Summary of Significant Accounting Principles - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
		Jan. 01, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Apr. 01, 2020	Mar. 31, 2019	Jan. 01, 2019
Significant Accounting Policies [Line Items]
Par value of common stock outstanding			$ 1.00
Statutory federal income tax rate			21.00%	21.00%	21.00%	35.00%
Total stockholders’ equity attributable to Alleghany stockholders			$ 8,755,720	$ 8,776,734	$ 7,692,710	$ 8,514,063
Right-of-use asset	[1]		227,700
Lease liability	[2]		255,500
Retained earnings			6,318,334	6,435,163
Cumulative Effect, Period of Adoption, Adjusted Balance
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			6,318,334	6,435,163	5,577,362	5,091,282
Retained Earnings | Cumulative Effect, Period of Adoption, Adjusted Balance
Significant Accounting Policies [Line Items]
Total stockholders’ equity attributable to Alleghany stockholders				(3,570)		600,508
Accumulated Other Comprehensive Income
Significant Accounting Policies [Line Items]
Total stockholders’ equity attributable to Alleghany stockholders			$ 452,402	$ 171,350	$ (202,003)	618,118
Accumulated Other Comprehensive Income | Cumulative Effect, Period of Adoption, Adjusted Balance
Significant Accounting Policies [Line Items]
Total stockholders’ equity attributable to Alleghany stockholders						(600,508)
Accounting Standards Update 2018-02 | Retained Earnings | Revision of Prior Period Accounting Standards Update Adjustment
Significant Accounting Policies [Line Items]
Total stockholders’ equity attributable to Alleghany stockholders						(135,000)
Accounting Standards Update 2018-02 | Accumulated Other Comprehensive Income | Revision of Prior Period Accounting Standards Update Adjustment
Significant Accounting Policies [Line Items]
Total stockholders’ equity attributable to Alleghany stockholders						135,000
Accounting Standards Update 2016-01 | Retained Earnings | Revision of Prior Period Accounting Standards Update Adjustment
Significant Accounting Policies [Line Items]
Total stockholders’ equity attributable to Alleghany stockholders						736,000
Accounting Standards Update 2016-01 | Accumulated Other Comprehensive Income | Revision of Prior Period Accounting Standards Update Adjustment
Significant Accounting Policies [Line Items]
Total stockholders’ equity attributable to Alleghany stockholders						$ (736,000)
Accounting Standards Update 2016-02 | Cumulative Effect, Period of Adoption, Adjusted Balance
Significant Accounting Policies [Line Items]
Right-of-use asset									$ 200,000
Lease liability									$ 200,000
Accounting Standards Update 2016-13 | Cumulative Effect, Period of Adoption, Adjusted Balance
Significant Accounting Policies [Line Items]
Allowances for credit losses on certain financial assets		$ 4,500
Retained earnings		$ (3,600)
Minimum
Significant Accounting Policies [Line Items]
Useful life of property and equipment			3 years
Lease term criteria to record lease liability and right-of-use asset			1 year
Maximum
Significant Accounting Policies [Line Items]
Useful life of property and equipment			20 years
Kentucky Trailer
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners			23.00%
Integrated Project Services LLC
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners			15.00%
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%
Precision Cutting Technologies, Inc
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners								11.00%
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 lease liabilities and is reported as a component of other liabilities on Alleghany’s consolidated balance sheet.