Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Common Stock Cumulative Effect, Period of Adoption, Adjustment	Contributed Capital	Contributed Capital Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (loss)	Accumulated Other Comprehensive Income (loss) Cumulative Effect, Period of Adoption, Adjustment	Treasury Stock	Treasury Stock Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment
Beginning Balance at Dec. 31, 2019	$ 8,776,734	$ (3,570)	$ 17,460	$ 0	$ 3,608,638	$ 0	$ 171,350	$ 0	$ (1,455,877)	$ 0	$ 6,435,163	$ (3,570)
Net earnings (losses)	(361,218)		0		0		0		0		(361,218)
Other comprehensive income (loss), net of tax:
Retirement plans	517		0		0		517		0		0
Change in unrealized appreciation of investments, net	(287,477)		0		0		(287,477)		0		0
Change in unrealized currency translation adjustment, net	(13,733)		0		0		(13,733)		0		0
Comprehensive income (loss) attributable to Alleghany stockholders	(661,911)		0		0		(300,693)		0		(361,218)
Dividends paid	(215,013)		0		0		0		0		(215,013)
Treasury stock repurchase	(44,268)		0		0		0		(44,268)		0
Other, net	5,090		0		1,499		0		3,591		0
Ending Balance at Mar. 31, 2020	7,857,062		17,460		3,610,137		(129,343)		(1,496,554)		5,855,362
Redeemable noncontrolling interests balance at Dec. 31, 2019	204,753
Redeemable Non-controlling Interests
Net earnings (losses) attributable to redeemable noncontrolling interests	(254)
Comprehensive income (loss) attributable to redeemable noncontrolling interests	(254)
Other net changes to redeemable noncontrolling interests	3,862
Redeemable noncontrolling interests balance at Mar. 31, 2020	208,361
Beginning Balance at Dec. 31, 2019	8,776,734	$ (3,570)	17,460	$ 0	3,608,638	$ 0	171,350	$ 0	(1,455,877)	$ 0	6,435,163	$ (3,570)
Net earnings (losses)	(183,840)
Other comprehensive income (loss), net of tax:
Retirement plans	936
Change in unrealized currency translation adjustment, net	1,081
Comprehensive income (loss) attributable to Alleghany stockholders	(63,196)
Treasury stock repurchase	(44,300)
Ending Balance at Jun. 30, 2020	8,456,239		17,460		3,609,521		291,993		(1,495,475)		6,032,740
Redeemable noncontrolling interests balance at Dec. 31, 2019	204,753
Redeemable Non-controlling Interests
Net earnings (losses) attributable to redeemable noncontrolling interests	(1,172)
Comprehensive income (loss) attributable to redeemable noncontrolling interests	(1,172)
Redeemable noncontrolling interests balance at Jun. 30, 2020	227,906
Beginning Balance at Mar. 31, 2020	7,857,062		17,460		3,610,137		(129,343)		(1,496,554)		5,855,362
Net earnings (losses)	177,378		0		0		0		0		177,378
Other comprehensive income (loss), net of tax:
Retirement plans	419		0		0		419		0		0
Change in unrealized appreciation of investments, net	406,103		0		0		406,103		0		0
Change in unrealized currency translation adjustment, net	14,814		0		0		14,814		0		0
Comprehensive income (loss) attributable to Alleghany stockholders	598,714		0		0		421,336		0		177,378
Treasury stock repurchase	0		0		0		0		0		0
Other, net	463		0		(616)		0		1,079		0
Ending Balance at Jun. 30, 2020	8,456,239		17,460		3,609,521		291,993		(1,495,475)		6,032,740
Redeemable noncontrolling interests balance at Mar. 31, 2020	208,361
Redeemable Non-controlling Interests
Net earnings (losses) attributable to redeemable noncontrolling interests	(918)
Comprehensive income (loss) attributable to redeemable noncontrolling interests	(918)
Other net changes to redeemable noncontrolling interests	20,463
Redeemable noncontrolling interests balance at Jun. 30, 2020	227,906
Beginning Balance at Dec. 31, 2020	8,755,720		17,460		3,613,454		452,402		(1,645,930)		6,318,334
Net earnings (losses)	230,041		0		0		0		0		230,041
Other comprehensive income (loss), net of tax:
Retirement plans	(965)		0		0		(965)		0		0
Change in unrealized appreciation of investments, net	(229,708)		0		0		(229,708)		0		0
Change in unrealized currency translation adjustment, net	2,461		0		0		2,461		0		0
Comprehensive income (loss) attributable to Alleghany stockholders	1,829		0		0		(228,212)		0		230,041
Treasury stock repurchase	(63,192)		0		0		0		(63,192)		0
Other, net	1,524		0		310		0		1,214		0
Ending Balance at Mar. 31, 2021	8,695,881		17,460		3,613,764		224,190		(1,707,908)		6,548,375
Redeemable noncontrolling interests balance at Dec. 31, 2020	233,809
Redeemable Non-controlling Interests
Net earnings (losses) attributable to redeemable noncontrolling interests	8,022
Comprehensive income (loss) attributable to redeemable noncontrolling interests	8,022
Other net changes to redeemable noncontrolling interests	(8,268)
Redeemable noncontrolling interests balance at Mar. 31, 2021	233,563
Beginning Balance at Dec. 31, 2020	8,755,720		17,460		3,613,454		452,402		(1,645,930)		6,318,334
Net earnings (losses)	633,692
Other comprehensive income (loss), net of tax:
Retirement plans	(852)
Change in unrealized currency translation adjustment, net	213
Comprehensive income (loss) attributable to Alleghany stockholders	484,486
Treasury stock repurchase	(99,100)
Ending Balance at Jun. 30, 2021	9,142,031		17,460		3,612,783		303,196		(1,743,434)		6,952,026
Redeemable noncontrolling interests balance at Dec. 31, 2020	233,809
Redeemable Non-controlling Interests
Net earnings (losses) attributable to redeemable noncontrolling interests	23,960
Comprehensive income (loss) attributable to redeemable noncontrolling interests	23,960
Redeemable noncontrolling interests balance at Jun. 30, 2021	240,722
Beginning Balance at Mar. 31, 2021	8,695,881		17,460		3,613,764		224,190		(1,707,908)		6,548,375
Net earnings (losses)	403,651		0		0		0		0		403,651
Other comprehensive income (loss), net of tax:
Retirement plans	113		0		0		113		0		0
Change in unrealized appreciation of investments, net	81,141		0		0		81,141		0		0
Change in unrealized currency translation adjustment, net	(2,249)		0		0		(2,249)		0		0
Comprehensive income (loss) attributable to Alleghany stockholders	482,656		0		0		79,005		0		403,651
Treasury stock repurchase	(35,925)		0		0		0		(35,925)		0
Other, net	(581)		0		(981)		1		399		0
Ending Balance at Jun. 30, 2021	9,142,031		$ 17,460		$ 3,612,783		$ 303,196		$ (1,743,434)		$ 6,952,026
Redeemable noncontrolling interests balance at Mar. 31, 2021	233,563
Redeemable Non-controlling Interests
Net earnings (losses) attributable to redeemable noncontrolling interests	15,938
Comprehensive income (loss) attributable to redeemable noncontrolling interests	15,938
Other net changes to redeemable noncontrolling interests	(8,779)
Redeemable noncontrolling interests balance at Jun. 30, 2021	$ 240,722