Summary of Significant Accounting Principles - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Jan. 01, 2018	Aug. 01, 2017	Jan. 01, 2017
Significant Accounting Policies [Line Items]
Statutory federal income tax rate		21.00%	35.00%	35.00%
Accumulated Other Comprehensive Income
Significant Accounting Policies [Line Items]
Cumulative effective of adoption of new accounting pronouncements	[1]				$ (600.5)		$ 12.9
Accounting Standards Update 2018-02 | Accumulated Other Comprehensive Income
Significant Accounting Policies [Line Items]
Cumulative effective of adoption of new accounting pronouncements	[1]				135.1
Accounting Standards Update 2018-02 | Retained Earnings
Significant Accounting Policies [Line Items]
Cumulative effective of adoption of new accounting pronouncements					(135.1)
Accounting Standards Update 2017-08 | Accumulated Other Comprehensive Income
Significant Accounting Policies [Line Items]
Cumulative effective of adoption of new accounting pronouncements							13.0
Accounting Standards Update 2017-08 | Retained Earnings
Significant Accounting Policies [Line Items]
Cumulative effective of adoption of new accounting pronouncements							$ (13.0)
Accounting Standards Update 2016-01 |Reclassification of net unrealized gains on equity securities, net of tax | Accumulated Other Comprehensive Income
Significant Accounting Policies [Line Items]
Cumulative effective of adoption of new accounting pronouncements	[1]				(735.6)
Accounting Standards Update 2016-01 |Reclassification of net unrealized gains on equity securities, net of tax | Retained Earnings
Significant Accounting Policies [Line Items]
Cumulative effective of adoption of new accounting pronouncements					$ 735.6
Wilbert Funeral Services, Inc
Significant Accounting Policies [Line Items]
Equity interest percentage		45.00%				45.00%
Pacific Comp
Significant Accounting Policies [Line Items]
Cash consideration from sale				$ 158.0
Disposal date				Dec. 31, 2017
Assets disposed of				$ 442.0
Liabilities disposed of				316.0
Gain on sale of business, after tax				16.0
Alleghany Corporation | Financial Guarantee | AIHL Re Limited Liability Company reinsurance of Pacific Compensation Insurance Company
Significant Accounting Policies [Line Items]
Guaranteed obligation, aggregate limit				150.0
Aihl Re Limited Liability Company | AIHL Re Limited Liability Company reinsurance of Pacific Compensation Insurance Company
Significant Accounting Policies [Line Items]
Aggregate limit				$ 150.0
Final commutation and settlement date				Dec. 31, 2024
WWSC Holdings, LLC
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners		20.00%
Kentucky Trailer
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners		21.00%
Integrated Project Services LLC
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners		16.00%
Bourn & Koch, Inc.
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]
Business acquisition date		Apr. 28, 2017
Hirschfeld Holdings, LP | WWSC Holdings, LLC
Significant Accounting Policies [Line Items]
Business acquisition date		Feb. 07, 2018

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