| Component: (Network and Table) | |
|---|---|
| Network | 177 - Disclosure - Difference between Federal Income Tax Rate and Effective Income Tax Rate (Detail) (http://www.alleghany.com/taxonomy/role/DisclosureDifferenceBetweenFederalIncomeTaxRateAndEffectiveIncomeTaxRate) |
| Table | Reconciliation of Statutory Federal Tax Rate [Table] |
| Reporting Entity [Axis] | 0000775368 (http://www.sec.gov/CIK) |
| Legal Entity [Axis] | Entity [Domain] |
| 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 |