Condensed Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Equity securities	$ 2,505,496	$ 3,572,790
Debt securities	14,211,745	11,823,968
Short-term investments	914,776	893,776
Other invested assets	573,605	555,972
Cash	1,179,098	1,038,763
Property and equipment at cost, net of accumulated depreciation and amortization	205,397	195,243
Other assets	1,183,633	1,012,379
Net deferred tax assets	5,860	164,890
Total assets	26,931,604	25,344,896
Liabilities, Redeemable Noncontrolling Interests and Stockholders’ Equity
Senior notes	1,751,113	1,669,039
Other liabilities	1,516,076	1,127,346
Total liabilities	17,950,117	17,482,424
Redeemable noncontrolling interest	204,753	169,762	$ 106,530	$ 74,720
Stockholders' equity attributable to Alleghany stockholders	8,776,734	7,692,710	$ 8,514,063	$ 7,939,945
Total liabilities, redeemable noncontrolling interests and stockholders’ equity	26,931,604	25,344,896
Parent Company
Assets
Equity securities	4,042	64,121
Debt securities	182,490	90
Short-term investments	125,412	287,573
Other invested assets	81,728	79,266
Cash	684	331
Property and equipment at cost, net of accumulated depreciation and amortization	4,201	4,588
Other assets	57,415	8,303
Net deferred tax assets	30,118	13,956
Investment in subsidiaries	9,638,153	8,491,404
Total assets	10,124,243	8,949,632
Liabilities, Redeemable Noncontrolling Interests and Stockholders’ Equity
Senior notes	994,241	993,286
Other liabilities	144,382	87,489
Current taxes payable	4,133	6,385
Total liabilities	1,142,756	1,087,160
Redeemable noncontrolling interest	204,753	169,762
Stockholders' equity attributable to Alleghany stockholders	8,776,734	7,692,710
Total liabilities, redeemable noncontrolling interests and stockholders’ equity	$ 10,124,243	$ 8,949,632