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, 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