Consolidated Statements of Earnings and Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues
Net premiums earned	$ 1,207,856	$ 1,209,188
Net investment income	124,126	115,538
Change in the fair value of equity securities	(42,649)
Net realized capital gains	44,505	59,651
Other than temporary impairment losses	(511)	(3,217)
Noninsurance revenue	251,627	151,292
Total revenues	1,584,954	1,532,452
Costs and Expenses
Net loss and loss adjustment expenses	670,578	699,305
Commissions, brokerage and other underwriting expenses	406,295	408,515
Other operating expenses	264,897	175,138
Corporate administration	7,785	16,885
Amortization of intangible assets	5,264	3,764
Interest expense	21,531	20,935
Total costs and expenses	1,376,350	1,324,542
(Losses) earnings before income taxes	208,604	207,910
Income taxes	37,422	58,550
Net earnings	171,182	149,360
Net earnings attributable to noncontrolling interest	(393)	184
Net earnings attributable to Alleghany stockholders	171,575	149,176
Net earnings	171,182	149,360
Other comprehensive income:
Change in unrealized gains (losses), net of deferred taxes of ($38,905) and $67,270 for 2018 and 2017, respectively	(146,358)	124,929
Less: reclassification for net realized capital gains and other than temporary impairment losses, net of taxes of $366 and ($19,752) for 2018 and 2017, respectively	1,379	(36,682)
Change in unrealized currency translation adjustment, net of deferred taxes of $1,356 and $2,719 for 2018 and 2017, respectively	5,100	5,049
Retirement plans	(1,322)	(395)
Comprehensive income	29,981	242,261
Comprehensive income attributable to noncontrolling interest	(393)	184
Comprehensive income attributable to Alleghany stockholders	$ 30,374	$ 242,077
Basic earnings per share attributable to Alleghany stockholders	$ 11.15	$ 9.68
Diluted earnings per share attributable to Alleghany stockholders	$ 11.04	$ 9.67