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Derivatives - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 6 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Jun. 30, 2012
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Series F Preferred Stock [Member]
Dec. 31, 2011
Series F Preferred Stock [Member]
Oct. 31, 2008
Series F Preferred Stock [Member]
Derivative [Line Items]                
Minimum fixed coupon rate of preferred stock           2.375%    
Number of years of U.S treasury rate one           30 years    
Number of years of U.S treasury rate two           10 years    
LIBOR rate           3 month LIBOR    
New coupon rate of preferred stock           5.065%    
Notional value of interest rate swap agreement               $ 50,000
Treasury Rate           5.2175%    
Unrealized mark to market loss on interest rate protection agreement 29 1,372   334 1,560      
Settlement payments of mark to market gains or losses 326 106   865 294      
Amortization to be reclassified from OCI into Income       2,406        
Outstanding settlement payment due on Series F Agreement           $ 326 $ 280  
Number of years of long dated treasuries     30 years