Amortized Cost and Fair Value of Available For Sale Securities (Parenthetical) (Detail) - USD ($) $ in Thousands		Mar. 31, 2020		Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities		$ 13,337,754	[1]	$ 14,211,745
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities	[2]	2,295,100		2,549,600
Other asset-backed securities | Collateralized loan obligations
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities		$ 732,300		$ 834,200

[1] 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.

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