Stockholders' Equity - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 31, 2012	Jul. 31, 2014
Stockholders Equity Note [Line Items]
Treasury stock acquired	$ 300,478,000	$ 40,389,000	$ 17,768,000
Amount of equity unavailable for dividends or advances from subsidiaries	6,300,000,000
Stockholders' equity	7,473,428,000	6,923,757,000
Amount available for dividends or advances, parent level	1,200,000,000
2012 Repurchase Program
Stockholders Equity Note [Line Items]
Aggregate amount of common stock authorized for repurchase				300,000,000
2014 Repurchase Program
Stockholders Equity Note [Line Items]
Aggregate amount of common stock authorized for repurchase					350,000,000
Treasury stock acquired	58,600,000
Operating Segments
Stockholders Equity Note [Line Items]
Statutory net income of insurance operating units	786,500,000	836,900,000
Combined statutory capital and surplus of insurance operating unit	6,600,000,000	6,500,000,000
Operating Segments | Reinsurance Segment
Stockholders Equity Note [Line Items]
Stockholders' equity	5,100,000,000
Transatlantic Reinsurance Company
Stockholders Equity Note [Line Items]
Dividends paid	400,000,000	200,000,000
Percentage of statutory surplus considered for dividend calculation	10.00%
Transatlantic Reinsurance Company | Reinsurance Segment | Maximum
Stockholders Equity Note [Line Items]
Amount Available for dividends without prior approval	78,600,000
Transatlantic Holdings Incorporated
Stockholders Equity Note [Line Items]
Dividends paid	300,000,000	150,000,000
Alleghany Insurance Holdings Limited Liability Company Operating Subsidiaries
Stockholders Equity Note [Line Items]
Stockholders' equity	2,000,000,000
Percentage of statutory surplus considered for dividend calculation	10.00%
Alleghany Insurance Holdings Limited Liability Company Operating Subsidiaries | Maximum
Stockholders Equity Note [Line Items]
Amount Available for dividends without prior approval	$ 9,700,000