Summary of Significant Accounting Principles - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
		Jan. 01, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Apr. 01, 2020	Jan. 01, 2019	Dec. 31, 2018
Significant Accounting Policies [Line Items]
Accretion reduced capital contribution			$ (5,300)	$ 0	$ 0
Par value of common stock outstanding			$ 1.00
Statutory federal income tax rate			21.00%	21.00%	21.00%
Total stockholders’ equity attributable to Alleghany stockholders			$ 9,186,882	$ 8,755,720	$ 8,776,734			$ 7,692,710
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]			Other Assets
Right-of-use asset	[1]		$ 240,500
Lease liability	[2]		268,500
Retained earnings			7,353,226	6,318,334
Cumulative Effect, Period of Adoption, Adjustment
Significant Accounting Policies [Line Items]
Total stockholders’ equity attributable to Alleghany stockholders					(3,570)
Retained Earnings
Significant Accounting Policies [Line Items]
Total stockholders’ equity attributable to Alleghany stockholders			7,353,226	6,318,334	6,435,163			5,577,362
Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment
Significant Accounting Policies [Line Items]
Total stockholders’ equity attributable to Alleghany stockholders					(3,570)
Accumulated Other Comprehensive Income
Significant Accounting Policies [Line Items]
Total stockholders’ equity attributable to Alleghany stockholders			$ 141,822	$ 452,402	171,350			$ (202,003)
Accumulated Other Comprehensive Income | Cumulative Effect, Period of Adoption, Adjustment
Significant Accounting Policies [Line Items]
Total stockholders’ equity attributable to Alleghany stockholders					$ 0
Accounting Standards Update 2016-02 | Cumulative Effect, Period of Adoption, Adjustment
Significant Accounting Policies [Line Items]
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]							Other Assets
Right-of-use asset							$ 200,000
Lease liability							$ 200,000
Accounting Standards Update 2016-13 | Cumulative Effect, Period of Adoption, Adjustment
Significant Accounting Policies [Line Items]
Allowances for credit losses on certain financial assets		$ 4,500
Retained earnings		$ (3,600)
Kentucky Trailer
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners			22.00%
Integrated Project Services LLC
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners			18.00%
Jazwares, LLC
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners			24.00%
WWSC Holdings, LLC
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners			20.00%
Concord
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners			15.00%
Wilbert Funeral Services, Inc
Significant Accounting Policies [Line Items]
Equity interest percentage acquired						55.00%
Equity interest percentage						100.00%

[1] Reported as a component of other assets on Alleghany’s consolidated balance sheet.

[2] Represents the present value of expected lease payments over the remaining lease term and is reported as a component of other liabilities on Alleghany’s consolidated balance sheet.