Consolidated Statements of Earnings and Comprehensive Income - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues
Net premiums earned		$ 6,000,161	$ 5,478,143	$ 4,976,190
Net investment income		490,856	550,241	500,534
Change in the fair value of equity securities		(110,459)	709,695	(228,994)
Net realized capital gains		3,098	(6,551)	(3,241)
Change in allowance for credit losses on available for sale securities		(8,029)
Change in allowance for credit losses on available for sale securities			(19,660)	(1,328)
Noninsurance revenue		2,521,092	2,328,848	1,643,999
Total revenues		8,896,719	9,040,716	6,887,160
Costs and Expenses
Net loss and loss adjustment expenses		4,339,046	3,686,435	3,520,431
Commissions, brokerage and other underwriting expenses	[1]	1,789,820	1,758,698	1,617,333
Other operating expenses		2,429,262	2,263,326	1,579,309
Corporate administration		48,581	74,830	15,732
Amortization of intangible assets		44,241	33,834	24,039
Interest expense		88,152	99,957	90,724
Total costs and expenses		8,739,102	7,917,080	6,847,568
Earnings (losses) before income taxes		157,617	1,123,636	39,592
Income taxes		30,734	233,435	(15,062)
Net earnings		126,883	890,201	54,654
Net earnings attributable to noncontrolling interests		25,129	32,400	15,115
Net earnings attributable to Alleghany stockholders		101,754	857,801	39,539
Net earnings		126,883	890,201	54,654
Other comprehensive income (loss):
Change in unrealized gains (losses), net of deferred taxes		277,040	402,957	(212,539)
Change in unrealized currency translation adjustment, net of deferred taxes		22,366	2,933	(21,902)
Retirement plans		14,754	(12,153)	3,071
Comprehensive income (loss)		407,935	1,263,554	(164,959)
Comprehensive income attributable to noncontrolling interests		25,129	32,400	15,115
Comprehensive income (loss) attributable to Alleghany stockholders		$ 382,806	$ 1,231,154	$ (180,074)
Basic earnings per share attributable to Alleghany stockholders		$ 7.14	$ 59.44	$ 2.62
Diluted earnings per share attributable to Alleghany stockholders		$ 7.04	$ 59.39	$ 2.62
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		$ (33,108)	$ (20,384)	$ 11,757

[1] Includes amortization associated with deferred acquisition costs of $1,457.0 million, $1,392.8 million and $1,271.4 million for the years ended December 31, 2020, 2019 and 2018, respectively.