Amortized Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Jun. 30, 2020		Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		$ 14,395,510			$ 13,798,576
Debt securities, gross unrealized gains		686,400			445,800
Debt securities, gross unrealized losses		(123,000)			(32,800)
Debt securities, allowance for credit losses		(8,904)	[1]	$ (31,400)
Debt securities, fair value		14,950,006			14,211,745
Amortized Cost or Cost		15,515,200			14,713,500
Gross Unrealized Gains		686,400			445,800
Gross Unrealized Losses		(123,000)			(32,800)
Allowance for Credit Losses	[1]	(8,900)
Fair Value		16,069,700			15,126,500
U.S. government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,102,200			1,198,500
Debt securities, gross unrealized gains		53,800			19,300
Debt securities, gross unrealized losses		(300)			(2,800)
Debt securities, fair value		1,155,700			1,215,000
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,246,800			2,190,500
Debt securities, gross unrealized gains		141,200			118,700
Debt securities, gross unrealized losses		(1,800)			(1,300)
Debt securities, fair value		2,386,200			2,307,900
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		733,700			675,900
Debt securities, gross unrealized gains		33,600			16,400
Debt securities, gross unrealized losses		(500)			(1,600)
Debt securities, fair value		766,800			690,700
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		3,758,500			3,206,200
Debt securities, gross unrealized gains		261,000			155,100
Debt securities, gross unrealized losses		(9,500)			(2,300)
Debt securities, allowance for credit losses	[1]	(7,600)
Debt securities, fair value		4,002,400			3,359,000
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,252,500			1,348,100
Debt securities, gross unrealized gains		42,500			32,200
Debt securities, gross unrealized losses		(6,000)			(2,900)
Debt securities, allowance for credit losses	[1]	(900)
Debt securities, fair value		1,288,100			1,377,400
RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,063,700			1,785,100
Debt securities, gross unrealized gains		82,700			56,100
Debt securities, gross unrealized losses		(600)			(800)
Debt securities, fair value	[2]	2,145,800			1,840,400
CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		862,500			849,900
Debt securities, gross unrealized gains		33,800			23,200
Debt securities, gross unrealized losses		(25,000)			(1,400)
Debt securities, fair value		871,300			871,700
Other Asset Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,375,600			2,544,500
Debt securities, gross unrealized gains		37,800			24,800
Debt securities, gross unrealized losses		(79,300)			(19,700)
Debt securities, allowance for credit losses	[1]	(400)
Debt securities, fair value	[3]	2,333,700			2,549,600
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		1,119,700			914,800
Fair Value		$ 1,119,700			$ 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] Primarily includes government agency pass-through securities guaranteed by a government agency or government sponsored enterprise, among other types of RMBS.

[3] Includes $748.5 million and $834.2 million of collateralized loan obligations as of June 30, 2020 and December 31, 2019, respectively.