Investments - Additional Information (Detail)				1 Months Ended		3 Months Ended		9 Months Ended
	Jul. 18, 2019 USD ($)	Sep. 24, 2018 USD ($)	Mar. 15, 2018 USD ($)	Jul. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Sep. 30, 2019 USD ($) Investment	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Investment	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Investments [Line Items]
Proceeds from sale of debt and equity securities						$ 1,100,000,000	$ 900,000,000	$ 4,900,000,000	$ 2,800,000,000
Net realized capital gains						3,957,000	16,230,000	$ 20,753,000	67,197,000
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						$ 3,597,000	3,000	$ 13,617,000	514,000
Percentage of debt securities owned with credit rating below investment grade or not rated						3.50%		3.50%		4.40%
Other invested assets						$ 610,234,000		$ 610,234,000		$ 555,972,000
Equity securities						2,092,267,000		2,092,267,000		3,572,790,000
Available-for-sale Securities						15,456,800,000		15,456,800,000		12,717,800,000
Net investment income						147,829,000	$ 127,329,000	413,623,000	377,728,000
Commercial mortgage loans						706,030,000		706,030,000		676,532,000
Allowance for loan losses on commercial mortgage loans						0		$ 0
Maximum [Member]
Investments [Line Items]
Term of commercial mortgage loans								10 years
Minimum [Member]
Investments [Line Items]
Term of commercial mortgage loans								2 years
Ares | Insurance Segment
Investments [Line Items]
Net realized capital gains			$ 45,700,000						$ 45,700,000
Investment in other invested asset				$ 250,000,000.0
Percentage of equity stake				6.25%
Ares | Insurance Segment | Maximum [Member]
Investments [Line Items]
Investment commitment in investment fund				$ 1,000,000,000.0
Pillar Capital Holdings Limited And Managed Funds
Investments [Line Items]
Other invested assets						203,600,000		$ 203,600,000
Pillar Capital Holdings Limited And Managed Funds | Insurance Segment
Investments [Line Items]
Investment in other invested asset					$ 25,000,000.0
Pillar Capital Holdings Limited And Managed Funds | Reinsurance Segment
Investments [Line Items]
Investment in other invested asset					$ 175,000,000.0
Limited partnership interests in certain subsidiaries of Ares | Insurance Segment
Investments [Line Items]
Net investment income			12,900,000
Put Option | Equity Contract
Investments [Line Items]
Cost of Put Option	$ 38,400,000
Portion of equity portfolio hedged for downside equity market risk	$ 1,000,000,000.0
Derivative, maturity date	Dec. 31, 2019
Derivative, fair value						32,600,000		32,600,000
Decrease in fair value of derivative recorded as a reduction to net realized capital gains						5,800,000		5,800,000
Put Option | Equity Contract | Insurance Segment
Investments [Line Items]
Decrease in fair value of derivative recorded as a reduction to net realized capital gains						$ 5,800,000		$ 5,800,000
Debt Securities
Investments [Line Items]
Number of securities in an unrealized loss position | Investment						580		580
Number of securities in an unrealized loss position for 12 months or more | Investment						295		295
Common Stock
Investments [Line Items]
Equity securities						$ 2,088,200,000		$ 2,088,200,000		$ 3,567,400,000
Common Stock | Ares | Insurance Segment
Investments [Line Items]
Equity securities			208,200,000
Reclassification of converted interests from other invested assets to equity securities		$ 56,900,000
Other invested assets | Limited partnership interests in certain subsidiaries of Ares | Insurance Segment
Investments [Line Items]
Available-for-sale Securities			$ 58,700,000