Annual report pursuant to Section 13 and 15(d)

Stockholders' Equity At-the-Market Offering (Details)

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Stockholders' Equity At-the-Market Offering (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Subsidiary, Sale of Stock [Line Items]      
Number of shares authorized to be sold under equity distribution agreement (in shares) 35,000,000    
Number of common shares issued under equity distribution agreement and outstanding as of period-end (in shares) 7,490,235    
Accumulated proceeds from issuance of common shares under equity distribution agreement $ 128.6    
Number of shares of stock issued during period (in shares)     0
Issuance of stock, net of offering costs     $ 0.0
At the Market Offering [Member]      
Subsidiary, Sale of Stock [Line Items]      
Number of shares of stock issued during period (in shares) 0 3,697,300  
Issuance of stock, net of offering costs $ 0.0 $ 51.0