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Stockholders' Equity - Narrative (Details) - USD ($)
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Aug. 20, 2018
Mar. 31, 2019
Sep. 30, 2019
Mar. 31, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Mar. 31, 2018
Conversion of Stock [Line Items]                  
Common stock, outstanding (shares)     89,458,340     89,458,340   76,080,625  
Preferred stock, authorized (shares)     16,670,000     16,670,000   16,670,000  
Common stock dividend rate (usd per share)   $ 2.13   $ 2.13          
Common stock, monthly dividend rate (usd per share)       $ 0.1775          
Public Offering                  
Conversion of Stock [Line Items]                  
Number of shares issued (shares) 4,600,000                
Gross proceeds from sale of stock $ 95,000,000.0                
Sale of stock (usd per share) $ 20.65                
Offering transaction costs $ 3,800,000                
Additional offering transaction costs $ 300,000                
Underwriters                  
Conversion of Stock [Line Items]                  
Number of shares issued (shares) 600,000                
Agent | At-the-Market Program                  
Conversion of Stock [Line Items]                  
Number of shares issued (shares)     5,596,452 7,759,322 164,927 13,555,774 164,927    
Gross proceeds from sale of stock     $ 109,900,000 $ 152,800,000 $ 3,500,000 $ 262,600,000 $ 3,500,000    
Commissions paid     1,600,000 1,500,000 35,140,000 3,100,000 35,140,000    
Payments of stock offering costs     $ 200,000 800,000 $ 300,000 $ 1,000,000.0 $ 300,000    
Aggregate common stock offering price       $ 175,000,000.0          
Redeemable Preferred Stock                  
Conversion of Stock [Line Items]                  
Preferred stock, authorized (shares)     13,409,650     13,409,650   13,409,650  
Preferred stock, issued (shares)     6,682,448     6,682,448   5,416,890  
Dividend payout (usd per share)           $ 0.453125      
Preferred stock, dividend rate           7.25%   7.25%  
Preferred stock, liquidation preference (usd per share)     $ 25.00     $ 25.00   $ 25  
Redeemable Preferred Stock | Maximum                  
Conversion of Stock [Line Items]                  
Periods for dividend payment           30 days      
Redeemable Preferred Stock | Minimum                  
Conversion of Stock [Line Items]                  
Periods for dividend payment           10 days      
Redeemable Preferred Stock | Agent | At-the-Market Program                  
Conversion of Stock [Line Items]                  
Number of shares issued (shares)     724,600   3,225 1,265,558 7,240    
Gross proceeds from sale of stock     $ 18,500,000   $ 81,009,000 $ 32,300,000      
Commissions paid     300,000   1,215,000 500,000 $ 2,724,000    
Payments of stock offering costs     $ 100,000   $ 100,000 $ 200,000 400,000    
Aggregate preferred stock offering price                 $ 200,000,000.0
Net proceeds             $ 200,000