Quarterly report pursuant to Section 13 or 15(d)

Subsequent Events - Monthly Distributions Declared by Company's Board of Directors (Detail)

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Subsequent Events - Monthly Distributions Declared by Company's Board of Directors (Detail) - $ / shares
 1 Months Ended
Oct. 05, 2018
Sep. 28, 2018
Sep. 19, 2018
Sep. 10, 2018
Aug. 31, 2018
Aug. 21, 2018
Aug. 07, 2018
Jul. 31, 2018
Jul. 20, 2018
Sep. 30, 2018
Aug. 31, 2018
Jul. 30, 2018
Jul. 10, 2018
Subsequent event | Series A Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)                         $ 0.4843749
Subsequent event | Series B Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)                         0.46875
Subsequent event | Series D Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)                         0.4374999
Subsequent event | Senior Common Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)                         0.2625
Subsequent event | Common Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)                         $ 0.375
Subsequent event | April 2018                          
Dividends Payable [Line Items]                          
Record Date                 Jul. 20, 2018        
Payment Date               Jul. 31, 2018          
Subsequent event | April 2018 | Series A Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)               $ 0.1614583          
Subsequent event | April 2018 | Series B Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)               0.15625          
Subsequent event | April 2018 | Series D Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)               0.1458333          
Subsequent event | April 2018 | Senior Common Stock                          
Dividends Payable [Line Items]                          
Payable to the Holders of Record During the Month of:                       July  
Payment Date             Aug. 07, 2018            
Distribution per share (in dollars per share)             $ 0.0875            
Subsequent event | April 2018 | Common Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)               $ 0.125          
Forecast | May 2018                          
Dividends Payable [Line Items]                          
Record Date           Aug. 21, 2018              
Payment Date         Aug. 31, 2018                
Forecast | May 2018 | Series A Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)         $ 0.1614583           $ 0.1614583    
Forecast | May 2018 | Series B Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)         0.15625           0.15625    
Forecast | May 2018 | Series D Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)         0.1458333           $ 0.1458333    
Forecast | May 2018 | Senior Common Stock                          
Dividends Payable [Line Items]                          
Payable to the Holders of Record During the Month of:                     August    
Payment Date       Sep. 10, 2018                  
Distribution per share (in dollars per share)       $ 0.0875                  
Forecast | May 2018 | Common Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)         $ 0.125           $ 0.125    
Forecast | June 2018                          
Dividends Payable [Line Items]                          
Record Date     Sep. 19, 2018                    
Payment Date   Sep. 28, 2018                      
Forecast | June 2018 | Series A Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)   $ 0.1614583                      
Forecast | June 2018 | Series B Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)   0.15625                      
Forecast | June 2018 | Series D Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)   0.1458333                      
Forecast | June 2018 | Senior Common Stock                          
Dividends Payable [Line Items]                          
Payable to the Holders of Record During the Month of:                   September      
Payment Date Oct. 05, 2018                        
Distribution per share (in dollars per share) $ 0.0875                        
Forecast | June 2018 | Common Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)   $ 0.125