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Stockholders' Equity - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2018
Jun. 30, 2018
Dec. 31, 2017
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)   123,020,301 107,121,235
Common shares outstanding (in shares)   123,020,301 107,121,235
Proceeds from issuance of common stock, net of offering costs $ 275.9    
Series B Preferred Stock [Member]      
Class of Stock [Line Items]      
Preferred stock, shares issued (in shares)   6,770,393 6,770,393
Preferred stock, shares outstanding (in shares)   6,770,393 6,770,393
Preferred stock dividend percentage   8.00%  
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
Preferred stock dividend percentage   8.00%  
Common Stock [Member]      
Class of Stock [Line Items]      
Issuance of stock (in shares)   15,525,000  
Common Stock [Member] | Follow-on public offering [Member]      
Class of Stock [Line Items]      
Issuance of stock (in shares) 15,525,000    
Price per share of issued stock (in dollars per share) $ 17.77