Common Stock (Details) - USD ($)		1 Months Ended			12 Months Ended
	Dec. 09, 2020	Dec. 31, 2020	Jun. 30, 2020	Feb. 29, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	May 31, 2021	Mar. 31, 2021	Mar. 16, 2021	Mar. 04, 2020	Jul. 02, 2015
Common Stock And Warrant Liability [Line Items]
Common stock, shares authorized (in shares)		151,875,000	101,250,000		227,812,500	151,875,000		151,875,000
Proceeds from issuance of common stock					$ 0	$ 159,447,000	$ 0
Common stock, shares issued (in shares)		91,502,461			91,645,369	91,502,461
Value of shares of common stock issued		$ 915,000			$ 916,000	$ 915,000
Private Placement
Common Stock And Warrant Liability [Line Items]
Proceeds from direct offering		169,600,000	$ 17,500,000
Fees payable expenses		$ 10,200,000	1,000,000
Other estimated offering expenses payable expense			$ 500,000
Over-Allotment Option [Member]
Common Stock And Warrant Liability [Line Items]
Number of shares sold (in shares)		3,847,826
Common Stock
Common Stock And Warrant Liability [Line Items]
Common stock authorized									$ 100,000,000
Compensation fee (percent of gross proceeds)										3.00%
Sale price per share (in dollars per share)		$ 5.75				$ 5.75
Common Stock | Private Placement
Common Stock And Warrant Liability [Line Items]
Common stock issued (in shares)		29,500,000	14,000,000
Sale price per share (in dollars per share)			$ 1.25
Demand Sales Agreement
Common Stock And Warrant Liability [Line Items]
Proceeds from issuance of common stock					0	$ 8,176,000	0
Demand Sales Agreement | Common Stock
Common Stock And Warrant Liability [Line Items]
Common stock authorized											$ 30,000,000
Compensation fee (percent of gross proceeds)											3.00%
Number of shares sold (in shares)	6,298,648
Proceeds from direct offering	$ 8,300,000
Remaining purchase program amount	21,700,000
Proceeds from issuance of common stock	$ 7,900,000
Aspire Capital Fund, LLC
Common Stock And Warrant Liability [Line Items]
Proceeds from issuance of common stock				$ 3,000,000	0	$ 8,099,000	$ 0
Stock Purchase Program Price, Period				30 months
Common stock, shares issued (in shares)				2,693,965
Additional shares authorized under the plan (in shares)						4,650,000
Value of shares of common stock issued		$ 5,400,000				$ 5,400,000
Aspire Capital Fund, LLC | Common Stock
Common Stock And Warrant Liability [Line Items]
Authorized purchase program amount				$ 30,000,000
Commitment fee (in shares)		646,551				646,551
Sale of stock remaining authorized amount		$ 21,600,000			$ 21,600,000	$ 21,600,000
Stock purchase program, authorized per day (in shares)					150,000
Stock purchase program, authorized per day, after mutual agreement (in shares)					2,000,000
Cowen | Maximum | At-the-market Agreement
Common Stock And Warrant Liability [Line Items]
Planned proceeds from sale of stock (up to)												$ 30,000,000