Disclosure regarding factors used to determine that the impairment of securities not categorized as either held-to-maturity securities or trading securities (hence equal to available for sale securities) where cost exceeds fair value is not an other than temporary impairment (OTTI). This item contains disclosure of the nature of the available-for-sale investments determined to be temporarily impaired.

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Investments - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022 USD ($) Investment	Jun. 30, 2021 USD ($)	Jun. 30, 2022 USD ($) Investment	Jun. 30, 2021 USD ($)	Dec. 31, 2021 USD ($)
Investments [Line Items]
Proceeds from sale of equity securities	$ 100,000	$ 100,000	$ 327,976	$ 350,913
Proceeds from sale of debt securities	200,000	1,200,000	$ 1,001,030	2,366,976
Securities impairment test description			Alleghany continually monitors the difference between amortized cost and the estimated fair value of its debt investments. The analysis of a security’s decline in value is performed in its functional currency. Debt securities in an unrealized loss position are evaluated for credit losses if they meet any of the following criteria: (i) they are trading at a discount of at least 20 percent to amortized cost and have a credit rating below investment grade or are not rated; (ii) there has been a negative credit or news event with respect to the issuer that could indicate the existence of a credit loss; or (iii) Alleghany intends to sell, or it is more likely than not that Alleghany will sell, the debt security before recovery of its amortized cost basis.
Change in allowance for credit losses on available for sale securities	7,595	$ (246)	$ 8,177	$ (2,218)
Other invested assets	538,302		538,302		$ 557,800
Commercial mortgage loans	451,404		451,404		475,860
Commercial mortgage loan investments in default or in arrears	0		$ 0
Commercial mortgage loan principal amount description			The principal amounts of the loans were no more than approximately two-thirds of the property’s appraised value at the time the loans were made and the estimated fair value of underlying collateral was approximately double that of the commercial mortgage loan portfolio carrying value as of June 30, 2022.
Minimum
Investments [Line Items]
Term of commercial mortgage loans			2 years
Maximum
Investments [Line Items]
Term of commercial mortgage loans			10 years
Pillar Capital Holdings Limited And Managed Funds
Investments [Line Items]
Other invested assets	$ 158,800		$ 158,800		$ 156,400
Debt Securities [Member]
Investments [Line Items]
Number of securities in an unrealized loss position \| Investment	4,159		4,159
Number of securities in an unrealized loss position for 12 months or more \| Investment	522		522