Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Date the entity disposed of a business or a group of assets, in CCYY-MM-DD format.

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Sum of investments and unrestricted cash as of the balance sheet date.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

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Amount of direct and assumed premiums written.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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Segments of Business - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($) Segment	Dec. 31, 2015 USD ($) Segment	Dec. 31, 2014 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments \| Segment	2	2	2
Gross premiums written	$ 5,696,900	$ 5,767,100	$ 5,122,200
Net premiums earned	4,954,990	4,975,777	4,230,286
Identifiable assets	25,384,317	23,756,591
Stockholders' equity attributable to Alleghany stockholders	$ 8,514,063	$ 7,939,945	$ 7,554,707	$ 7,473,428
ORX
Segment Reporting Information [Line Items]
Disposal date	Dec. 23, 2016
Pacific Comp
Segment Reporting Information [Line Items]
Disposal date	Dec. 31, 2017
Reinsurance Segment \| Gross Premiums Written \| Largest Broker Concentration Risk
Segment Reporting Information [Line Items]
Percentage of gross premiums written	24.00%	25.00%	26.00%
Reinsurance Segment \| Gross Premiums Written \| Second Largest Broker Concentration Risk
Segment Reporting Information [Line Items]
Percentage of gross premiums written	19.00%	20.00%	20.00%
Reinsurance Segment \| Gross Premiums Written \| Third Largest Broker Concentration Risk
Segment Reporting Information [Line Items]
Percentage of gross premiums written	16.00%	16.00%	10.00%
Reinsurance Segment \| Gross Premiums Written \| Large Whole Account Quota Share Treaty
Segment Reporting Information [Line Items]
Percentage of gross premiums written	19.00%	20.00%	6.00%
Reinsurance Segment \| Non-US
Segment Reporting Information [Line Items]
Gross premiums written	$ 1,500,000	$ 1,500,000	$ 1,600,000
Net premiums earned	1,400,000	1,400,000	1,400,000
Reinsurance Segment \| U.K.
Segment Reporting Information [Line Items]
Net premiums earned	612,100	622,300	640,400
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written	5,720,200	5,793,000	5,150,200
Net premiums earned	4,955,000	4,975,800	4,230,300
Operating Segments \| Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written	4,210,600	4,330,300	3,662,100
Net premiums earned	3,808,700	3,845,000	3,115,500
Identifiable assets	16,700,000
Cash and invested assets	13,600,000
Stockholders' equity attributable to Alleghany stockholders	5,200,000
Operating Segments \| Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written	1,509,600	1,462,700	1,488,100
Net premiums earned	1,146,300	$ 1,130,800	$ 1,114,800
Identifiable assets	6,900,000
Cash and invested assets	5,300,000
Stockholders' equity attributable to Alleghany stockholders	3,000,000
Other activities
Segment Reporting Information [Line Items]
Identifiable assets	1,800,000
Cash and invested assets	600,000
Stockholders' equity attributable to Alleghany stockholders	$ 300,000