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
 9 Months Ended
Oct. 10, 2017
Sep. 30, 2017
October 31, 2017 | Senior Common Stock    
Dividends Payable [Line Items]    
Distributions Payable To Holders Of Records Months   October
November 30, 2017 | Senior Common Stock    
Dividends Payable [Line Items]    
Distributions Payable To Holders Of Records Months   November
December 29, 2017 | Senior Common Stock    
Dividends Payable [Line Items]    
Distributions Payable To Holders Of Records Months   December
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 | October 31, 2017    
Dividends Payable [Line Items]    
Record Date Oct. 20, 2017  
Payment Date Oct. 31, 2017  
Subsequent event | October 31, 2017 | Series A Preferred Stock    
Dividends Payable [Line Items]    
Distribution per Share (in dollars per share) $ 0.1614583  
Subsequent event | October 31, 2017 | Series B Preferred Stock    
Dividends Payable [Line Items]    
Distribution per Share (in dollars per share) 0.15625  
Subsequent event | October 31, 2017 | Series D Preferred Stock    
Dividends Payable [Line Items]    
Distribution per Share (in dollars per share) $ 0.1458333  
Subsequent event | October 31, 2017 | Senior Common Stock    
Dividends Payable [Line Items]    
Payment Date Nov. 07, 2017  
Distribution per Share (in dollars per share) $ 0.0875  
Subsequent event | October 31, 2017 | Common Stock    
Dividends Payable [Line Items]    
Distribution per Share (in dollars per share) $ 0.125  
Subsequent event | November 30, 2017    
Dividends Payable [Line Items]    
Record Date Nov. 20, 2017  
Payment Date Nov. 30, 2017  
Subsequent event | November 30, 2017 | Series A Preferred Stock    
Dividends Payable [Line Items]    
Distribution per Share (in dollars per share) $ 0.1614583  
Subsequent event | November 30, 2017 | Series B Preferred Stock    
Dividends Payable [Line Items]    
Distribution per Share (in dollars per share) 0.15625  
Subsequent event | November 30, 2017 | Series D Preferred Stock    
Dividends Payable [Line Items]    
Distribution per Share (in dollars per share) $ 0.1458333  
Subsequent event | November 30, 2017 | Senior Common Stock    
Dividends Payable [Line Items]    
Payment Date Dec. 07, 2017  
Distribution per Share (in dollars per share) $ 0.0875  
Subsequent event | November 30, 2017 | Common Stock    
Dividends Payable [Line Items]    
Distribution per Share (in dollars per share) $ 0.125  
Subsequent event | December 29, 2017    
Dividends Payable [Line Items]    
Record Date Dec. 19, 2017  
Payment Date Dec. 29, 2017  
Subsequent event | December 29, 2017 | Series A Preferred Stock    
Dividends Payable [Line Items]    
Distribution per Share (in dollars per share) $ 0.1614583  
Subsequent event | December 29, 2017 | Series B Preferred Stock    
Dividends Payable [Line Items]    
Distribution per Share (in dollars per share) 0.15625  
Subsequent event | December 29, 2017 | Series D Preferred Stock    
Dividends Payable [Line Items]    
Distribution per Share (in dollars per share) $ 0.1458333  
Subsequent event | December 29, 2017 | Senior Common Stock    
Dividends Payable [Line Items]    
Payment Date Jan. 08, 2018  
Distribution per Share (in dollars per share) $ 0.0875  
Subsequent event | December 29, 2017 | Common Stock    
Dividends Payable [Line Items]    
Distribution per Share (in dollars per share) $ 0.125