Summary of Significant Accounting Policies (Details)	1 Months Ended		3 Months Ended	9 Months Ended
	Jul. 31, 2020	Feb. 29, 2020	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) Institution shares	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Other Income
Expected service period		18 months
Cash and cash equivalents and restricted cash reported within the condensed balance sheets
Cash and cash equivalents				$ 133,072,000		$ 157,954,000
Restricted cash				1,652,000		1,282,000
Cash, cash equivalents and restricted cash shown in the consolidated statements of cash flows			$ 167,426,000	$ 134,724,000	$ 167,426,000	159,236,000	$ 114,504,000
Convertible Preferred Stock Warrants
Warrants to purchase (in shares) | shares				49,997
Reclassification of redeemable convertible preferred stock warrant liability to additional paid-in capital			$ 133,000	$ 500,000	$ 503,000
Concentration of credit risk
Number of financial institutions holding cash and cash and cash equivalents | Institution				2
Collaboration and License Agreement, Percentage Of Profits And Costs Sharing	50.00%
Research Cost Sharing Provision Receivable				$ 110,000
Prepaid expenses and other current assets
Other Income
Receivables under service agreements				1,000		237,000
Other current liabilities and other non-current liabilities
Other Income
Contract liabilities						$ 0
Other current liabilities
Other Income
Contract liabilities				$ 1,200,000