Investments - Additional Information (Detail) $ in Thousands				1 Months Ended				12 Months Ended
	Mar. 15, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 29, 2017 USD ($)	Sep. 24, 2018 USD ($)	Apr. 15, 2016 USD ($)	Jul. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2018 USD ($) Investment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jul. 31, 2014
Net Investment Income [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								$ 1,328
Other than temporary impairment losses									$ 16,871	$ 45,165
Percentage of debt securities owned with credit rating below investment grade or not rated		5.30%						4.40%	5.30%
Statutory deposit, investments at fair value								$ 2,000,000
Commercial mortgage loans		$ 658,364						676,532	$ 658,364
Other invested assets		743,358						555,972	743,358
Proceeds From Sale Of Debt And Equity Securities								5,300,000	8,300,000	7,000,000
Net realized capital gains								(3,241)	107,222	63,205
Net Investment Income								500,534	451,016	438,455
Equity Securities, FV-NI								3,572,790
Available-for-sale Securities		17,399,000						12,717,800	17,399,000
Available-for-sale Securities, Equity Securities		4,099,467							4,099,467
Jazwares, LLC
Net Investment Income [Line Items]
Pre-existing equity ownership					30.00%
Stranded Oil Resources Corporation
Net Investment Income [Line Items]
Loss on sale of oil field			$ (4,800)						(4,800)
Write-down of assets								35,400		98,800
Write-down of assets								35,400		98,800
Jazwares, LLC
Net Investment Income [Line Items]
Gain on settlement of contingent consideration liabilities		20,900
Gain on acquisition					$ 13,200					13,200
Reinsurance Segment
Net Investment Income [Line Items]
Write-down of assets									7,900
Alleghany Capital Corporation
Net Investment Income [Line Items]
Gain on settlement of contingent consideration liabilities									20,900
Alleghany Capital Corporation | Jazwares, LLC
Net Investment Income [Line Items]
Gain on settlement of contingent consideration liabilities		20,900
Ownership interest acquired					50.00%						30.00%
Pacific Comp
Net Investment Income [Line Items]
Gain on disposition of business									8,400
Pillar Capital Holdings Limited And Managed Funds
Net Investment Income [Line Items]
Other invested assets								193,600
Pillar Capital Holdings Limited And Managed Funds | Reinsurance Segment
Net Investment Income [Line Items]
Investment in other invested asset							$ 175,000
Pillar Capital Holdings Limited And Managed Funds | Insurance Segment
Net Investment Income [Line Items]
Investment in other invested asset							$ 25,000
Limited partnership interests in certain subsidiaries of Ares Management LLC | Insurance Segment
Net Investment Income [Line Items]
Net Investment Income	$ 12,900
Ares | Insurance Segment
Net Investment Income [Line Items]
Investment in other invested asset						$ 250,000
Percentage of equity stake						6.25%
Net realized capital gains	45,700							45,700
Equity Securities
Net Investment Income [Line Items]
Other than temporary impairment losses									15,100	23,300
Debt Securities
Net Investment Income [Line Items]
Other than temporary impairment losses								$ 1,300
Other than temporary impairment losses									1,800	$ 21,900
Number of securities in an unrealized loss position | Investment								2,226
Number of securities in an unrealized loss position for 12 months or more | Investment								850
Number of securities in an unrealized loss position | Investment								2,226
Other invested assets | Limited partnership interests in certain subsidiaries of Ares Management LLC | Insurance Segment
Net Investment Income [Line Items]
Available-for-sale Securities	58,700
Common Stock
Net Investment Income [Line Items]
Equity Securities, FV-NI								$ 3,567,400
Available-for-sale Securities, Equity Securities		$ 4,094,500							$ 4,094,500
Common Stock | Ares | Insurance Segment
Net Investment Income [Line Items]
Reclassification of converted interests from other invested assets to equity securities				$ 56,900
Equity Securities, FV-NI	$ 208,200
Maximum [Member]
Net Investment Income [Line Items]
Term of commercial mortgage loans								10 years
Percentage of principal amounts of loans to the property's appraised value								0.667%
Maximum [Member] | Ares | Insurance Segment
Net Investment Income [Line Items]
Investment commitment in investment fund						$ 1,000,000
Minimum [Member]
Net Investment Income [Line Items]
Term of commercial mortgage loans								2 years
Minimum [Member] | Debt Securities
Net Investment Income [Line Items]
Percentage of unrealized loss to cost where a security would be evaluated for other than temporarily impairment								20.00%