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Investments - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2013
Security
Mar. 31, 2012
Dec. 31, 2012
Investments [Line Items]      
Unrealized losses of one security under closed-block life insurance issuance $ 3,431    
12 Months or Longer, Gross Unrealized Loss 61,709   70,164
Unrealized losses of one security of large diversified financial services company 2,375    
Period of unrealized loss position on large diversified financial services company 69 months    
Period of unrealized loss position on security secured by IRS facility lease payments 63 months    
Number of individual Alt-A securities 15    
Unrealized losses on security secured by IRS facility lease payments 28,912    
Investment realized losses 46,060 392  
Securities fair value 6,929   6,945
Fair value of securities held by bankruptcy remote trust 218,735   201,329
Other investments (543)    
Corporate obligations [Member]
      
Investments [Line Items]      
12 Months or Longer, Gross Unrealized Loss 6,868    
National Century Financial Enterprises, Inc. [Member]
      
Investments [Line Items]      
Investment realized losses 150,201    
Investment in asset backed notes 174,500    
Cash recoveries 39,978    
Minimum [Member]
      
Investments [Line Items]      
Period of unrealized loss position on closed-block life insurance issuance 45 months    
Maximum [Member]
      
Investments [Line Items]      
Period of unrealized loss position on closed-block life insurance issuance 63 months    
Below investment grade securities and non-rated securities [Member]
      
Investments [Line Items]      
Fair value of below investment grade securities and non-rated securities 143,145   192,190
Gross unrealized loss 38,994   43,934
Residential mortgage-backed securities [Member]
      
Investments [Line Items]      
12 Months or Longer, Gross Unrealized Loss 10,328   15,236
Asset-backed Securities [Member]
      
Investments [Line Items]      
12 Months or Longer, Gross Unrealized Loss $ 44,426