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Fair Value Measurements - Information about Valuation Inputs for Fixed Income Securities Classified as Level 3 (Detail)
 3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Corporate obligations [Member]
    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Coupon rate 0.345% 0.345%
Maturity 20 years 10 months 21 days 21 years 1 month 21 days
Yield 6.03% 6.08%
Collateralized debt obligations [Member]
    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Coupon rate 0.85% 1.05%
Maturity 1 year 2 months 27 days 1 year 6 months 4 days
Yield 3.85% 5.92%
Other commercial asset-backed [Member]
    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Coupon rate 0.69% 0.71%
Maturity 7 years 7 months 13 days 7 years 10 months 10 days
Yield 7.50% 7.50%
European ABS transactions [Member]
    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Coupon rate 1.59% 1.64%
Maturity 18 years 3 months 4 days 12 years 4 months 2 days
Yield 3.98% 4.02%