted-Average Exercise Price per Share Options vested and expected to vest	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
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life (in Years) Outstanding	5 years 10 months 28 days	6 years 10 months 13 days	6 years 4 months 24 days	7 years 3 months 18 days
Weighted Average Remaining Contract Life (in Years) Options vested and expected to vest	5 years 5 months 4 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 46,617	$ 33,290	$ 33,290
Aggregate Intrinsic Value of In-The-Money Options Exercised	89	283
Aggregate Intrinsic Value of In-The-Money Options Outstanding	46,276	$ 38,235	$ 46,617	$ 33,290
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 46,276
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 Forfeitured/cancelled	(1,250)	(12,600)
Options to Purchase Common Stock Expired	(8,550)	(4,475)
Options to Purchase Common Stock Exercised	(3,500)	(11,385)
Options to Purchase Common Stock Outstanding Ending Balance	1,842,169	1,871,944	1,855,469	1,900,404
Options to Purchase Common Stock Options vested and expected to vest	1,842,169