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Investments - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Security
Dec. 31, 2011
Investments [Line Items]    
Unrealized losses of one security under closed-block life insurance issuance $ 3,548  
12 Months or Longer, Gross Unrealized Loss 70,164 114,011
Unrealized losses of one security of large diversified financial services company 3,363  
Period of unrealized loss position on large diversified financial services company 66 months  
Unrealized loss position of Alt-A securities greater than twelve months 15,236  
Percentage of unrealized losses on mortgage-backed securities attributable to Alt-A Securities 100.00%  
Number of individual Alt-A securities 16  
Unrealized losses on security secured by IRS facility lease payments 29,838  
Percentage of permitted policy claim 25.00%  
Securities fair value 6,945 7,132
Fair value of securities held by bankruptcy remote trust 201,329 172,880
Corporate obligations [Member]
    
Investments [Line Items]    
12 Months or Longer, Gross Unrealized Loss 8,036  
Minimum [Member]
    
Investments [Line Items]    
Period of unrealized loss position on closed-block life insurance issuance 42 months  
Period of unrealized loss position on security secured by IRS facility lease payments 19 months  
Maximum [Member]
    
Investments [Line Items]    
Period of unrealized loss position on closed-block life insurance issuance 60 months  
Period of unrealized loss position on security secured by IRS facility lease payments 60 months  
Below investment grade securities and non rated securities [Member]
    
Investments [Line Items]    
Fair value of below investment grade securities and non-rated securities 192,190 268,633
Gross unrealized loss 43,934 77,947
Residential mortgage-backed securities [Member]
    
Investments [Line Items]    
12 Months or Longer, Gross Unrealized Loss 15,236 43,321
Asset-backed Securities [Member]
    
Investments [Line Items]    
12 Months or Longer, Gross Unrealized Loss $ 46,503