Amounts of Gross Realized Capital Gains and Gross Realized Capital Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Gross realized capital gains	$ 134,600	[1]	$ 62,400		$ 86,900	[2]
Gross realized capital losses	(131,500)	[3]	(68,900)	[4]	(90,100)	[5]
Net realized capital gains	$ 3,098		$ (6,551)		$ (3,241)

[1] Gross realized capital gains in 2020 include (i) a $15.0 million realized gain on a partial settlement and remeasurement of fair value of certain outstanding contingent consideration liabilities by Alleghany Capital in connection with its 2018 acquisition of Concord (the “Concord Settlement Gain”); (ii) a gain of $16.3 million on April 1, 2020 in connection with Alleghany Capital’s acquisition of an additional approximately 55 percent of Wilbert that it did not previously own, and the remeasurement of its pre-existing approximately 45 percent equity ownership to estimated fair value (the “Wilbert Remeasurement Gain”); and (iii) a $5.0 million realized gain from a reduction of certain contingent consideration liabilities at the PCT-level in connection with its acquisition of a provider of high-performance solid carbide end mills in June 2019.

[2] Gross realized capital gains in 2018 include a $45.7 million gain on AIHL’s conversion of its limited partnership interests in certain subsidiaries of Ares Management LLC (“Ares”) into Ares common units in 2018.

[3] Gross realized capital losses in 2020 include (i) impairment charges of $76.0 million from write-downs of SORC oil field assets prior to SORC’s December 31, 2020 sale; and (ii) a $7.1 million realized loss as a result of an early redemption of debt (see Note 8 for further information on this early redemption).

[4] Gross realized capital losses in 2019 include $38.4 million from a derivative.  Specifically, on July 18, 2019, AIHL purchased an exchange-traded equity derivative index put option (the “Put Option”) for $38.4 million to hedge the downside equity market risk on approximately $1.0 billion of Alleghany’s equity portfolio. The Put Option did not qualify for hedge accounting. The Put Option expired worthless on December 31, 2019, and the resulting $38.4 million decline in value of the Put Option was recorded as a gross realized capital loss. Gross realized capital losses in 2019 also includes a $13.6 million loss from the December 2019 sale of a privately held investment accounted for under the equity method.

[5] Gross realized capital losses in 2018 include a $35.4 million capital loss due to an impairment charge from the write-down of certain SORC assets arising from a decline in energy prices as of December 31, 2018