Summary of Significant Accounting Policies - Fair Value of Financial Instruments (Details)	9 Months Ended
	Sep. 30, 2020 USD ($) item	Dec. 31, 2019 USD ($)	Dec. 31, 2017	Dec. 31, 2016 USD ($)
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)	4.50%	4.50%
Number of trades | item	0
Convertible Notes
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)	4.50%		4.50%
Principal amount of debt for conversion calculations				$ 1,000
Significant Other Observable Inputs (Level 2) | Convertible Notes | Face Value
Fair value hierarchy for the Company's financial assets measured at fair value
Convertible debt fair value	$ 2,100,000	$ 3,000,000.0
Significant Other Observable Inputs (Level 2) | Convertible Notes | Estimated fair value
Fair value hierarchy for the Company's financial assets measured at fair value
Convertible 4.5% senior notes gross carrying value	2,100,000	2,100,000
Recurring basis
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	168,967,000	163,674,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	168,967,000	163,674,000
Recurring basis | Significant Other Observable Inputs (Level 2)
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	0	0
Recurring basis | Significant Unobservable Inputs (Level 3)
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	$ 0	$ 0