Amortized Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Dec. 31, 2020		Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		$ 14,898,522		$ 13,798,576
Debt securities, gross unrealized gains		762,200		445,800
Debt securities, gross unrealized losses		(39,600)		(32,800)
Debt securities, allowance for credit losses		(2,579)	[1]	0
Debt securities, fair value		15,618,470		14,211,745
Amortized Cost or Cost		15,612,700		14,713,500
Gross Unrealized Gains		762,200		445,800
Gross Unrealized Losses		(39,600)		(32,800)
Allowance for Credit Losses	[1]	(2,600)
Fair Value		16,332,700		15,126,500
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,317,600		1,198,500
Debt securities, gross unrealized gains		44,000		19,300
Debt securities, gross unrealized losses		(1,500)		(2,800)
Debt securities, allowance for credit losses	[1]	0
Debt securities, fair value		1,360,100		1,215,000
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,489,100		2,190,500
Debt securities, gross unrealized gains		168,300		118,700
Debt securities, gross unrealized losses		(600)		(1,300)
Debt securities, allowance for credit losses	[1]	0
Debt securities, fair value		2,656,800		2,307,900
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		846,600		675,900
Debt securities, gross unrealized gains		33,400		16,400
Debt securities, gross unrealized losses		(500)		(1,600)
Debt securities, allowance for credit losses	[1]	0
Debt securities, fair value		879,500		690,700
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		3,262,000		3,206,200
Debt securities, gross unrealized gains		289,700		155,100
Debt securities, gross unrealized losses		(3,500)		(2,300)
Debt securities, allowance for credit losses	[1]	(2,500)
Debt securities, fair value		3,545,700		3,359,000
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,268,300		1,348,100
Debt securities, gross unrealized gains		55,600		32,200
Debt securities, gross unrealized losses		(700)		(2,900)
Debt securities, allowance for credit losses	[1]	(100)
Debt securities, fair value		1,323,100		1,377,400
RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,533,600		1,785,100
Debt securities, gross unrealized gains		78,300		56,100
Debt securities, gross unrealized losses		(500)		(800)
Debt securities, allowance for credit losses	[1]	0
Debt securities, fair value	[2]	2,611,400		1,840,400
CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		852,600		849,900
Debt securities, gross unrealized gains		45,300		23,200
Debt securities, gross unrealized losses		(7,600)		(1,400)
Debt securities, allowance for credit losses	[1]	0
Debt securities, fair value		890,300		871,700
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost	[3]	2,328,700		2,544,500
Debt securities, gross unrealized gains	[3]	47,600		24,800
Debt securities, gross unrealized losses	[3]	(24,700)		(19,700)
Debt securities, allowance for credit losses	[1],[3]	0
Debt securities, fair value	[3]	2,351,600		2,549,600
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		714,200		914,800
Gross Unrealized Gains		0		0
Gross Unrealized Losses		0		0
Allowance for Credit Losses	[1]	0
Fair Value		$ 714,200		$ 914,800

[1] See Note 1(r) for further 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 $878.4 million and $834.2 million of collateralized loan obligations as of December 31, 2020 and 2019, respectively.