Condensed Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets
Equity securities	$ 2,718,902	$ 2,505,496
Debt securities	15,618,470	14,211,745
Short-term investments	714,208	914,776
Other invested assets	465,153	573,605
Cash	791,442	1,179,098
Property and equipment at cost, net of accumulated depreciation and amortization	267,872	205,397
Other assets	1,559,245	1,183,633
Net deferred tax assets	0	5,860
Current taxes receivable	3,189	4,081
Total assets	28,927,010	26,931,604
Liabilities, Redeemable Noncontrolling Interests and Stockholders’ Equity
Senior notes	2,135,946	1,751,113
Other liabilities	1,594,918	1,516,076
Total liabilities	19,937,481	17,950,117
Redeemable noncontrolling interest	233,809	204,753	$ 169,762	$ 106,530
Stockholders' equity attributable to Alleghany stockholders	8,755,720	8,776,734	$ 7,692,710	$ 8,514,063
Total liabilities, redeemable noncontrolling interests and stockholders’ equity	28,927,010	26,931,604
Parent Company
Assets
Equity securities	571,314	4,042
Debt securities	48,753	182,490
Short-term investments	76,259	125,412
Other invested assets	91,873	81,728
Cash	17,243	684
Property and equipment at cost, net of accumulated depreciation and amortization	3,707	4,201
Other assets	60,089	57,415
Net deferred tax assets	21,060	30,118
Current taxes receivable	17,649	0
Investment in subsidiaries	9,361,261	9,638,153
Total assets	10,269,208	10,124,243
Liabilities, Redeemable Noncontrolling Interests and Stockholders’ Equity
Senior notes	1,190,192	994,241
Other liabilities	89,487	144,382
Current taxes payable	0	4,133
Total liabilities	1,279,679	1,142,756
Redeemable noncontrolling interest	233,809	204,753
Stockholders' equity attributable to Alleghany stockholders	8,755,720	8,776,734
Total liabilities, redeemable noncontrolling interests and stockholders’ equity	$ 10,269,208	$ 10,124,243