Investments (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) item	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) item	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) item
Schedule of Investments [Line Items]
Cash, cash equivalents & restricted cash, Amortized Cost	$ 24,314		$ 24,314		$ 67,782
Total cash, cash equivalents, restricted cash and investments, Gross Unrealized Gains	28		28		56
Total cash, cash equivalents, restricted cash and investments, Gross Unrealized Losses	(2)		(2)		$ (3)
Total cash, cash equivalents, restricted cash and investments, Fair Value	103,400		103,400
Realized gains or losses on investments	$ 0	$ 0	$ 0	$ 0
Number of investments in unrealized loss position | item	2		2		1
Number of investments in unrealized loss position for more than 12 months | item	0		0		0
Unrealized loss on fair value	$ 9,800		$ 9,800		$ 6,000
Amortized Cost
Schedule of Investments [Line Items]
Total cash, cash equivalents, restricted cash and investments, Amortized Cost	103,631		103,631		147,409
Fair Value
Schedule of Investments [Line Items]
Total cash, cash equivalents, restricted cash and investments, Fair Value	103,657		$ 103,657		$ 147,462
Corporate bonds and commercial paper
Schedule of Investments [Line Items]
Maturity period, investments			1 year		1 year
Corporate bonds and commercial paper | Minimum
Schedule of Investments [Line Items]
Maturity period, investments					1 year
Corporate bonds and commercial paper | Maximum
Schedule of Investments [Line Items]
Maturity period, investments					5 years
Corporate bonds and commercial paper | Amortized Cost
Schedule of Investments [Line Items]
Due within 1 year, Amortized Cost	79,076		$ 79,076		$ 73,389
Due between 1 and 5 years, Amortized Cost					5,998
Corporate bonds and commercial paper | Fair Value
Schedule of Investments [Line Items]
Due within 1 year, Fair Value	79,102		79,102		73,444
Due between 1 and 5 years, Fair Value					5,995
Corporate bonds and commercial paper due within 1 year
Schedule of Investments [Line Items]
Gross Unrealized Gains	28		28		55
Gross Unrealized Losses	(2)		(2)
Corporate bonds and commercial paper due between 1 and 5 years
Schedule of Investments [Line Items]
Gross Unrealized Losses					(3)
Investments
Schedule of Investments [Line Items]
Gross Unrealized Gains	28		28		55
Gross Unrealized Losses	(2)		(2)		(3)
Investments | Amortized Cost
Schedule of Investments [Line Items]
Investments, Amortized Cost	79,076		79,076		79,387
Investments | Fair Value
Schedule of Investments [Line Items]
Investments, Fair Value	79,102		79,102		79,439
Cash and money market accounts | Amortized Cost
Schedule of Investments [Line Items]
Cash, cash equivalents & restricted cash, Amortized Cost	24,555		24,555		63,024
Cash and money market accounts | Fair Value
Schedule of Investments [Line Items]
Cash, cash equivalents & restricted cash, Fair Value	24,555		24,555		63,024
Corporate bonds, agency bonds and commercial paper
Schedule of Investments [Line Items]
Gross Unrealized Gains					$ 1
Corporate bonds, agency bonds and commercial paper | Maximum
Schedule of Investments [Line Items]
Original maturity period, cash and cash equivalents					90 days
Corporate bonds, agency bonds and commercial paper | Amortized Cost
Schedule of Investments [Line Items]
Cash, cash equivalents & restricted cash, Amortized Cost					$ 4,998
Corporate bonds, agency bonds and commercial paper | Fair Value
Schedule of Investments [Line Items]
Cash, cash equivalents & restricted cash, Fair Value					4,999
Cash and cash equivalents
Schedule of Investments [Line Items]
Gross Unrealized Gains					1
Cash and cash equivalents | Amortized Cost
Schedule of Investments [Line Items]
Cash, cash equivalents & restricted cash, Amortized Cost	24,555		24,555		68,022
Cash and cash equivalents | Fair Value
Schedule of Investments [Line Items]
Cash, cash equivalents & restricted cash, Fair Value	$ 24,555		$ 24,555		$ 68,023