Investments - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) Investment	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) Investment	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Investments [Line Items]
Proceeds from sale of equity securities	$ 100,000	$ 800,000	$ 350,913	$ 1,391,193
Proceeds from sale of debt securities	1,200,000	400,000	$ 2,366,976	2,380,250
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. If Alleghany intends to sell, or it is more likely than not that Alleghany will sell, a debt security before recovery of its amortized cost basis, the total amount of the unrealized loss position is recognized as a credit loss in earnings. To the extent that a debt security that is in an unrealized loss position is not impaired based on the preceding, Alleghany will consider a debt security to be impaired when it believes it to be probable that Alleghany will not be able to collect the entire amortized cost basis. For debt securities in an unrealized loss position as of the end of each quarter, Alleghany develops a best estimate of the present value of expected cash flows. If the results of the cash flow analysis indicate that Alleghany will not recover the full amount of its amortized cost basis in the debt security, Alleghany records a credit loss in earnings equal to the difference between the present value of expected cash flows and the amortized cost basis of the debt security. If applicable, the difference between the total unrealized loss position on the debt security and the total loss recognized in earnings is the non-credit related portion, which is recorded as a component of other comprehensive income. In developing the cash flow analyses for debt securities, Alleghany considers various factors for the different categories of debt securities. For municipal bonds, Alleghany takes into account the taxing power of the issuer, source of revenue, credit risk and enhancements and pre-refunding. For mortgage and asset-backed securities, Alleghany discounts its best estimate of future cash flows at an effective rate equal to the original effective yield of the security or, in the case of floating rate securities, at the current coupon. Alleghany’s models include assumptions about prepayment speeds, default and delinquency rates, underlying collateral (if any), credit ratings, credit enhancements and other observable market data. For corporate bonds, Alleghany reviews business prospects, credit ratings and available information from asset managers and rating agencies for individual securities.
Changes in allowance for credit losses (reduction) for available for sale securities	(246)	$ (17,021)	$ (2,218)	$ 14,354
Other invested assets	538,857		538,857		$ 465,153
Commercial mortgage loans	543,106		543,106		670,239
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, 2021.
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	$ 167,700		$ 167,700		$ 173,300
Debt Securities
Investments [Line Items]
Number of securities in an unrealized loss position | Investment	951		951
Number of securities in an unrealized loss position for 12 months or more | Investment	109		109