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, 2018	$ 7,692,710	$ 17,460	$ 3,612,830	$ (202,003)	$ (1,312,939)	$ 5,577,362
Net (losses) earnings	440,227					440,227
Other comprehensive income (loss), net of tax:
Retirement plans	937			937
Change in unrealized appreciation of investments, net	187,794			187,794
Change in unrealized currency translation adjustment, net	(885)			(885)
Comprehensive (loss) income attributable to Alleghany stockholders	628,073			187,846		440,227
Treasury stock repurchase	(80,486)				(80,486)
Other, net	(3,810)		(4,176)		366
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, 2019	8,776,734	17,460	3,608,638	171,350	(1,455,877)	6,435,163
Cumulative effect of adoption of new accounting pronouncements at Dec. 31, 2019	(3,570)					(3,570)
Net (losses) earnings	(361,218)					(361,218)
Other comprehensive income (loss), net of tax:
Retirement plans	517			517
Change in unrealized appreciation of investments, net	(287,477)			(287,477)
Change in unrealized currency translation adjustment, net	(13,733)			(13,733)
Comprehensive (loss) income attributable to Alleghany stockholders	(661,911)			(300,693)		(361,218)
Dividends paid	(215,013)					(215,013)
Treasury stock repurchase	(44,268)				(44,268)
Other, net	5,090		1,499		3,591
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