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Quarterly Results for 2013 and 2012 - Additional Information (Detail) (USD $)
In Millions, except Per Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]          
Increase in revenue during period $ 2.4        
Decrease in operating expenses 14.3        
Increase in net investment income 1.1        
Increase decrease in net unrealized loss 2.0        
Decrease in losses and loss adjustment expenses during period 5.7        
Increase decrease in general and administrative expenses during period $ 8.6        
Effective tax rate 40.00% 52.90% 41.30% 42.30% 40.40%
Diluted earnings per share $ 0.08