Amortized Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Mar. 31, 2020		Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		$ 13,320,808		$ 13,798,576
Debt securities, gross unrealized gains		424,200		445,800
Debt securities, gross unrealized losses		(375,800)		(32,800)
Debt securities, allowance for credit losses	[1]	(31,375)
Debt securities, fair value		13,337,754	[2]	14,211,745
Amortized Cost or Cost		13,856,900		14,713,500
Gross Unrealized Gains		424,200		445,800
Gross Unrealized Losses		(375,800)		(32,800)
Allowance for Credit Losses	[1]	(31,400)
Fair Value		13,873,900		15,126,500
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		793,500		1,198,500
Debt securities, gross unrealized gains		53,000		19,300
Debt securities, gross unrealized losses		(200)		(2,800)
Debt securities, fair value		846,300		1,215,000
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,158,300		2,190,500
Debt securities, gross unrealized gains		117,200		118,700
Debt securities, gross unrealized losses		(2,900)		(1,300)
Debt securities, fair value		2,272,600		2,307,900
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		669,100		675,900
Debt securities, gross unrealized gains		27,600		16,400
Debt securities, gross unrealized losses		(700)		(1,600)
Debt securities, fair value		696,000		690,700
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		3,386,800		3,206,200
Debt securities, gross unrealized gains		103,500		155,100
Debt securities, gross unrealized losses		(69,100)		(2,300)
Debt securities, allowance for credit losses	[1]	(26,500)
Debt securities, fair value		3,394,700		3,359,000
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,311,200		1,348,100
Debt securities, gross unrealized gains		14,500		32,200
Debt securities, gross unrealized losses		(32,700)		(2,900)
Debt securities, allowance for credit losses	[1]	(2,400)
Debt securities, fair value		1,290,600		1,377,400
RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,613,700		1,785,100
Debt securities, gross unrealized gains		86,900		56,100
Debt securities, gross unrealized losses		(200)		(800)
Debt securities, allowance for credit losses	[1]	(500)
Debt securities, fair value	[3]	1,699,900		1,840,400
CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		870,600		849,900
Debt securities, gross unrealized gains		15,900		23,200
Debt securities, gross unrealized losses		(43,900)		(1,400)
Debt securities, fair value		842,600		871,700
Other Asset Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,517,600		2,544,500
Debt securities, gross unrealized gains		5,600		24,800
Debt securities, gross unrealized losses		(226,100)		(19,700)
Debt securities, allowance for credit losses	[1]	(2,000)
Debt securities, fair value	[4]	2,295,100		2,549,600
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		536,100		914,800
Fair Value		$ 536,100		$ 914,800

[1] See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance.

[2] Includes allowance for credit losses as of March 31, 2020.  See Note 3(f) of this Form 10-Q for further information on Alleghany’s allowance for credit losses for AFS securities.

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

[4] Includes $732.3 million and $834.2 million of collateralized loan obligations as of March 31, 2020 and December 31, 2019, respectively.