Condensed Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Equity securities (cost: 2017 - $397,906; 2016 - $171,860)	$ 4,099,467	$ 3,109,523
Debt securities (amortized cost: 2017 - $26,236; 2016 - $31,505)	12,721,399	12,983,213
Short-term investments	578,054	778,410
Other invested assets	743,358	645,245
Cash	838,375	594,091	$ 475,267	$ 605,259
Property and equipment at cost, net of accumulated depreciation and amortization	125,337	112,920
Other assets	659,096	522,922
Net deferred tax assets	136,489	354,852
Total assets	25,384,317	23,756,591
Liabilities, Redeemable Noncontrolling Interests and Stockholders' Equity
Senior notes	1,484,897	1,476,489
Other liabilities	1,068,907	912,081
Total liabilities	16,763,724	15,741,926
Redeemable noncontrolling interest	106,530	74,720	25,719	8,616
Stockholders' equity attributable to Alleghany stockholders	8,514,063	7,939,945	7,554,707	7,473,428
Total liabilities, redeemable noncontrolling interest and stockholders' equity	25,384,317	23,756,591
Parent Company
Assets
Equity securities (cost: 2017 - $397,906; 2016 - $171,860)	438,355	187,413
Debt securities (amortized cost: 2017 - $26,236; 2016 - $31,505)	26,572	31,564
Short-term investments	91,590	235,510
Other invested assets	76,953	36,741
Cash	21,280	6,139	$ 2,477	$ 10,535
Property and equipment at cost, net of accumulated depreciation and amortization	5,101	5,537
Other assets	5,518	14,359
Net deferred tax assets	14,454	39,503
Investment in subsidiaries	9,119,885	8,611,000
Total assets	9,799,708	9,167,766
Liabilities, Redeemable Noncontrolling Interests and Stockholders' Equity
Senior notes	992,331	991,376
Other liabilities	119,852	112,342
Current taxes payable	66,932	49,383
Total liabilities	1,179,115	1,153,101
Redeemable noncontrolling interest	106,530	74,720
Stockholders' equity attributable to Alleghany stockholders	8,514,063	7,939,945
Total liabilities, redeemable noncontrolling interest and stockholders' equity	$ 9,799,708	$ 9,167,766