Condensed Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Assets
Equity securities (cost: 2016 - $171,860; 2015 - $217,039)	$ 3,109,523	$ 3,005,908
Debt securities (amortized cost: 2016 - $31,505; 2015 - $31,746)	12,983,213	13,605,963
Short-term investments	778,410	365,810
Other invested assets	645,245	676,811
Cash	594,091	475,267	$ 605,259	$ 498,315
Property and equipment at cost, net of accumulated depreciation and amortization	112,920	101,306
Other assets	522,922	633,964
Net deferred tax assets	354,852	468,440
Total assets	23,756,591	22,839,079
Liabilities, Redeemable Noncontrolling Interests and Stockholders' Equity
Senior notes	1,476,489	1,419,363
Other liabilities	912,081	894,690
Total liabilities	15,741,926	15,258,653
Redeemable noncontrolling interest	74,720	25,719	8,616	23,764
Stockholders' equity attributable to Alleghany stockholders	7,939,945	7,554,707	7,473,428	6,923,757
Total liabilities, redeemable noncontrolling interest and stockholders' equity	23,756,591	22,839,079
Parent Company
Assets
Equity securities (cost: 2016 - $171,860; 2015 - $217,039)	187,413	216,776
Debt securities (amortized cost: 2016 - $31,505; 2015 - $31,746)	31,564	30,483
Short-term investments	235,510	83,135
Other invested assets	36,741
Cash	6,139	2,477	$ 10,535	$ 17,356
Property and equipment at cost, net of accumulated depreciation and amortization	5,537	259
Other assets	14,359	18,920
Net deferred tax assets	39,503	45,775
Investment in subsidiaries	8,611,000	8,325,073
Total assets	9,167,766	8,722,898
Liabilities, Redeemable Noncontrolling Interests and Stockholders' Equity
Senior notes	991,376	990,421
Other liabilities	112,342	110,058
Current taxes payable	49,383	41,993
Total liabilities	1,153,101	1,142,472
Redeemable noncontrolling interest	74,720	25,719
Stockholders' equity attributable to Alleghany stockholders	7,939,945	7,554,707
Total liabilities, redeemable noncontrolling interest and stockholders' equity	$ 9,167,766	$ 8,722,898