Stockholders' Equity - At-the-market equity and follow on offering programs (Details) - USD ($)	1 Months Ended		9 Months Ended
	Jun. 30, 2020	Apr. 30, 2020	Sep. 30, 2021	May 31, 2020	Jul. 31, 2018
2018 ATM
Offering program
Amount of shares authorized to be offered and sold					$ 75,000,000
Proceeds from issuance of common stock, gross		$ 65,153,000
Proceeds from stock issuance, net of issuance costs		$ 62,976,000
At Market Equity Offering Program, 5.59 per Share
Offering program
Number of shares sold (in shares)		8,938,599
Shares sold, price per share (in dollars per share)		$ 5.59
At Market Equity Offering Program, 7.74 per Share
Offering program
Number of shares sold (in shares)		1,962,000
Shares sold, price per share (in dollars per share)		$ 7.74
2020 ATM
Offering program
Amount of shares authorized to be offered and sold				$ 100,000,000
Number of shares sold (in shares)			2,271,074
Shares sold, price per share (in dollars per share)			$ 14.99
Proceeds from issuance of common stock, gross			$ 34,033,000
Proceeds from stock issuance, net of issuance costs			$ 33,287,000
Public Stock Offering
Offering program
Number of shares sold (in shares)	9,200,000
Shares sold, price per share (in dollars per share)	$ 19.00
Proceeds from stock issuance, net of issuance costs	$ 163,990,000