Summary of Rating of Debt securities (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020		Dec. 31, 2019
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 13,337,754	[1]	$ 14,211,745
Percentage of debt securities, before allowance for credit losses		100.00%
AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 2,111,700
Percentage of debt securities, before allowance for credit losses		15.80%
AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 5,387,200
Percentage of debt securities, before allowance for credit losses		40.40%
A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 3,394,700
Percentage of debt securities, before allowance for credit losses		25.50%
BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 1,972,700
Percentage of debt securities, before allowance for credit losses		14.80%
Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[2]	$ 471,500
Percentage of debt securities, before allowance for credit losses	[2]	3.50%
U.S. Government obligations
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 846,300		1,215,000
U.S. Government obligations | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		846,300
Municipal bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		2,272,600		2,307,900
Municipal bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		184,200
Municipal bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,526,500
Municipal bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		489,400
Municipal bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		71,600
Municipal bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[2]	900
Foreign government obligations
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		696,000		690,700
Foreign government obligations | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		445,400
Foreign government obligations | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		153,800
Foreign government obligations | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		95,800
Foreign government obligations | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,000
U.S. corporate bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		3,394,700		3,359,000
U.S. corporate bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		9,000
U.S. corporate bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		158,000
U.S. corporate bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,586,400
U.S. corporate bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,298,200
U.S. corporate bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[2]	343,100
Foreign corporate bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,290,600		1,377,400
Foreign corporate bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		225,200
Foreign corporate bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		95,900
Foreign corporate bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		607,400
Foreign corporate bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		325,900
Foreign corporate bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[2]	36,200
RMBS
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[3]	1,699,900		1,840,400
RMBS | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		2,500
RMBS | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,692,300
RMBS | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,000
RMBS | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[2]	4,100
CMBS
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		842,600		871,700
CMBS | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		283,600
CMBS | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		400,800
CMBS | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		157,800
CMBS | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		400
Other asset-backed securities
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[4]	2,295,100		$ 2,549,600
Other asset-backed securities | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		961,800
Other asset-backed securities | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		513,600
Other asset-backed securities | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		457,900
Other asset-backed securities | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		274,600
Other asset-backed securities | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[2]	$ 87,200

[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] Consists of $109.3 million of securities rated BB / Ba, $187.5 million of securities rated B, $26.2 million of securities rated CCC, $0.4 million of securities rated CC, $2.1 million of securities rated below CC and $146.0 million of not rated 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.