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
Jan. 07, 2019
Dec. 31, 2018
Dec. 20, 2018
Dec. 07, 2018
Nov. 30, 2018
Nov. 20, 2018
Nov. 07, 2018
Oct. 31, 2018
Oct. 19, 2018
Dec. 31, 2018
Nov. 30, 2018
Oct. 31, 2018
Oct. 09, 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                 Oct. 19, 2018        
Payment Date               Oct. 31, 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:                       October  
Payment Date             Nov. 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           Nov. 20, 2018              
Payment Date         Nov. 30, 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:                     November    
Payment Date       Dec. 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     Dec. 20, 2018                    
Payment Date   Dec. 31, 2018                      
Forecast | June 2018 | Series A Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)   $ 0.1614583               $ 0.1614583      
Forecast | June 2018 | Series B Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)   0.15625               0.15625      
Forecast | June 2018 | Series D Preferred Stock                          
Dividends Payable [Line Items]                          
Distribution per share (in dollars per share)   0.1458333               $ 0.1458333      
Forecast | June 2018 | Senior Common Stock                          
Dividends Payable [Line Items]                          
Payable to the Holders of Record During the Month of:                   December      
Payment Date Jan. 07, 2019                        
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               $ 0.125