Investments - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2010	Jun. 30, 2011 Investment	Jun. 30, 2010
Schedule of Available-for-sale Securities [Line Items]
Proceed from sale of available-for-sale securities		$ 541,941	$ 1,078,492
Equity instrument, impairment test description		To the extent that an equity security in an unrealized loss position is not impaired based on the initial review described above, Alleghany then further evaluates such equity security and deems it to be other-than-temporarily impaired if its decline in fair value has existed for twelve months or more or if its decline in fair value from its cost is greater than 50 percent, absent compelling evidence to the contrary. Alleghany then evaluates all remaining equity securities that are in an unrealized loss position the cost of which: (i) exceeds their fair value by 20 percent or more as of the balance sheet date; or (ii) has exceeded their fair value continuously for six (6) months or more preceding the balance sheet date.
Other-than-temporary impairment losses recognized in earnings	5,703		6,780
Number of debt and equity securities in an unrealized loss position		147
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairment losses recognized in earnings			6,500
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairment losses recognized in earnings			$ 300
Number of debt and equity securities in an unrealized loss position		15