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1080 - 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)
Reporting Entity [Axis]0000775368 (http://www.sec.gov/CIK)
Legal Entity [Axis]Entity [Domain]
Reconciliation of Statutory Federal Tax Rate [Line Items]Period [Axis]
2016-01-01 - 2016-12-31
2015-01-01 - 2015-12-31
2014-01-01 - 2014-12-31
Federal income tax rate
35.00%  
35.00%  
35.00%  
Foreign tax credits
(0.60%) 
(0.40%) 
 
Income subject to dividends-received deduction
(1.60%) 
(1.60%) 
(1.30%) 
Tax-exempt interest
(6.50%) 
(6.80%) 
(6.50%) 
State taxes, net of federal tax benefit
0.60%  
0.20%  
0.40%  
Prior period adjustment
2.40%  
(0.20%) 
0.10%  
Other, net
(0.40%) 
(0.40%) 
(0.70%) 
Effective tax rate
28.90%  
 
25.80%  
 
27.00%