Summary of Significant Accounting Principles - Additional Information (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Mar. 31, 2019	Jan. 01, 2019
Significant Accounting Policies [Line Items]
Statutory federal income tax rate		21.00%	35.00%
Right-of-use asset	[1]			$ 212,100
Lease liability	[2]			$ 240,300
Retained Earnings
Significant Accounting Policies [Line Items]
Cumulative effect of adoption of new accounting pronouncements			$ 600,508
Accumulated Other Comprehensive Income
Significant Accounting Policies [Line Items]
Cumulative effect of adoption of new accounting pronouncements	[3]		(600,508)
Accounting Standards Update 2018-02 | Retained Earnings
Significant Accounting Policies [Line Items]
Cumulative effect of adoption of new accounting pronouncements			(135,000)
Accounting Standards Update 2018-02 | Accumulated Other Comprehensive Income
Significant Accounting Policies [Line Items]
Cumulative effect of adoption of new accounting pronouncements	[3]		135,100
Accounting Standards Update 2016-02
Significant Accounting Policies [Line Items]
Right-of-use asset						$ 200,000
Lease liability						$ 200,000
Accounting Standards Update 2016-01 | Retained Earnings
Significant Accounting Policies [Line Items]
Cumulative effect of adoption of new accounting pronouncements			736,000
Accounting Standards Update 2016-01 | Accumulated Other Comprehensive Income
Significant Accounting Policies [Line Items]
Cumulative effect of adoption of new accounting pronouncements	[3]		$ (735,600)
Wilbert Funeral Services, Inc
Significant Accounting Policies [Line Items]
Equity interest percentage				45.00%
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%
Precision Cutting Technologies
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners					11.00%
Jazwares, LLC
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners				23.00%
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%

[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.

[3] See Note 1(c) of this Form 10-Q for additional information on Alleghany’s adoption of new investment accounting guidance and new guidance on certain tax effects caused by the Tax Act.