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177 - Disclosure - Difference between Federal Income Tax Rate and Effective Income Tax Rate (Detail)
(http://www.alleghany.com/taxonomy/role/DisclosureDifferenceBetweenFederalIncomeTaxRateAndEffectiveIncomeTaxRate)
TableReconciliation of Statutory Federal Tax Rate [Table]
Slicers (applies to each fact value in each table cell)
Reconciliation of Statutory Federal Tax Rate [Line Items]Period [Axis]
2012-01-01 - 2012-12-31
2011-01-01 - 2011-12-31
2010-01-01 - 2010-12-31
Federal income tax rate
.350  
.350  
.350  
Foreign tax credit and other adjustments
 
(.009) 
(.017) 
Income subject to dividends-received deduction
(.005) 
(.040) 
(.018) 
Tax-exempt interest
(.089) 
(.057) 
(.043) 
State taxes, net of federal tax benefit
.001  
.004  
.010  
Non-taxable gain on bargain purchase
(.241) 
 
 
Non-deductible transaction costs
.010  
 
 
Other, net
(.002) 
.001  
.002  
Effective tax rate
.024  
 
.249  
 
.284