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
Jul. 09, 2018
Jun. 29, 2018
Jun. 20, 2018
Jun. 07, 2018
May 31, 2018
May 22, 2018
May 07, 2018
Apr. 30, 2018
Apr. 20, 2018
Jun. 30, 2018
May 31, 2018
Apr. 30, 2018
Apr. 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                 Apr. 20, 2018        
Payment Date               Apr. 30, 2018          
Subsequent event | April 2018 | Series A Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)               $ 0.1614583       $ 0.1614583  
Subsequent event | April 2018 | Series B Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)               0.15625       0.15625  
Subsequent event | April 2018 | Series D Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)               0.1458333       $ 0.1458333  
Subsequent event | April 2018 | Senior Common Stock                          
Dividends Payable [Line Items]                          
Payable to the Holders of Record During the Month of:                       April  
Payment Date             May 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       $ 0.125  
Forecast | May 2018                          
Dividends Payable [Line Items]                          
Record Date           May 22, 2018              
Payment Date         May 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:                     May    
Payment Date       Jun. 07, 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     Jun. 20, 2018                    
Payment Date   Jun. 29, 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:                   June      
Payment Date Jul. 09, 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