Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjusted Balance	Common Stock	Contributed Capital	Accumulated Other Comprehensive Income (loss)	Accumulated Other Comprehensive Income (loss) Cumulative Effect, Period of Adoption, Adjusted Balance	Treasury Stock	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjusted Balance
Beginning Balance at Dec. 31, 2017	$ 8,514,063		$ 17,460	$ 3,612,109	$ 618,118	$ (600,508)	$ (824,906)	$ 5,091,282	$ 600,508
Net earnings (losses)	39,539							39,539
Other comprehensive income (loss), net of tax:
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 noncontrolling interests balance at Dec. 31, 2017	106,530
Redeemable Non-controlling Interests
Net earnings attributable to redeemable noncontrolling interests	15,115
Comprehensive income (loss) attributable to noncontrolling interests	15,115
Other net changes to redeemable noncontrolling interests	48,117
Redeemable noncontrolling interests balance at Dec. 31, 2018	169,762
Net earnings (losses)	857,801							857,801
Other comprehensive income (loss), net of tax:
Retirement plans	(12,153)				(12,153)
Change in unrealized appreciation of investments, net	382,573				382,573
Change in unrealized currency translation adjustment, net	2,933				2,933
Comprehensive income (loss) attributable to Alleghany stockholders	1,231,154				373,353			857,801
Treasury stock repurchase	(144,422)						(144,422)
Other, net	(2,708)			(4,192)			1,484
Ending Balance at Dec. 31, 2019	8,776,734	$ (3,570)	17,460	3,608,638	171,350		(1,455,877)	6,435,163	$ (3,570)
Redeemable Non-controlling Interests
Net earnings attributable to redeemable noncontrolling interests	32,400
Comprehensive income (loss) attributable to noncontrolling interests	32,400
Other net changes to redeemable noncontrolling interests	2,591
Redeemable noncontrolling interests balance at Dec. 31, 2019	204,753
Net earnings (losses)	101,754							101,754
Other comprehensive income (loss), net of tax:
Retirement plans	14,754				14,754
Change in unrealized appreciation of investments, net	243,932				243,932
Change in unrealized currency translation adjustment, net	22,366				22,366
Comprehensive income (loss) attributable to Alleghany stockholders	382,806				281,052			101,754
Dividends paid	(215,013)							(215,013)
Treasury stock repurchase	(194,762)						(194,762)
Other, net	9,525			4,816			4,709
Ending Balance at Dec. 31, 2020	8,755,720		$ 17,460	$ 3,613,454	$ 452,402		$ (1,645,930)	$ 6,318,334
Redeemable Non-controlling Interests
Net earnings attributable to redeemable noncontrolling interests	25,129
Comprehensive income (loss) attributable to noncontrolling interests	25,129
Other net changes to redeemable noncontrolling interests	3,927
Redeemable noncontrolling interests balance at Dec. 31, 2020	$ 233,809