Condensed Statements of Earnings (Detail) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues
Net investment income																	$ 500,534	$ 451,016	$ 438,455
Change in the fair value of equity securities																	(228,994)
Net realized capital gains																	(3,241)	107,222	63,205
Other than temporary impairment losses																	(1,328)
Total revenues	$ 1,227,100		$ 2,177,400		$ 1,897,700		$ 1,585,000		$ 1,670,800		$ 1,667,500		$ 1,553,900		$ 1,532,400		6,887,160	6,424,655	6,131,019
Costs and Expenses
Interest expense																	90,724	83,070	81,599
Corporate administration																	15,732	46,998	42,960
Total costs and expenses																	6,847,568	6,387,965	5,483,214
Earnings before income taxes																	39,592	36,690	647,805
Income taxes																	(15,062)	(63,802)	187,141
Net earnings																	54,654	100,492	460,664
Net earnings attributable to noncontrolling interest																	15,115	10,359	3,743
Net earnings attributable to Alleghany stockholders	$ (712,100)	[1]	$ 284,900	[1]	$ 295,100	[1]	$ 171,600	[1]	$ 153,300	[1]	$ (314,200)	[1]	$ 101,800	[1]	$ 149,200	[1]	39,539	90,133	456,921
Non insurance Revenue [Member]
Revenues
Noninsurance revenue																	1,643,999	928,298	698,747
Parent Company
Revenues
Net investment income																	16,548	13,659	7,688
Change in the fair value of equity securities																	(44,780)	0	0
Net realized capital gains																	(265)	3,326	(10,674)
Other than temporary impairment losses																	0	0	0
Total revenues																	(28,433)	17,668	(2,827)
Costs and Expenses
Interest expense																	53,121	52,103	52,470
Corporate administration																	17,565	45,308	42,035
Total costs and expenses																	70,686	97,411	94,505
Operating (losses)																	(99,119)	(79,743)	(97,332)
Equity in earnings of consolidated subsidiaries																	138,711	116,433	745,137
Earnings before income taxes																	39,592	36,690	647,805
Income taxes																	(15,062)	(63,802)	187,141
Net earnings																	54,654	100,492	460,664
Net earnings attributable to noncontrolling interest																	15,115	10,359	3,743
Net earnings attributable to Alleghany stockholders																	39,539	90,133	456,921
Parent Company | Non insurance Revenue [Member]
Revenues
Noninsurance revenue																	$ 64	$ 683	$ 159

[1] Attributable to Alleghany stockholders.