Stockholders' Equity - Additional Information (Detail) - USD ($)		1 Months Ended	12 Months Ended
	Mar. 16, 2020	Feb. 29, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Stockholders Equity Note [Line Items]
Remaining authorized repurchases under share repurchase program			$ 141,900,000
Amount of equity unavailable for dividends or advances from subsidiaries			7,300,000,000
Stockholders' equity attributable to Alleghany stockholders			9,186,882,000	$ 8,755,720,000	$ 8,776,734,000		$ 7,692,710,000
Amount available for dividends or advances, parent level			1,900,000,000
Dividends paid			$ 0	215,013,000	0
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			$ 9,495,400,000
Operating Segments | Reinsurance Segment and Insurance Segment
Stockholders Equity Note [Line Items]
Stockholders' equity attributable to Alleghany stockholders			8,157,500,000
Statutory net income of insurance operating units			681,400,000	274,800,000
Combined statutory capital and surplus of insurance operating unit			7,200,000,000	6,800,000,000
Transatlantic Reinsurance Company
Stockholders Equity Note [Line Items]
Amount of dividend that can be declared without regulatory approval			108,600,000
Dividends paid			$ 290,000,000.0	$ 245,000,000.0	$ 220,000,000.0
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