Consolidated Statements of Earnings and Comprehensive Income - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues
Net premiums earned		$ 5,478,143	$ 4,976,190	$ 4,954,990
Net investment income		550,241	500,534	451,016
Change in the fair value of equity securities		709,695	(228,994)
Net realized capital gains		(6,551)	(3,241)	107,222
Other than temporary impairment losses		(19,660)	(1,328)
Other than temporary impairment losses				(16,871)
Noninsurance revenue		2,328,848	1,643,999	928,298
Total revenues		9,040,716	6,887,160	6,424,655
Costs and Expenses
Net loss and loss adjustment expenses		3,686,435	3,520,431	3,620,197
Commissions, brokerage and other underwriting expenses	[1]	1,758,698	1,617,333	1,651,177
Other operating expenses		2,263,326	1,579,309	967,104
Corporate administration		74,830	15,732	46,998
Amortization of intangible assets		33,834	24,039	19,419
Interest expense		99,957	90,724	83,070
Total costs and expenses		7,917,080	6,847,568	6,387,965
Earnings (losses) before income taxes		1,123,636	39,592	36,690
Income taxes		233,435	(15,062)	(63,802)
Net earnings		890,201	54,654	100,492
Net earnings attributable to noncontrolling interests		32,400	15,115	10,359
Net earnings attributable to Alleghany stockholders		857,801	39,539	90,133
Net earnings		890,201	54,654	100,492
Other comprehensive income (loss):
Change in unrealized gains (losses), net of deferred taxes		402,957	(212,539)	520,976
Less: reclassification for net realized capital gains and other than temporary impairments, net of taxes		(20,384)	11,757	(47,925)
Change in unrealized currency translation adjustment, net of deferred taxes		2,933	(21,902)	26,639
Retirement plans		(12,153)	3,071	(3,755)
Comprehensive income (loss)		1,263,554	(164,959)	596,427
Comprehensive income attributable to noncontrolling interests		32,400	15,115	10,359
Comprehensive income (loss) attributable to Alleghany stockholders		$ 1,231,154	$ (180,074)	$ 586,068
Basic earnings per share attributable to Alleghany stockholders		$ 59.44	$ 2.62	$ 5.85
Diluted earnings per share attributable to Alleghany stockholders		$ 59.39	$ 2.62	$ 5.85

[1] Includes amortization associated with deferred acquisition costs of $1,392.8 million, $1,271.4 million and $1,261.7 million for the years ended December 31, 2019, 2018 and 2017, respectively.