Aggregate dividends declared during the period for each share of common stock outstanding.

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Describes the declaration as being a special dividend (over and above the normal dividend); as the annual, semi-annual, quarterly dividend, or other normal periodic dividend; or as some other type of dividend.

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Cash outflow in the form of capital distributions and dividends to common shareholders, preferred shareholders and noncontrolling interests.

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Amount of dividends that can be declared or paid without approval of regulatory agency.

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Amount of net income for the period determined using accounting principles prescribed or permitted by insurance regulators.

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Amount of statutory capital and surplus computed using statutory accounting practices prescribed by the National Association of Insurance Commissioners.

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Amount of stock repurchase plan authorized.

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Amount remaining of a stock repurchase plan authorized.

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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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Stockholders' Equity - Additional Information (Detail) - USD ($)		1 Months Ended		12 Months Ended
	Mar. 15, 2018	Feb. 29, 2020	Feb. 28, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Jun. 30, 2018	Dec. 31, 2016	Nov. 30, 2015
Stockholders Equity Note [Line Items]
Remaining authorized repurchases under share repurchase program				$ 627,100,000
Amount of equity unavailable for dividends or advances from subsidiaries				6,900,000,000
Stockholders' equity attributable to Alleghany stockholders				8,776,734,000	$ 7,692,710,000	$ 8,514,063,000			$ 7,939,945,000
Amount available for dividends or advances, parent level				$ 1,900,000,000
Dividends paid	$ 154,000,000.0				153,967,000
Dividend Declared in 2018
Stockholders Equity Note [Line Items]
Special dividend declared per share			$ 10.00
Dividend record date			Mar. 05, 2018
Dividend payable nature				special
Dividend Declared in 2020
Stockholders Equity Note [Line Items]
Dividend payable nature				special
Dividend Declared in 2020 \| Subsequent Event
Stockholders Equity Note [Line Items]
Special dividend declared per share		$ 15.00
Dividend record date		Mar. 05, 2020
Dividend payable date		Mar. 16, 2020
Operating Segments
Stockholders Equity Note [Line Items]
Stockholders' equity attributable to Alleghany stockholders				$ 9,317,200,000
Operating Segments \| Reinsurance Segment and Insurance Segment
Stockholders Equity Note [Line Items]
Stockholders' equity attributable to Alleghany stockholders				8,416,300,000
Statutory net income of insurance operating units				690,900,000	263,600,000
Combined statutory capital and surplus of insurance operating unit				7,000,000,000.0	6,500,000,000
Transatlantic Reinsurance Company
Stockholders Equity Note [Line Items]
Dividends paid				220,000,000.0	$ 280,000,000.0	$ 270,000,000.0
Transatlantic Reinsurance Company \| Maximum
Stockholders Equity Note [Line Items]
Amount of dividend that can be declared without regulatory approval				$ 41,500,000
Stock Repurchase Programs 2015
Stockholders Equity Note [Line Items]
Aggregate amount of common stock authorized for repurchase										$ 400,000,000.0
Stock Repurchase Programs 2018
Stockholders Equity Note [Line Items]
Aggregate amount of common stock authorized for repurchase								$ 400,000,000.0
Stock Repurchase Programs 2019
Stockholders Equity Note [Line Items]
Aggregate amount of common stock authorized for repurchase							$ 500,000,000.0