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Debt Securities - Overall Statistics for Company's AAA-rated CMBS Investments Calculated on Weighted Average Basis (Detail)
 3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Investments Debt And Equity Securities [Abstract]    
Coupon 5.60% 5.60%
Yield 4.60% 4.10%
Weighted Average Life 1 year 9 months 18 days 1 year 9 months 18 days