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, 2017	$ 8,514,063	$ 17,460	$ 3,612,109	$ 618,118	$ (824,906)	$ 5,091,282
Cumulative effect of adoption of new accounting pronouncements on beginning balance at Dec. 31, 2017				(600,508)		600,508
Net earnings	171,575					171,575
Other comprehensive income (loss), net of tax:
Retirement plans	(1,322)			(1,322)
Change in unrealized appreciation of investments, net	(144,979)			(144,979)
Change in unrealized currency translation adjustment, net	5,100			5,100
Comprehensive income attributable to Alleghany stockholders	30,374			(141,201)		171,575
Dividends paid	(153,967)					(153,967)
Treasury stock repurchase	(21,268)				(21,268)
Other, net	4,198		1,521	(29)	2,677	29
Ending Balance at Mar. 31, 2018	8,373,400	17,460	3,613,630	(123,620)	(843,497)	5,709,427
Redeemable noncontrolling interests balance at Dec. 31, 2017	106,530
Redeemable Non-controlling Interest
Net earnings attributable to redeemable noncontrolling interest	(393)
Comprehensive income (loss) attributable to noncontrolling interest	(393)
Other net changes to redeemable noncontrolling interest	31,494
Redeemable noncontrolling interests balance at Mar. 31, 2018	137,631
Beginning Balance at Dec. 31, 2017	8,514,063	17,460	3,612,109	618,118	(824,906)	5,091,282
Cumulative effect of adoption of new accounting pronouncements on beginning balance at Dec. 31, 2017				(600,508)		600,508
Net earnings	466,691
Other comprehensive income (loss), net of tax:
Retirement plans	(1,113)
Change in unrealized currency translation adjustment, net	(5,423)
Comprehensive income attributable to Alleghany stockholders	276,766
Treasury stock repurchase	(236,100)
Ending Balance at Jun. 30, 2018	8,404,389	17,460	3,612,615	(172,307)	(1,057,885)	6,004,506
Redeemable noncontrolling interests balance at Dec. 31, 2017	106,530
Redeemable Non-controlling Interest
Net earnings attributable to redeemable noncontrolling interest	2,895
Comprehensive income (loss) attributable to noncontrolling interest	2,895
Redeemable noncontrolling interests balance at Jun. 30, 2018	135,372
Beginning Balance at Mar. 31, 2018	8,373,400	17,460	3,613,630	(123,620)	(843,497)	5,709,427
Net earnings	295,116					295,116
Other comprehensive income (loss), net of tax:
Retirement plans	209			209
Change in unrealized appreciation of investments, net	(38,410)			(38,410)
Change in unrealized currency translation adjustment, net	(10,523)			(10,523)
Comprehensive income attributable to Alleghany stockholders	246,392			(48,724)		295,116
Treasury stock repurchase	(214,835)				(214,835)
Other, net	(568)		(1,015)	37	447	(37)
Ending Balance at Jun. 30, 2018	8,404,389	17,460	3,612,615	(172,307)	(1,057,885)	6,004,506
Redeemable noncontrolling interests balance at Mar. 31, 2018	137,631
Redeemable Non-controlling Interest
Net earnings attributable to redeemable noncontrolling interest	3,288
Comprehensive income (loss) attributable to noncontrolling interest	3,288
Other net changes to redeemable noncontrolling interest	(5,547)
Redeemable noncontrolling interests balance at Jun. 30, 2018	135,372
Beginning Balance at Dec. 31, 2018	7,692,710	17,460	3,612,830	(202,003)	(1,312,939)	5,577,362
Net 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 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 Interest
Net earnings attributable to redeemable noncontrolling interest	7,675
Comprehensive income (loss) attributable to noncontrolling interest	7,675
Other net changes to redeemable noncontrolling interest	(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 earnings	735,686
Other comprehensive income (loss), net of tax:
Retirement plans	1,041
Change in unrealized currency translation adjustment, net	2,290
Comprehensive income attributable to Alleghany stockholders	1,089,653
Treasury stock repurchase	(93,200)
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 Dec. 31, 2018	169,762
Redeemable Non-controlling Interest
Net earnings attributable to redeemable noncontrolling interest	17,049
Comprehensive income (loss) attributable to noncontrolling interest	17,049
Redeemable noncontrolling interests balance at Jun. 30, 2019	175,572
Beginning Balance at Mar. 31, 2019	8,236,487	17,460	3,608,654	(14,157)	(1,393,059)	6,017,589
Net earnings	295,459					295,459
Other comprehensive income (loss), net of tax:
Retirement plans	104			104
Change in unrealized appreciation of investments, net	162,842			162,842
Change in unrealized currency translation adjustment, net	3,175			3,175
Comprehensive income attributable to Alleghany stockholders	461,580			166,121		295,459
Treasury stock repurchase	(12,666)				(12,666)
Other, net	513		(585)		1,098
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 Interest
Net earnings attributable to redeemable noncontrolling interest	9,374
Comprehensive income (loss) attributable to noncontrolling interest	9,374
Other net changes to redeemable noncontrolling interest	(9,523)
Redeemable noncontrolling interests balance at Jun. 30, 2019	$ 175,572