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Stockholders' Equity - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
 1 Months Ended 3 Months Ended 12 Months Ended
Oct. 31, 2017
Aug. 31, 2017
Mar. 31, 2018
Jun. 30, 2017
Dec. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Class of Stock [Line Items]                
Common stock, shares authorized (in shares)           450,000,000 450,000,000  
Common stock, par value (in dollars per share)           $ 0.01 $ 0.01  
Preferred stock, shares authorized (in shares)           50,000,000 50,000,000  
Preferred stock, par value (in dollars per share)           $ 0.01 $ 0.01  
Common stock, shares issued (in shares)           133,853,565 107,121,235  
Common shares outstanding (in shares)           133,853,565 107,121,235  
Share price (in dollars per share)           $ 16.66    
Proceeds from issuance of common stock, net of offering costs     $ 275.9 $ 248.9 $ 177.8      
Series B Preferred Stock [Member]                
Class of Stock [Line Items]                
Preferred stock, par value (in dollars per share) $ 0.01              
Preferred stock, shares issued (in shares)           6,770,393 6,770,393  
Preferred stock, shares outstanding (in shares)           6,770,393 6,770,393  
Stock repurchased (in shares) 1,229,607              
Value of stock repurchased $ 30.8              
Share price (in dollars per share) $ 25.04              
Preferred stock, liquidation value per share (in dollars per share) 25.00              
Preferred stock, accumulated and unpaid dividends (in dollars per share) 0.04              
Series C Preferred Stock [Member]                
Class of Stock [Line Items]                
Preferred stock, shares issued (in shares)           6,900,000 6,900,000  
Preferred stock, shares outstanding (in shares)           6,900,000 6,900,000  
Series A Preferred Stock [Member]                
Class of Stock [Line Items]                
Preferred stock, shares issued (in shares)   3,450,000            
Preferred stock, accumulated and unpaid dividends (in dollars per share)   $ 0.1079            
Preferred stock, redemption price (in dollars per share)   $ 25.00            
Common Stock [Member]                
Class of Stock [Line Items]                
Issuance of stock (in shares)           15,525,000 15,470,000 10,500,000
Exchange of convertible senior notes for common stock (in shares)           10,828,475    
Common Stock [Member] | Follow-on public offering [Member]                
Class of Stock [Line Items]                
Issuance of stock (in shares)     15,525,000 13,800,000 10,500,000      
Price per share of issued stock (in dollars per share)     $ 17.77 $ 18.05 $ 16.97     $ 16.97
QHREAC [Member]                
Class of Stock [Line Items]                
Common stock, par value (in dollars per share) $ 0.01              
Stock repurchased (in shares) 1,670,000              
Value of stock repurchased $ 30.8              
Share price (in dollars per share) $ 18.44