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Fair Value Measurements - Information about Valuation Inputs for Fixed Income Securities Classified as Level 3 (Detail)
 9 Months Ended 12 Months Ended
Sep. 30, 2012
Dec. 31, 2011
U.S. Agency Obligations [Member]
    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Coupon rate 0.00% 6.91%
Maturity 0 years 1 year 3 months 29 days
Yield 0.00% 2.13%
Corporate Obligations [Member]
    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Coupon rate 0.35% 0.60%
Maturity 21 years 4 months 21 days 21 years 5 months 9 days
Yield 6.36% 7.20%
Collateralized debt obligations [Member]
    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Coupon rate 1.06% 0.86%
Maturity 1 year 6 months 1 year 6 months 18 days
Yield 8.80% 11.79%
Other commercial asset-backed securities [Member]
    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Coupon rate 0.82% 1.41%
Maturity 8 years 7 days 3 years
Yield 7.50% 4.50%
European ABS transactions [Member]
    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Coupon rate 1.82% 1.56%
Maturity 12 years 7 months 2 days 11 years 11 months 27 days
Yield 4.39% 3.90%