Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Mar. 15, 2018	Feb. 28, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2018	Dec. 31, 2015	Nov. 30, 2015	Jul. 31, 2014
Stockholders Equity Note [Line Items]
Amount of equity unavailable for dividends or advances from subsidiaries			$ 6,000,000
Stockholders' equity attributable to Alleghany stockholders			7,692,710	$ 8,514,063	$ 7,939,945		$ 7,554,707
Amount available for dividends or advances, parent level			1,700,000
Dividends paid	$ 154,000		153,967
Dividend Declared [Member]
Stockholders Equity Note [Line Items]
Dividends Payable, Date of Record		Mar. 05, 2018
Dividends Payable, Nature		special
Common Stock, Dividends, Per Share, Declared		$ 10.00
Operating Segments
Stockholders Equity Note [Line Items]
Stockholders' equity attributable to Alleghany stockholders			8,299,200
Operating Segments | Reinsurance Segment And Insurance Segment [Member]
Stockholders Equity Note [Line Items]
Stockholders' equity attributable to Alleghany stockholders			7,419,900
Statutory net income of insurance operating units			263,600	144,400
Combined statutory capital and surplus of insurance operating unit			6,500,000	6,900,000
Transatlantic Reinsurance Company
Stockholders Equity Note [Line Items]
Dividends paid			280,000	$ 270,000	$ 350,000
Transatlantic Reinsurance Company | Maximum
Stockholders Equity Note [Line Items]
Amount of dividend that can be declared without regulatory approval			64,900
2014 Repurchase Program
Stockholders Equity Note [Line Items]
Aggregate amount of common stock authorized for repurchase									$ 350,000
2015 Repurchase Program
Stockholders Equity Note [Line Items]
Aggregate amount of common stock authorized for repurchase								$ 400,000
2018 Repurchase Program
Stockholders Equity Note [Line Items]
Aggregate amount of common stock authorized for repurchase						$ 400,000
Stock repurchase program, remaining share authorized for repurchase			$ 271,500