Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Contributed Capital	Accumulated Other Comprehensive Income (loss)	Treasury Stock	Retained Earnings
Beginning Balance at Dec. 31, 2020	$ 8,755,720	$ 17,460	$ 3,613,454	$ 452,402	$ (1,645,930)	$ 6,318,334
Net earnings	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 (loss) income 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
Retirement plans attributable to redeemable noncontrolling interests	0
Change in unrealized appreciation of investments, net, attributable to redeemable noncontrolling interests	0
Change in unrealized currency translation adjustment, net, attributable to redeemable noncontrolling interests	0
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	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	633,692
Other comprehensive income (loss), net of tax:
Retirement plans	(852)
Change in unrealized currency translation adjustment, net	213
Comprehensive (loss) income 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), Net of Tax, Attributable to Noncontrolling Interest	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	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			81,141
Change in unrealized currency translation adjustment, net	(2,249)			(2,249)
Comprehensive (loss) income attributable to Alleghany stockholders	482,656		0	79,005	0	403,651
Treasury stock repurchase	(35,925)	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
Retirement plans attributable to redeemable noncontrolling interests	0
Change in unrealized appreciation of investments, net, attributable to redeemable noncontrolling interests	0
Change in unrealized currency translation adjustment, net, attributable to redeemable noncontrolling interests	0
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	15,938
Other net changes to redeemable noncontrolling interests	(8,779)
Redeemable noncontrolling interests balance at Jun. 30, 2021	240,722
Beginning Balance at Dec. 31, 2021	9,186,882	17,460	3,608,905	141,822	(1,934,531)	7,353,226
Net earnings	125,681	0	0	0	0	125,681
Other comprehensive income (loss), net of tax:
Retirement plans	130	0	0	130	0	0
Change in unrealized appreciation of investments, net	(605,889)	0	0	(605,889)	0	0
Change in unrealized currency translation adjustment, net	(523)	0	0	(523)	0	0
Comprehensive (loss) income attributable to Alleghany stockholders	(480,601)	0	0	(606,282)	0	125,681
Treasury stock repurchase	(95,987)	0	0	0	(95,987)	0
Other, net	(14,921)	0	(15,535)	0	614	0
Ending Balance at Mar. 31, 2022	8,595,373	17,460	3,593,370	(464,460)	(2,029,904)	7,478,907
Redeemable noncontrolling interests balance at Dec. 31, 2021	317,346
Redeemable Non-controlling Interests
Net earnings (losses) attributable to redeemable noncontrolling interests	43,212
Retirement plans attributable to redeemable noncontrolling interests	0
Change in unrealized appreciation of investments, net, attributable to redeemable noncontrolling interests	0
Change in unrealized currency translation adjustment, net, attributable to redeemable noncontrolling interests	0
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	43,212
Other net changes to redeemable noncontrolling interests	5,235
Redeemable noncontrolling interests balance at Mar. 31, 2022	365,793
Beginning Balance at Dec. 31, 2021	9,186,882	17,460	3,608,905	141,822	(1,934,531)	7,353,226
Net earnings	(45,933)
Other comprehensive income (loss), net of tax:
Retirement plans	239
Change in unrealized currency translation adjustment, net	(26,230)
Comprehensive (loss) income attributable to Alleghany stockholders	(1,152,036)
Treasury stock repurchase	(96,000)
Ending Balance at Jun. 30, 2022	7,906,670	17,460	3,575,815	(964,281)	(2,029,617)	7,307,293
Redeemable noncontrolling interests balance at Dec. 31, 2021	317,346
Redeemable Non-controlling Interests
Net earnings (losses) attributable to redeemable noncontrolling interests	90,939
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	90,939
Redeemable noncontrolling interests balance at Jun. 30, 2022	395,449
Beginning Balance at Mar. 31, 2022	8,595,373	17,460	3,593,370	(464,460)	(2,029,904)	7,478,907
Net earnings	(171,614)	0	0	0	0	(171,614)
Other comprehensive income (loss), net of tax:
Retirement plans	109	0	0	109	0	0
Change in unrealized appreciation of investments, net	(474,223)	0	0	(474,223)	0	0
Change in unrealized currency translation adjustment, net	(25,707)	0	0	(25,707)	0	0
Comprehensive (loss) income attributable to Alleghany stockholders	(671,435)	0	0	(499,821)	0	(171,614)
Treasury stock repurchase	0	0	0	0	0	0
Other, net	(17,268)	0	(17,555)	0	287	0
Ending Balance at Jun. 30, 2022	7,906,670	$ 17,460	$ 3,575,815	$ (964,281)	$ (2,029,617)	$ 7,307,293
Redeemable noncontrolling interests balance at Mar. 31, 2022	365,793
Redeemable Non-controlling Interests
Net earnings (losses) attributable to redeemable noncontrolling interests	47,727
Retirement plans attributable to redeemable noncontrolling interests	0
Change in unrealized appreciation of investments, net, attributable to redeemable noncontrolling interests	0
Change in unrealized currency translation adjustment, net, attributable to redeemable noncontrolling interests	0
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	47,727
Other net changes to redeemable noncontrolling interests	(18,071)
Redeemable noncontrolling interests balance at Jun. 30, 2022	$ 395,449