Amounts of Gross Realized Capital Gains and Gross Realized Capital Losses (Detail) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020		Jun. 30, 2021	Jun. 30, 2020
Investments Debt And Equity Securities [Abstract]
Gross realized capital gains	$ 21,300	$ 28,500	[1]	$ 42,300	$ 87,400	[2]
Gross realized capital losses	(8,400)	(66,600)	[3]	(16,400)	(112,500)	[4]
Net realized capital gains	$ 12,942	$ (38,100)		$ 25,873	$ (25,074)

[1] Gross realized capital gains in the three months ended June 30, 2020 include 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”).

[2] Gross realized capital gains in the six months ended June 30, 2020 include (i) the $16.3 million Wilbert Remeasurement Gain; and (ii) 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.

[3] Gross realized capital losses in the three months ended June 30, 2020 include impairment charges of $44.5 million from write-downs of SORC oil field assets prior to SORC’s December 31, 2020 sale.

[4] Gross realized capital losses in the six months ended June 30, 2020 include (i) impairment charges of $74.4 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 to Notes to Consolidated Financial Statements set forth in Part II, Item 8, “Financial Statements and Supplementary Data” of the 2020 Form 10-K).