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Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities by Contractual Maturity (Total Held-to-maturity Securities) (Detail) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Investment [Line Items]    
Total, Amortized Cost Basis $ 565,874 $ 583,287
Total, Fair Value 594,531 627,349
Carrying Value [Member]
    
Investment [Line Items]    
Less than one year, Amortized Cost Basis 66,726  
One to five years, Amortized Cost Basis 143,021  
Five to ten years, Amortized Cost Basis 132,814  
More than ten years, Amortized Cost Basis 141,335  
Total, Amortized Cost Basis 565,874  
Fair Value [Member]
    
Investment [Line Items]    
Less than one year, Fair Value 67,210  
One to five years, Fair Value 150,147  
Five to ten years, Fair Value 143,055  
More than ten years, Fair Value 145,696  
Total, Fair Value 594,531 627,349
U.S. agency-based mortgage-backed securities [Member]
    
Investment [Line Items]    
Total, Amortized Cost Basis 26,732 32,984
Total, Fair Value 28,911 36,042
U.S. agency-based mortgage-backed securities [Member] | Carrying Value [Member]
    
Investment [Line Items]    
Held-to-maturity, Amortized Cost 26,732  
U.S. agency-based mortgage-backed securities [Member] | Fair Value [Member]
    
Investment [Line Items]    
Held-to-maturity, Fair Value 28,911  
Commercial mortgage-backed securities [Member]
    
Investment [Line Items]    
Total, Amortized Cost Basis 51,502 51,529
Total, Fair Value 55,732 57,962
Commercial mortgage-backed securities [Member] | Carrying Value [Member]
    
Investment [Line Items]    
Held-to-maturity, Amortized Cost 51,502  
Commercial mortgage-backed securities [Member] | Fair Value [Member]
    
Investment [Line Items]    
Held-to-maturity, Fair Value 55,732  
Asset-backed securities [Member]
    
Investment [Line Items]    
Total, Amortized Cost Basis 3,744 4,229
Total, Fair Value 3,780 4,245
Asset-backed securities [Member] | Carrying Value [Member]
    
Investment [Line Items]    
Held-to-maturity, Amortized Cost 3,744  
Asset-backed securities [Member] | Fair Value [Member]
    
Investment [Line Items]    
Held-to-maturity, Fair Value $ 3,780