Investments - Additional Information (Detail) $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 01, 2020 USD ($)	Dec. 31, 2012 USD ($)	Sep. 30, 2020 USD ($) Investment	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) Investment	Sep. 30, 2019 USD ($)	Mar. 31, 2020	Dec. 31, 2019 USD ($)
Investments [Line Items]
Proceeds from sale of equity securities			$ 100,000	$ 300,000	$ 1,442,025	$ 2,261,863
Proceeds from sale of debt securities			400,000	800,000	2,906,720	2,649,257
Realized loss on early redemption of debt					(7,100)
Realized gain on reduction of contingent consideration liabilities					$ 5,000
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			(3,454)		$ 10,900
Change in allowance for credit losses on available for sale securities				$ 3,597		$ 13,617
Other invested assets			445,828		445,828			$ 573,605
Commercial mortgage loans			691,881		691,881			686,206
Commercial mortgage loan investments in default or in arrears			0		$ 0
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 Fund
Investments [Line Items]
Other invested assets			$ 170,500		$ 170,500			$ 205,500
Reinsurance Segment | Pillar Capital Holdings Limited And Managed Fund
Investments [Line Items]
Investment in other invested asset		$ 175,000
Insurance Segment | Pillar Capital Holdings Limited And Managed Fund
Investments [Line Items]
Investment in other invested asset		$ 25,000
Debt Securities
Investments [Line Items]
Number of securities in an unrealized loss position | Investment			843		843
Number of securities in an unrealized loss position for 12 months or more | Investment			122		122
Wilbert Funeral Services, Inc
Investments [Line Items]
Gain on acquisition of equity ownership	$ 16,300
Equity interest percentage acquired	55.00%
Equity interest percentage							45.00%
Concord | Alleghany Capital Corporation
Investments [Line Items]
Gain on partial settlement and remeasurement of fair value of outstanding contingent consideration liabilities			$ 13,800		$ 13,800
Stranded Oil Resources Corporation
Investments [Line Items]
Write down of oil field assets			$ 1,600		76,000
Precision Cutting Technologies, Inc
Investments [Line Items]
Realized gain on reduction of contingent consideration liabilities					$ 5,000