Amortized Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Dec. 31, 2021		Dec. 31, 2020		Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		$ 15,727,143		$ 14,898,522
Debt securities, gross unrealized gains		433,800		762,200
Debt securities, gross unrealized losses		(98,800)		(39,600)
Debt securities, allowance for credit losses		(502)	[1]	(2,579)	[1]	$ 0
Debt securities, fair value		16,061,560		15,618,470
Amortized Cost or Cost		16,869,400		15,612,700
Gross Unrealized Gains		433,800		762,200
Gross Unrealized Losses		(98,800)		(39,600)
Allowance for Credit Losses	[1]	(500)		(2,600)
Fair Value		17,203,900		16,332,700
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,039,700		1,317,600
Debt securities, gross unrealized gains		27,900		44,000
Debt securities, gross unrealized losses		(16,900)		(1,500)
Debt securities, allowance for credit losses	[1]	0		0
Debt securities, fair value		2,050,700		1,360,100
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,412,700		2,489,100
Debt securities, gross unrealized gains		127,300		168,300
Debt securities, gross unrealized losses		(4,100)		(600)
Debt securities, allowance for credit losses	[1]	0		0
Debt securities, fair value		2,535,900		2,656,800
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		850,800		846,600
Debt securities, gross unrealized gains		12,000		33,400
Debt securities, gross unrealized losses		(7,900)		(500)
Debt securities, allowance for credit losses	[1]	0		0
Debt securities, fair value		854,900		879,500
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		3,336,900		3,262,000
Debt securities, gross unrealized gains		159,200		289,700
Debt securities, gross unrealized losses		(18,300)		(3,500)
Debt securities, allowance for credit losses	[1]	(500)		(2,500)
Debt securities, fair value		3,477,300		3,545,700
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,216,600		1,268,300
Debt securities, gross unrealized gains		19,900		55,600
Debt securities, gross unrealized losses		(9,600)		(700)
Debt securities, allowance for credit losses	[1]	0		(100)
Debt securities, fair value		1,226,900		1,323,100
RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,993,600		2,533,600
Debt securities, gross unrealized gains		37,200		78,300
Debt securities, gross unrealized losses		(21,800)		(500)
Debt securities, allowance for credit losses	[1]	0		0
Debt securities, fair value	[2]	2,009,000		2,611,400
CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		879,800		852,600
Debt securities, gross unrealized gains		28,300		45,300
Debt securities, gross unrealized losses		(2,200)		(7,600)
Debt securities, allowance for credit losses	[1]	0		0
Debt securities, fair value		905,900		890,300
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	[3]	2,997,000		2,328,700
Debt securities, gross unrealized gains	[3]	22,000		47,600
Debt securities, gross unrealized losses	[3]	(18,000)		(24,700)
Debt securities, allowance for credit losses	[1],[3]	0		0
Debt securities, fair value	[4]	3,001,000		2,351,600
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		1,142,300		714,200
Gross Unrealized Gains		0		0
Gross Unrealized Losses		0		0
Allowance for Credit Losses	[1]	0		0
Fair Value		$ 1,142,300		$ 714,200

[1] See Note 1(r) for additional information regarding Alleghany’s adoption of new credit loss accounting guidance.

[2] Primarily includes government agency pass-through securities guaranteed by a government agency or government sponsored enterprise, among other types of RMBS.

[3] Includes $1,320.4 million and $878.4 million of collateralized loan obligations as of December 31, 2021 and 2020, respectively.

[4] Includes $1,320.4 million and $878.4 million of collateralized loan obligations as of December 31, 2021 and 2020, respectively.