Annual report pursuant to Section 13 and 15(d)

Subsequent Events - Series B Preferred Stock Offering (Details)

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Subsequent Events - Series B Preferred Stock Offering (Details) - USD ($)
 2 Months Ended 12 Months Ended
Jan. 10, 2018
Feb. 20, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Subsequent Event [Line Items]          
Proceeds from issuance of preferred stock     $ 0 $ 28,750,000 $ 0
Stock issuance costs     1,977,000 $ 272,000 $ 1,482,000
Preferred Stock | Subsequent Event          
Subsequent Event [Line Items]          
Maximum amount of securities allowed to be sold   6,500,000      
Series B Preferred Stock | Preferred Stock          
Subsequent Event [Line Items]          
Stock issuance costs     $ 165,000    
Series B Preferred Stock | Preferred Stock | Subsequent Event          
Subsequent Event [Line Items]          
Maximum amount of securities allowed to be sold 6,000,000        
Preferred stock, dividend rate (as percent) 6.00%        
Sale of stock (in dollars per share) $ 25        
Value of securities allowed for issuance $ 150,000,000 $ 162,500,000      
Proceeds from issuance of preferred stock $ 131,300,000        
Series B Preferred Stock | Preferred Stock | Over-Allotment Option | Subsequent Event          
Subsequent Event [Line Items]          
Stock Issued During Period, Shares, Dividend Reinvestment Plan 500,000