Stockholders' Equity - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 30, 2015	Dec. 31, 2014	Jul. 31, 2014
Stockholders Equity Note [Line Items]
Amount of equity unavailable for dividends or advances from subsidiaries	$ 6,600,000
Stockholders' equity attributable to Alleghany stockholders	8,514,063	$ 7,939,945	$ 7,554,707		$ 7,473,428
Amount available for dividends or advances, parent level	1,900,000
Operating Segments
Stockholders Equity Note [Line Items]
Statutory net income of insurance operating units	144,400	688,900
Combined statutory capital and surplus of insurance operating unit	6,900,000	$ 6,700,000
Transatlantic Reinsurance Company | Maximum
Stockholders Equity Note [Line Items]
Amount of dividend that can be declared without regulatory approval	37,100
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
Stock repurchase program, remaining share authorized for repurchase	$ 363,200