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Investments - Additional Information (Detail) (USD $)
6 Months Ended 12 Months Ended
Jun. 30, 2013
Securities
Dec. 31, 2012
Securities
Schedule of Investments [Line Items]    
Percentage of which the company held security of any issuer 10.00% 10.00%
Number of fixed maturity securities classified as available for sale 190 46
Fair value of fixed maturity securities classified as available for sale $ 271,100,000 $ 72,200,000
Unrealized loss of fixed maturity securities classified as available for sale 16,300,000 825,168
Percentage decrease in value of individual equity security to assume other than temporary decline in value 20.00%  
Time period for which individual security should be in unrealized position of twenty percentage or more to assume it as other than temporary decline More than six months  
Percentage of ownership interest 48.20%  
Equity securities [Member]
   
Schedule of Investments [Line Items]    
Number of fixed maturity securities classified as available for sale 7  
General obligation bonds [Member] | Obligations of states and political subdivisions [Member]
   
Schedule of Investments [Line Items]    
Aggregate fair value of bond held 336,600,000 358,500,000
Amortized cost of bond held 328,900,000 332,400,000
Special revenue bond [Member]
   
Schedule of Investments [Line Items]    
Aggregate fair value of bond held 104,600,000 101,000,000
Amortized cost of bond held $ 105,300,000 $ 93,900,000
Education Bond [Member]
   
Schedule of Investments [Line Items]    
Percentage of investments in special revenue bonds 57.00% 54.00%
Water And Sewer Utility Bonds [Member]
   
Schedule of Investments [Line Items]    
Percentage of investments in special revenue bonds 21.00% 19.00%