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, 2018	$ 7,692,710		$ 17,460		$ 3,612,830		$ (202,003)		$ (1,312,939)		$ 5,577,362
Net earnings (losses)	857,801		0		0		0		0		857,801
Other comprehensive income (loss), net of tax:
Retirement plans	(12,153)		0		0		(12,153)		0		0
Change in unrealized appreciation of investments, net	382,573		0		0		382,573		0		0
Change in unrealized currency translation adjustment, net	2,933		0		0		2,933		0		0
Comprehensive income attributable to Alleghany stockholders	1,231,154		0		0		373,353		0		857,801
Treasury stock repurchase	(144,422)		0		0		0		(144,422)		0
Other, net	(2,708)		0		(4,192)		0		1,484		0
Ending 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)
Redeemable noncontrolling interests balance at Dec. 31, 2018	169,762
Redeemable Non-controlling Interests
Net earnings attributable to redeemable noncontrolling interests	32,400
Comprehensive (loss) income 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		0		0		0		0		101,754
Other comprehensive income (loss), net of tax:
Retirement plans	14,754		0		0		14,754		0		0
Change in unrealized appreciation of investments, net	243,932		0		0		243,932		0		0
Change in unrealized currency translation adjustment, net	22,366		0		0		22,366		0		0
Comprehensive income attributable to Alleghany stockholders	382,806		0		0		281,052		0		101,754
Dividends paid	(215,013)		0		0		0		0		(215,013)
Treasury stock repurchase	(194,762)		0		0		0		(194,762)		0
Other, net	9,525		0		4,816		0		4,709		0
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 (loss) income 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
Net earnings (losses)	1,034,892		0		0		0		0		1,034,892
Other comprehensive income (loss), net of tax:
Retirement plans	(463)		0		0		(463)		0		0
Change in unrealized appreciation of investments, net	(305,812)		0		0		(305,812)		0		0
Change in unrealized currency translation adjustment, net	(4,305)		0		0		(4,305)		0		0
Comprehensive income attributable to Alleghany stockholders	724,312		0		0		(310,580)		0		1,034,892
Treasury stock repurchase	(290,521)		0		0		0		(290,521)		0
Other, net	(2,629)		0		(4,549)		0		1,920		0
Ending Balance at Dec. 31, 2021	9,186,882		$ 17,460		$ 3,608,905		$ 141,822		$ (1,934,531)		$ 7,353,226
Redeemable Non-controlling Interests
Net earnings attributable to redeemable noncontrolling interests	95,886
Comprehensive (loss) income attributable to noncontrolling interests	95,886
Other net changes to redeemable noncontrolling interests	(12,349)
Redeemable noncontrolling interests balance at Dec. 31, 2021	$ 317,346