Schedule Carrying Amount of Mortgage Loans by Maturity (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2012
Jun. 30, 2012
Dec. 31, 2011
Mortgage Loans on Real Estate [Line Items]      
Mortgages loans Carrying Amount $ 563,214 $ 563,214 $ 604,772
Percentage of Total Carrying Amount 100.00% 100.00% 100.00%
Due in one year or less
     
Mortgage Loans on Real Estate [Line Items]      
Mortgages loans Carrying Amount 335   2,356
Due after one year through five years
     
Mortgage Loans on Real Estate [Line Items]      
Mortgages loans Carrying Amount 174,437   153,822
Percentage of Total Carrying Amount 31.00%   25.00%
Due after five years through ten years
     
Mortgage Loans on Real Estate [Line Items]      
Mortgages loans Carrying Amount 226,483   255,615
Percentage of Total Carrying Amount 40.00%   42.00%
Due after ten years
     
Mortgage Loans on Real Estate [Line Items]      
Mortgages loans Carrying Amount $ 161,959   $ 192,979
Percentage of Total Carrying Amount 29.00%   33.00%