Consolidated Statements of Earnings and Comprehensive Income - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Net premiums earned		$ 4,954,990	$ 4,975,777	$ 4,230,286
Net investment income		451,016	438,455	438,817
Net realized capital gains		107,222	63,205	213,897
Other than temporary impairment losses		(16,871)	(45,165)	(133,868)
Other revenue		928,298	698,747	250,346
Total revenues		6,424,655	6,131,019	4,999,478
Costs and Expenses
Net loss and loss adjustment expenses		3,620,197	2,917,166	2,339,790
Commissions, brokerage and other underwriting expenses	[1]	1,651,177	1,657,251	1,423,889
Other operating expenses		967,104	765,226	342,361
Corporate administration		46,998	42,960	46,503
Amortization of intangible assets		19,419	19,012	(2,211)
Interest expense		83,070	81,599	91,778
Total costs and expenses		6,387,965	5,483,214	4,242,110
(Losses) earnings before income taxes		36,690	647,805	757,368
Income taxes		(63,802)	187,141	195,173
Net earnings		100,492	460,664	562,195
Net earnings attributable to noncontrolling interest		10,359	3,743	1,880
Net earnings attributable to Alleghany stockholders		90,133	456,921	560,315
Net earnings		100,492	460,664	562,195
Other comprehensive income:
Change in unrealized gains (losses), net of deferred taxes of $280,526, $36,468 and ($83,332) for 2017, 2016 and 2015, respectively		520,976	67,726	(154,759)
Less: reclassification for net realized capital gains and other than temporary impairment losses, net of taxes of ($25,806), ($36,281) and ($37,044) for 2017, 2016 and 2015, respectively		(47,925)	(67,380)	(68,796)
Change in unrealized currency translation adjustment, net of deferred taxes of $14,344, ($3,889) and ($7,940) for 2017, 2016 and 2015, respectively		26,639	(7,223)	(14,746)
Retirement plans		(3,755)	(112)	990
Comprehensive income		596,427	453,675	324,884
Comprehensive income attributable to noncontrolling interest		10,359	3,743	1,880
Comprehensive (loss)income attributable to Alleghany stockholders		$ 586,068	$ 449,932	$ 323,004
Basic earnings per share attributable to Alleghany stockholders		$ 5.85	$ 29.60	$ 35.14
Diluted earnings per share attributable to Alleghany stockholders		$ 5.85	$ 29.59	$ 35.13

[1] Includes amortization associated with deferred acquisition costs of $1,261.7 million, $1,253.2 million and $1,024.5 million for the years ended December 31, 2017, 2016 and 2015, respectively.