Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjusted Balance	Common Stock	Common Stock Cumulative Effect, Period of Adoption, Adjusted Balance	Contributed Capital	Contributed Capital Cumulative Effect, Period of Adoption, Adjusted Balance	Accumulated Other Comprehensive Income (loss)	Accumulated Other Comprehensive Income (loss) Cumulative Effect, Period of Adoption, Adjusted Balance	Treasury Stock	Treasury Stock Cumulative Effect, Period of Adoption, Adjusted Balance	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjusted Balance
Beginning Balance at Dec. 31, 2018	$ 7,692,710		$ 17,460		$ 3,612,830		$ (202,003)		$ (1,312,939)		$ 5,577,362
Net (losses) earnings	440,227		0		0		0		0		440,227
Other comprehensive income (loss), net of tax:
Retirement plans	937		0		0		937		0		0
Change in unrealized appreciation of investments, net	187,794		0		0		187,794		0		0
Change in unrealized currency translation adjustment, net	(885)		0		0		(885)		0		0
Comprehensive (loss) income attributable to Alleghany stockholders	628,073		0		0		187,846		0		440,227
Treasury stock repurchase	(80,486)		0		0		0		(80,486)		0
Other, net	(3,810)		0		(4,176)		0		366		0
Ending Balance at Mar. 31, 2019	8,236,487		17,460		3,608,654		(14,157)		(1,393,059)		6,017,589
Redeemable noncontrolling interests balance at Dec. 31, 2018	169,762
Redeemable Non-controlling Interests
Net (losses) earnings attributable to redeemable noncontrolling interests	7,675
Comprehensive (loss) income attributable to noncontrolling interests	7,675
Other net changes to redeemable noncontrolling interests	(1,716)
Redeemable noncontrolling interests balance at Mar. 31, 2019	175,721
Beginning Balance at Dec. 31, 2018	7,692,710		17,460		3,612,830		(202,003)		(1,312,939)		5,577,362
Net (losses) earnings	826,118
Other comprehensive income (loss), net of tax:
Retirement plans	1,363
Change in unrealized currency translation adjustment, net	(8,054)
Comprehensive (loss) income attributable to Alleghany stockholders	1,251,386
Treasury stock repurchase	(112,400)
Ending Balance at Sep. 30, 2019	8,828,658		17,460		3,608,353		223,266		(1,423,901)		6,403,480
Redeemable noncontrolling interests balance at Dec. 31, 2018	169,762
Redeemable Non-controlling Interests
Net (losses) earnings attributable to redeemable noncontrolling interests	27,909
Comprehensive (loss) income attributable to noncontrolling interests	27,909
Redeemable noncontrolling interests balance at Sep. 30, 2019	180,853
Beginning Balance at Mar. 31, 2019	8,236,487		17,460		3,608,654		(14,157)		(1,393,059)		6,017,589
Net (losses) earnings	295,459		0		0		0		0		295,459
Other comprehensive income (loss), net of tax:
Retirement plans	104		0		0		104		0		0
Change in unrealized appreciation of investments, net	162,842		0		0		162,842		0		0
Change in unrealized currency translation adjustment, net	3,175		0		0		3,175		0		0
Comprehensive (loss) income attributable to Alleghany stockholders	461,580		0		0		166,121		0		295,459
Treasury stock repurchase	(12,666)		0		0		0		(12,666)		0
Other, net	513		0		(585)		0		1,098		0
Ending Balance at Jun. 30, 2019	8,685,914		17,460		3,608,069		151,964		(1,404,627)		6,313,048
Redeemable noncontrolling interests balance at Mar. 31, 2019	175,721
Redeemable Non-controlling Interests
Net (losses) earnings attributable to redeemable noncontrolling interests	9,374
Comprehensive (loss) income attributable to noncontrolling interests	9,374
Other net changes to redeemable noncontrolling interests	(9,523)
Redeemable noncontrolling interests balance at Jun. 30, 2019	175,572
Net (losses) earnings	90,432		0		0		0		0		90,432
Other comprehensive income (loss), net of tax:
Retirement plans	322		0		0		322		0		0
Change in unrealized appreciation of investments, net	81,323		0		0		81,323		0		0
Change in unrealized currency translation adjustment, net	(10,343)		0		0		(10,343)		0		0
Comprehensive (loss) income attributable to Alleghany stockholders	161,734		0		0		71,302		0		90,432
Treasury stock repurchase	(19,294)		0		0		0		(19,294)		0
Other, net	304		0		284		0		20		0
Ending Balance at Sep. 30, 2019	8,828,658		17,460		3,608,353		223,266		(1,423,901)		6,403,480
Redeemable Non-controlling Interests
Net (losses) earnings attributable to redeemable noncontrolling interests	10,860
Comprehensive (loss) income attributable to noncontrolling interests	10,860
Other net changes to redeemable noncontrolling interests	(5,579)
Redeemable noncontrolling interests balance at Sep. 30, 2019	180,853
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 (losses) earnings	(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 (loss) income 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 (losses) earnings attributable to redeemable noncontrolling interests	(254)
Comprehensive (loss) income attributable to 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 (losses) earnings	(57,318)
Other comprehensive income (loss), net of tax:
Retirement plans	1,356
Change in unrealized currency translation adjustment, net	13,190
Comprehensive (loss) income attributable to Alleghany stockholders	146,642
Treasury stock repurchase	(114,300)
Ending Balance at Sep. 30, 2020	8,596,395		17,460		3,609,789		375,310		(1,565,426)		6,159,262
Redeemable noncontrolling interests balance at Dec. 31, 2019	204,753
Redeemable Non-controlling Interests
Net (losses) earnings attributable to redeemable noncontrolling interests	9,953
Comprehensive (loss) income attributable to noncontrolling interests	9,953
Redeemable noncontrolling interests balance at Sep. 30, 2020	236,879
Beginning Balance at Mar. 31, 2020	7,857,062		17,460		3,610,137		(129,343)		(1,496,554)		5,855,362
Net (losses) earnings	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 (loss) income 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 (losses) earnings attributable to redeemable noncontrolling interests	(918)
Comprehensive (loss) income attributable to noncontrolling interests	(918)
Other net changes to redeemable noncontrolling interests	20,463
Redeemable noncontrolling interests balance at Jun. 30, 2020	227,906
Net (losses) earnings	126,522		0		0		0		0		126,522
Other comprehensive income (loss), net of tax:
Retirement plans	420		0		0		420		0		0
Change in unrealized appreciation of investments, net	70,788		0		0		70,788		0		0
Change in unrealized currency translation adjustment, net	12,109		0		0		12,109		0		0
Comprehensive (loss) income attributable to Alleghany stockholders	209,839		0		0		83,317		0		126,522
Treasury stock repurchase	(69,989)		0		0		0		(69,989)		0
Other, net	306		0		268		0		38		0
Ending Balance at Sep. 30, 2020	8,596,395		$ 17,460		$ 3,609,789		$ 375,310		$ (1,565,426)		$ 6,159,262
Redeemable Non-controlling Interests
Net (losses) earnings attributable to redeemable noncontrolling interests	11,125
Comprehensive (loss) income attributable to noncontrolling interests	11,125
Other net changes to redeemable noncontrolling interests	(2,152)
Redeemable noncontrolling interests balance at Sep. 30, 2020	$ 236,879