Consolidated Statements of Earnings and Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenues
Net premiums earned	$ 1,239,721	$ 1,253,515	$ 3,692,838	$ 3,736,596
Net investment income	104,663	120,603	321,857	332,326
Net realized capital gains	32,921	27,221	101,840	117,126
Other than temporary impairment losses	(6,131)	(11,729)	(13,095)	(38,216)
Other revenue	296,309	225,006	650,413	527,765
Total revenues	1,667,483	1,614,616	4,753,853	4,675,597
Costs and Expenses
Net loss and loss adjustment expenses	1,491,848	718,556	2,926,039	2,198,512
Commissions, brokerage and other underwriting expenses	398,163	423,042	1,220,415	1,238,712
Other operating expenses	277,918	228,408	678,226	575,527
Corporate administration	(4,689)	10,745	26,601	33,938
Amortization of intangible assets	5,765	6,008	14,140	14,490
Interest expense	20,804	20,682	62,728	61,384
Total costs and expenses	2,189,809	1,407,441	4,928,149	4,122,563
(Losses) earnings before income taxes	(522,326)	207,175	(174,296)	553,034
Income taxes	(212,379)	48,328	(116,368)	162,274
Net (losses) earnings	(309,947)	158,847	(57,928)	390,760
Net earnings attributable to noncontrolling interest	4,210	3,016	5,242	3,353
Net (losses) earnings attributable to Alleghany stockholders	(314,157)	155,831	(63,170)	387,407
Net (losses) earnings	(309,947)	158,847	(57,928)	390,760
Other comprehensive (loss) income :
Change in unrealized gains (losses), net of deferred taxes of $52,766 and $25,123 for 2017 and 2016, for the three months ended and $196,336 and $128,379 for 2017 and 2016, for the nine months ended respectively	97,994	46,657	364,623	238,418
Less: reclassification for net realized capital gains and other than temporary impairment losses, net of taxes of ($9,377) and ($5,422) for 2017 and 2016, for the three months ended and ($31,061) and ($22,999) for 2017 and 2016, for the nine months ended respectively	(17,414)	(10,070)	(57,684)	(42,712)
Change in unrealized currency translation adjustment, net of deferred taxes of $3,967 and ($1,637) for 2017 and 2016, for the three months ended and $12,050 and $10,318 for 2017 and 2016, for the nine months ended respectively	7,368	(3,041)	22,379	19,162
Retirement plans	98	95	(199)	452
Comprehensive (loss) income	(221,901)	192,488	271,191	606,080
Comprehensive income attributable to noncontrolling interest	4,210	3,016	5,242	3,353
Comprehensive (loss) income attributable to Alleghany stockholders	$ (226,111)	$ 189,472	$ 265,949	$ 602,727
Basic (losses) earnings per share attributable to Alleghany stockholders	$ (20.38)	$ 10.09	$ (4.10)	$ 25.09
Diluted (losses) earnings per share attributable to Alleghany stockholders	$ (20.90)	$ 10.09	$ (4.10)	$ 25.08