Investments - Additional Information (Detail) $ in Thousands			1 Months Ended		3 Months Ended		6 Months Ended
	Sep. 24, 2018 USD ($)	Mar. 15, 2018 USD ($)	Jul. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Jun. 30, 2019 USD ($) Investment	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) Investment	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Investments [Line Items]
Securities impairment test description							Debt securities in an unrealized loss position are evaluated for OTTI if they meet any of the following criteria: (i) they are trading at a discount of at least 20 percent to amortized cost for an extended period of time (nine consecutive months or more); (ii) there has been a negative credit or news event with respect to the issuer that could indicate the existence of an OTTI; 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 an OTTI 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 an OTTI 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 OTTI 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.
Other than temporary impairment losses						$ 0	$ 10,020	$ 511
Percentage of debt securities owned with credit rating below investment grade or not rated					3.50%		3.50%		4.40%
Commercial mortgage loans					$ 691,437		$ 691,437		$ 676,532
Allowance for loan losses on commercial mortgage loans					0		0
Other invested assets					579,018		579,018		555,972
Proceeds From Sale Of Debt And Equity Securities					1,300,000	800,000	3,800,000	1,900,000
Net realized capital gains					12,354	6,462	16,796	50,967
Net Investment Income					142,745	$ 126,273	265,794	250,399
Available-for-sale Securities					15,184,400		15,184,400		12,717,800
Equity securities					2,292,803		2,292,803		3,572,790
Pillar Capital Holdings Limited And Managed Funds
Investments [Line Items]
Other invested assets					$ 198,000		$ 198,000
Pillar Capital Holdings Limited And Managed Funds | Reinsurance Segment
Investments [Line Items]
Investment in other invested asset				$ 175,000
Pillar Capital Holdings Limited And Managed Funds | Insurance Segment
Investments [Line Items]
Investment in other invested asset				$ 25,000
Limited partnership interests in certain subsidiaries of Ares Management LLC | Insurance Segment
Investments [Line Items]
Net Investment Income		$ 12,900
Ares | Insurance Segment
Investments [Line Items]
Investment in other invested asset			$ 250,000
Percentage of equity stake			6.25%
Net realized capital gains		45,700						$ 45,700
Debt Securities
Investments [Line Items]
Number of securities in an unrealized loss position for 12 months or more | Investment					502		502
Number of securities in an unrealized loss position | Investment					785		785
Other invested assets | Limited partnership interests in certain subsidiaries of Ares Management LLC | Insurance Segment
Investments [Line Items]
Available-for-sale Securities		58,700
Common Stock
Investments [Line Items]
Equity securities					$ 2,288,700		$ 2,288,700		$ 3,567,400
Common Stock | Ares | Insurance Segment
Investments [Line Items]
Reclassification of converted interests from other invested assets to equity securities	$ 56,900
Equity securities		$ 208,200
Maximum [Member]
Investments [Line Items]
Term of commercial mortgage loans							10 years
Percentage of principal amounts of commercial mortgage loans to the property's appraised value					66.70%		66.70%
Maximum [Member] | Ares | Insurance Segment
Investments [Line Items]
Investment commitment in investment fund			$ 1,000,000
Minimum [Member]
Investments [Line Items]
Term of commercial mortgage loans							2 years
Minimum [Member] | Debt Securities
Investments [Line Items]
Percentage of unrealized loss to cost where a security would be evaluated for other than temporarily impairment					20.00%		20.00%