Investments - Additional Information (Detail)			1 Months Ended		3 Months Ended
		Mar. 15, 2018 USD ($)	Jul. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Mar. 31, 2018 USD ($) Investment	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Investment [Line Items]
Proceeds from sales of debt and equity securities					$ 1,200,000,000	$ 2,700,000,000
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					$ 511,000	3,217,000
Percentage of debt securities owned with credit rating below investment grade or not rated					4.40%		5.30%
Other invested assets					$ 601,555,000		$ 743,358,000
Fair value of common units					4,545,899,000		4,099,467,000
Net realized capital gains					44,505,000	59,651,000
Fair value of investment					12,677,900,000		17,399,000,000
Net investment income					124,126,000	115,538,000
Pillar Capital Holdings Limited And Managed Funds | Pillar Holdings and Funds
Investment [Line Items]
Other invested assets					217,900,000
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
Ares | Insurance Segment
Investment [Line Items]
Investment in other invested asset			$ 250,000,000
Percentage of equity stake			6.25%
Net realized capital gains		$ 45,700,000			45,700,000
Net investment income		12,900,000
Commercial Mortgage Loan Portfolio
Investment [Line Items]
Commercial mortgage loans					663,933,000		658,364,000
Allowance for loan losses on commercial mortgage loans					0
Equity Securities
Investment [Line Items]
Other than temporary impairment losses						3,100,000
Equity Securities | Common Stock
Investment [Line Items]
Fair value of common units					4,540,700,000		4,094,500,000
Equity Securities | Ares | Insurance Segment | Common Stock
Investment [Line Items]
Fair value of common units		208,200,000
Debt Securities
Investment [Line Items]
Other than temporary impairment losses					$ 500,000	$ 100,000
Number of securities in an unrealized loss position | Investment					1,820
Number of securities in an unrealized loss position for 12 months or more | Investment					390
Other invested assets
Investment [Line Items]
Fair value of investment	[1]				$ 60,300,000		$ 7,500,000
Other invested assets | Ares | Insurance Segment
Investment [Line Items]
Fair value of investment		$ 58,700,000
Minimum | Commercial Mortgage Loan Portfolio
Investment [Line Items]
Term of commercial mortgage loans					2 years
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 | Insurance Segment
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 amounts of loans to the property's appraised value					0.667%

[1] Includes partnership and non-marketable equity investments accounted for at fair value, and excludes investments accounted for using the equity method.