Stock‑Based Compensation - Restricted common stock award activity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Restricted stock awards
Shares
Unvested restricted common stock at beginning of period		209,742
Vested		(87,986)
Unvested restricted common stock at end of period	121,756	121,756	209,742
Weighted Average Grant Date Fair Value
Unvested restricted common stock at beginning of period		$ 0.0443
Vested		0.0400
Unvested restricted common stock at end of period	$ 0.0500	$ 0.0500	$ 0.0443
Restricted stock awards \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Liability for unvested awards	$ 0.1	$ 0.1	$ 0.1
Weighted Average Grant Date Fair Value
Total fair value of restricted common stock vested	$ 0.1	$ 0.1
Service‑based restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		4 years
Issued in period		0	0

Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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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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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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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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