Consolidated Statements of Earnings and Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues
Net premiums earned	$ 1,842,894	$ 1,556,859	$ 5,232,008	$ 4,396,147
Net investment income	132,965	129,265	413,380	359,938
Change in the fair value of equity securities	(136,042)	92,799	180,588	(187,991)
Net realized capital gains	20,172	16,751	46,045	(8,323)
Change in allowance for credit losses on available for sale securities	54	3,454	2,272	(10,900)
Product and service revenues	1,006,570	723,785	2,574,830	1,684,335
Total revenues	2,866,613	2,522,913	8,449,123	6,233,206
Costs and Expenses
Net loss and loss adjustment expenses	1,513,877	1,173,655	3,713,153	3,225,461
Commissions, brokerage and other underwriting expenses	529,247	464,522	1,528,796	1,315,935
Other operating expenses	890,972	669,423	2,354,453	1,646,390
Corporate administration	6,225	12,871	35,931	17,143
Amortization of intangible assets	12,637	11,223	36,546	32,884
Interest expense	25,659	23,319	73,448	63,869
Total costs and expenses	2,978,617	2,355,013	7,742,327	6,301,682
(Losses) earnings before income taxes	(112,004)	167,900	706,796	(68,476)
Income taxes	(28,509)	30,253	132,639	(21,111)
Net (losses) earnings	(83,495)	137,647	574,157	(47,365)
Net earnings attributable to noncontrolling interest	31,471	11,125	55,431	9,953
Net (losses) earnings attributable to Alleghany stockholders	(114,966)	126,522	518,726	(57,318)
Net earnings (losses)	(83,495)	137,647	574,157	(47,365)
Other comprehensive income (loss):
Change in unrealized gains, net of deferred taxes	(55,616)	77,133	(181,991)	212,175
Change in unrealized currency translation adjustment, net of deferred taxes	(3,370)	12,109	(3,158)	13,190
Retirement plans	117	420	(735)	1,356
Comprehensive (loss) income	(158,342)	220,964	350,103	156,595
Comprehensive income attributable to noncontrolling interests	31,471	11,125	55,431	9,953
Comprehensive (loss) income attributable to Alleghany stockholders	$ (189,813)	$ 209,839	$ 294,672	$ 146,642
Basic (losses) earnings per share attributable to Alleghany stockholders	$ (8.31)	$ 8.86	$ 37.28	$ (4.01)
Diluted (losses) earnings per share attributable to Alleghany stockholders	$ (8.60)	$ 8.86	$ 37.22	$ (4.43)
Accumulated Net Unrealized Investment Gain Loss
Other comprehensive income (loss):
Less: reclassification for net realized capital gains and change in allowance for credit losses on available for sale securities, net of taxes	$ (15,978)	$ (6,345)	$ (38,170)	$ (22,761)