v2.4.0.6
Investments - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2012
Investment
Derivative
Jun. 30, 2011
Jun. 30, 2012
Investment
Derivative
Jun. 30, 2011
Dec. 31, 2011
Investment [Line Items]          
Proceeds from the sale of investments in available-for-sale securities     $ 380,614 $ 738,742  
Number of securities account for gross unrealized loss 150   150    
Number of interest rate swaps 2   2    
Interest rate swaps liability 4,472   4,472    
Fair value of securities sold but not yet purchased 56,978   56,978   55,942
Securities sold under agreements to repurchase, principal amount 231,919   231,919   191,718
Securities sold under agreements to repurchase, interest expense 200 221 335 475  
Collateral pledged in support of the repurchase agreements 253,600   253,600    
Reverse repurchase agreement 57,000   57,000    
Securities received as collateral 57,000   57,000    
Minimum
          
Investment [Line Items]          
Securities sold under agreements to repurchase, interest rates     0.38%    
Maximum
          
Investment [Line Items]          
Securities sold under agreements to repurchase, interest rates     0.47%    
Securities sold under agreements to repurchase, at contract value
          
Investment [Line Items]          
Securities sold under agreements to repurchase, accrued interest 0   0    
Corporate Bonds
          
Investment [Line Items]          
Fair value of securities sold but not yet purchased 56,920   56,920    
Equity securities
          
Investment [Line Items]          
Fair value of securities sold but not yet purchased $ 58   $ 58