Investments - Additional Information (Detail)		1 Months Ended			12 Months Ended
	Apr. 15, 2016 USD ($)	Apr. 30, 2014 USD ($)	Jul. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2016 USD ($) Investment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Apr. 14, 2016	Oct. 15, 2014 USD ($)	May 31, 2014
Investment [Line Items]
Proceeds from sales of AFS securities					$ 7,000,000,000	$ 9,600,000,000	$ 6,600,000,000
Pre-tax net realized capital gains (loss)					$ 63,205,000	213,897,000	247,058,000
Securities impairment test description					Management’s assessment of equity securities initially involves an evaluation of all securities that are in an unrealized loss position, regardless of the duration or severity of the loss, as of the applicable balance sheet date. Such initial review consists primarily of assessing whether: (i) there has been a negative credit or news event with respect to the issuer that could indicate the existence of an OTTI; and (ii) Alleghany has the ability and intent to hold an equity security for a period of time sufficient to allow for an anticipated recovery (generally considered to be 12 months from the balance sheet date). To the extent that an equity security in an unrealized loss position is not impaired based on the initial review described above, Alleghany then further evaluates such equity security and deems it to be other than temporarily impaired if it has been in an unrealized loss position for 12 months or more or if its unrealized loss position is greater than 50 percent of its cost, absent compelling evidence to the contrary.Alleghany then evaluates those equity securities where the unrealized loss is at least 20 percent of cost as of the balance sheet date or that have been in an unrealized loss position continuously for six months or more preceding the balance sheet date. This evaluation takes into account quantitative and qualitative factors in determining whether such securities are other than temporarily impaired including: (i) market valuation metrics associated with the equity security (such as dividend yield and price-to-earnings ratio); (ii) current views on the equity security, as expressed by either Alleghany’s internal stock analysts and/or by third-party stock analysts or rating agencies; and (iii) credit or news events associated with a specific issuer, such as negative news releases and rating agency downgrades with respect to the issuer of the equity security. 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					$ 45,165,000	$ 133,868,000	36,294,000
Number of debt and equity securities in an unrealized loss position | Investment					1,317
Percentage of debt securities owned with credit rating below investment grade or not rated					5.10%	3.60%
Statutory deposit, investments at fair value					$ 1,700,000,000
Other invested assets					645,245,000	$ 676,811,000
Stranded Oil Resources Corporation
Investment [Line Items]
Pre-tax net realized capital gains (loss)					(98,800,000)
2015 Notes | Transatlantic Holdings Incorporated
Investment [Line Items]
Loss on early extinguishment of debt							(9,400,000)
Senior notes, face value repurchased							$ 300,000,000		$ 300,000,000
Senior notes, interest rate							5.75%
Senior notes, maturity date							Dec. 14, 2015
Pillar Capital Holdings Limited And Managed Funds | Pillar Holdings and Funds
Investment [Line Items]
Other invested assets					233,700,000
Pillar Capital Holdings Limited And Managed Funds | Pillar Holdings and Funds | Reinsurance Segment
Investment [Line Items]
Investment in other invested asset				$ 175,000,000
Pillar Capital Holdings Limited And Managed Funds | Pillar Holdings and Funds | Insurance Segment
Investment [Line Items]
Investment in other invested asset				$ 25,000,000
ORX
Investment [Line Items]
Pre-tax net realized capital gains (loss)						(25,800,000)
Ares
Investment [Line Items]
Other invested assets					$ 224,000,000
Investment in other invested asset			$ 250,000,000
Percentage of equity stake			6.25%
Potential ownership interest, upon conversion of limited partner equity interests into common stock										5.90%
Conversion of Investment Interest Description					These interests may be converted at any time at Alleghany's discretion.
Jazwares, LLC
Investment [Line Items]
Ownership interest acquired	50.00%
Gain on acquisition	$ 13,200,000				$ 13,200,000
Commercial Mortgage Loan Portfolio
Investment [Line Items]
Commercial mortgage loans					594,878,000	177,947,000
Allowance for loan losses on commercial mortgage loans					0
Jazwares, LLC
Investment [Line Items]
Ownership interest percentage								30.00%
Equity Securities
Investment [Line Items]
Other than temporary impairment losses					23,300,000	115,600,000	$ 28,700,000
Debt Securities
Investment [Line Items]
Other than temporary impairment losses					$ 21,900,000	$ 18,300,000	$ 7,600,000
Number of securities in an unrealized loss position for 12 months or more | Investment					138
Debt Securities | U.S. Government obligations
Investment [Line Items]
Pre-tax net realized capital gains (loss)		$ 34,000,000
Minimum | Commercial Mortgage Loan Portfolio
Investment [Line Items]
Term of commercial mortgage loans					2 years
Minimum | Equity Securities
Investment [Line Items]
Percentage of unrealized loss to cost where a security would be deemed to be other than temporarily impaired, absent compelling evidence to the contrary					50.00%
Percentage of unrealized loss to cost where a security would be evaluated for other than temporarily impairment					20.00%
Minimum | Debt Securities
Investment [Line Items]
Percentage of unrealized loss to cost where a security would be evaluated for other than temporarily impairment					20.00%
Maximum | Ares
Investment [Line Items]
Investment commitment in investment fund			$ 1,000,000,000
Maximum | Commercial Mortgage Loan Portfolio
Investment [Line Items]
Term of commercial mortgage loans					10 years
Percentage of principal amount of loans to the property's appraised value					66.67%