Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Contributed Capital	Accumulated Other Comprehensive Income	Treasury Stock	Retained Earnings
Beginning Balance at Dec. 31, 2015	$ 7,554,707	$ 17,460	$ 3,611,631	$ 116,273	$ (747,784)	$ 4,557,127
Net earnings	456,921					456,921
Other comprehensive income (loss), net of tax:
Retirement plans	(112)			(112)
Change in unrealized appreciation of investments, net	346			346
Change in unrealized currency translation adjustment, net	(7,223)			(7,223)
Comprehensive income (loss) attributable to Alleghany stockholders	449,932			(6,989)		456,921
Treasury stock repurchase	(68,320)				(68,320)
Other, net	3,626		362		3,264
Ending Balance at Dec. 31, 2016	7,939,945	17,460	3,611,993	109,284	(812,840)	5,014,048
Redeemable noncontrolling interests balance at Dec. 31, 2015	25,719
Redeemable non-controlling Interest
Net earnings attributable to redeemable noncontrolling interest	3,743
Comprehensive income (loss) attributable to noncontrolling interest	3,743
Other net changes to redeemable noncontrolling interest	45,258
Redeemable noncontrolling interests balance at Dec. 31, 2016	74,720
Other comprehensive income (loss), net of tax:
Cumulative effect of adoption of new accounting pronouncements				12,900
Cumulative effect of adoption of new accounting pronouncements | Accounting Standards Update 2017-08				12,899		(12,899)
Net earnings	90,133					90,133
Retirement plans	(3,755)			(3,755)
Change in unrealized appreciation of investments, net	473,051			473,051
Change in unrealized currency translation adjustment, net	26,639			26,639
Comprehensive income (loss) attributable to Alleghany stockholders	586,068			495,935		90,133
Treasury stock repurchase	(16,048)				(16,048)
Other, net	4,098		116		3,982
Ending Balance at Dec. 31, 2017	8,514,063	17,460	3,612,109	618,118	(824,906)	5,091,282
Redeemable non-controlling Interest
Net earnings attributable to redeemable noncontrolling interest	10,359
Comprehensive income (loss) attributable to noncontrolling interest	10,359
Other net changes to redeemable noncontrolling interest	21,451
Redeemable noncontrolling interests balance at Dec. 31, 2017	106,530
Other comprehensive income (loss), net of tax:
Cumulative effect of adoption of new accounting pronouncements				(600,508)		600,508
Cumulative effect of adoption of new accounting pronouncements | Accounting Standards Update 2017-08				135,100
Cumulative effect of adoption of new accounting pronouncements | Accounting Standards Update 2016-01				(735,600)		735,558
Cumulative effect of adoption of new accounting pronouncements | Accounting Standards Update 2018-02				135,050		(135,050)
Net earnings	39,539					39,539
Retirement plans	3,071			3,071
Change in unrealized appreciation of investments, net	(200,782)			(200,782)
Change in unrealized currency translation adjustment, net	(21,902)			(21,902)
Comprehensive income (loss) attributable to Alleghany stockholders	(180,074)			(219,613)		39,539
Dividends paid	(153,967)					(153,967)
Treasury stock repurchase	(491,633)				(491,633)
Other, net	4,321		721		3,600
Ending Balance at Dec. 31, 2018	7,692,710	$ 17,460	$ 3,612,830	$ (202,003)	$ (1,312,939)	$ 5,577,362
Redeemable non-controlling Interest
Net earnings attributable to redeemable noncontrolling interest	15,115
Comprehensive income (loss) attributable to noncontrolling interest	15,115
Other net changes to redeemable noncontrolling interest	48,117
Redeemable noncontrolling interests balance at Dec. 31, 2018	$ 169,762