Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Contributed Capital	Accumulated Other Comprehensive Income	Treasury Stock	Retained Earnings
Beginning Balance at Dec. 31, 2013	$ 6,923,757	$ 17,460	$ 3,613,151	$ 186,930	$ (213,911)	$ 3,320,127
Net earnings	679,239					679,239
Other comprehensive income (loss), net of tax:
Retirement plans	(10,473)			(10,473)
Change in unrealized appreciation of investments, net	216,988			216,988
Change in unrealized currency translation adjustment, net	(39,861)			(39,861)
Comprehensive income attributable to Alleghany stockholders	845,893			166,654		679,239
Dividends paid	0	0	0	0	0	0
Treasury stock repurchase	(300,478)				(300,478)
Other, net	4,256		(2,434)		6,690
Ending Balance at Dec. 31, 2014	7,473,428	17,460	3,610,717	353,584	(507,699)	3,999,366
Redeemable noncontrolling interests balance at Dec. 31, 2013	23,764
Other comprehensive income (loss), net of tax:
Net earnings attributable to redeemable noncontrolling interest	893
Comprehensive income attributable to noncontrolling interest	893
Other net changes to redeemable noncontrolling interest	(16,041)
Redeemable noncontrolling interests balance at Dec. 31, 2014	8,616
Net earnings	560,315					560,315
Other comprehensive income (loss), net of tax:
Retirement plans	990			990
Change in unrealized appreciation of investments, net	(223,555)			(223,555)
Change in unrealized currency translation adjustment, net	(14,746)			(14,746)
Comprehensive income attributable to Alleghany stockholders	323,004			(237,311)		560,315
Dividends paid	0	0	0	0	0	0
Treasury stock repurchase	(243,814)				(243,814)
Other, net	2,089		914		3,729	(2,554)
Ending Balance at Dec. 31, 2015	7,554,707	17,460	3,611,631	116,273	(747,784)	4,557,127
Other comprehensive income (loss), net of tax:
Net earnings attributable to redeemable noncontrolling interest	1,880
Comprehensive income attributable to noncontrolling interest	1,880
Other net changes to redeemable noncontrolling interest	15,223
Redeemable noncontrolling interests balance at Dec. 31, 2015	25,719
Net earnings	456,921					456,921
Other comprehensive income (loss), net of tax:
Retirement plans	(112)			(112)
Change in unrealized appreciation of investments, net	346			346
Change in unrealized currency translation adjustment, net	(7,223)			(7,223)
Comprehensive income attributable to Alleghany stockholders	449,932			(6,989)		456,921
Dividends paid	0	0	0	0	0	0
Treasury stock repurchase	(68,320)				(68,320)
Other, net	3,626		362		3,264
Ending Balance at Dec. 31, 2016	7,939,945	$ 17,460	$ 3,611,993	$ 109,284	$ (812,840)	$ 5,014,048
Other comprehensive income (loss), net of tax:
Net earnings attributable to redeemable noncontrolling interest	3,743
Comprehensive income attributable to noncontrolling interest	3,743
Other net changes to redeemable noncontrolling interest	45,258
Redeemable noncontrolling interests balance at Dec. 31, 2016	$ 74,720