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Preferred Stock - Additional Information (Detail) (USD $)
9 Months Ended 9 Months Ended
Sep. 30, 2012
Dec. 31, 2011
Sep. 30, 2012
Series F
Jan. 23, 2012
Series F
Sep. 30, 2012
Series G
Apr. 20, 2012
Series G
Class of Stock [Line Items]            
Number of preferred stock, shares authorized 50,000,000 50,000,000        
Par value of Preference shares issued and outstanding $ 1.00 $ 1.00        
Preferred Stock, Shares Issued     44,000   43,400  
Liquidation preference per share       $ 25,000   $ 25,000
Preferred stock, dividend rate, fixed percentage     6.50%   6.00%  
Preferred stock, dividend rate, variable percentage     4.468%   4.86125%  
Maximum period to redeem preferred stock     90 days   90 days  
Preferred stock, dividend rate, period     Three-month LIBOR plus   Three-month LIBOR plus