Condensed Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Assets
Equity securities (cost: 2014 - $47,387; 2013 - $23,576)	$ 2,815,484	$ 2,229,453
Debt securities (amortized cost: 2014 - $70,282; 2013 - $70,758)	14,598,641	14,802,890
Short-term investments	715,553	1,317,895
Cash	605,259	498,315	649,524	84,749
Property and equipment at cost, net of accumulated depreciation and amortization	88,910	58,974
Other assets	514,797	408,539
Net deferred tax assets	389,597	469,787
Total assets	23,489,436	23,361,088
Liabilities and Stockholders' Equity
Senior Notes	1,767,125	1,794,407
Other liabilities	729,767	810,507
Total liabilities	16,007,392	16,413,567
Stockholders' equity attributable to Alleghany stockholders	7,473,428	6,923,757
Noncontrolling interest	8,616	23,764
Stockholders' equity	7,482,044	6,947,521	6,403,787	2,925,677
Total liabilities and stockholders' equity	23,489,436	23,361,088
Parent Company
Assets
Equity securities (cost: 2014 - $47,387; 2013 - $23,576)	49,432	44,536
Debt securities (amortized cost: 2014 - $70,282; 2013 - $70,758)	71,262	68,731
Short-term investments	152,943	78,101
Cash	10,535	17,356	1,379	3,826
Property and equipment at cost, net of accumulated depreciation and amortization	498	584
Other assets	24,694	22,712
Net deferred tax assets	41,803	31,097
Investment in subsidiaries	8,241,843	7,471,271
Total assets	8,593,010	7,734,388
Liabilities and Stockholders' Equity
Senior Notes	997,025	698,910
Other liabilities	101,081	78,825
Current taxes payable	12,860	9,132
Total liabilities	1,110,966	786,867
Stockholders' equity attributable to Alleghany stockholders	7,473,428	6,923,757
Noncontrolling interest	8,616	23,764
Stockholders' equity	7,482,044	6,947,521
Total liabilities and stockholders' equity	$ 8,593,010	$ 7,734,388