Stockholders' Equity - Additional Information (Detail) - USD ($)			1 Months Ended		12 Months Ended
	Mar. 16, 2020	Mar. 15, 2018	Feb. 29, 2020	Feb. 28, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2019	Jun. 30, 2018	Dec. 31, 2017	Nov. 30, 2015
Stockholders Equity Note [Line Items]
Remaining authorized repurchases under share repurchase program					$ 432,400,000
Amount of equity unavailable for dividends or advances from subsidiaries					7,200,000,000
Stockholders' equity attributable to Alleghany stockholders					8,755,720,000	$ 8,776,734,000	$ 7,692,710,000			$ 8,514,063,000
Amount available for dividends or advances, parent level					1,600,000,000
Dividends paid					$ 215,013,000	0	153,967,000
Dividend Declared in 2018
Stockholders Equity Note [Line Items]
Dividends paid		$ 154,000,000.0
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]
Dividends paid	$ 215,000,000.0
Special dividend declared per share			$ 15.00
Dividend record date			Mar. 05, 2020
Dividend payable nature					special
Operating Segments
Stockholders Equity Note [Line Items]
Stockholders' equity attributable to Alleghany stockholders					$ 9,096,500,000
Operating Segments | Reinsurance Segment and Insurance Segment
Stockholders Equity Note [Line Items]
Stockholders' equity attributable to Alleghany stockholders					7,987,400,000
Statutory net income of insurance operating units					274,800,000	690,900,000
Combined statutory capital and surplus of insurance operating unit					6,800,000,000	7,000,000,000.0
Transatlantic Reinsurance Company
Stockholders Equity Note [Line Items]
Dividends paid					245,000,000.0	220,000,000.0	280,000,000.0
Transatlantic Reinsurance Company | Maximum
Stockholders Equity Note [Line Items]
Amount of dividend that can be declared without regulatory approval					72,800,000
Alleghany Capital Corporation Subsidiaries
Stockholders Equity Note [Line Items]
Dividends paid					$ 60,900,000	$ 82,400,000	$ 26,600,000
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