Reconciliation of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		$ 8,685,914	$ 8,404,389	$ 7,692,710	$ 8,514,063
Other comprehensive income (loss), net of tax:
Ending Balance		8,828,658	8,595,085	8,828,658	8,595,085
Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		151,964	(172,307)	(202,003)	618,118
Cumulative effect of adoption of new accounting pronouncements	[1]					$ (600,508)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications				435,500	(221,900)
Reclassifications from accumulated other comprehensive income				(10,200)	(16,500)
Total				425,300	(238,400)
Ending Balance		223,266	(220,808)	223,266	(220,808)
Accumulated Other Comprehensive Income | Accounting Standards Update 2016-01
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of new accounting pronouncements	[1]					(735,600)
Accumulated Other Comprehensive Income | Accounting Standards Update 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of new accounting pronouncements	[1]					135,100
Unrealized Appreciation of Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				(61,600)	718,200
Cumulative effect of adoption of new accounting pronouncements	[1]					(579,000)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications				442,200	(213,300)
Reclassifications from accumulated other comprehensive income		(4,900)	(12,800)	(10,200)	(16,500)
Total				432,000	(229,800)
Ending Balance		370,400	(90,600)	370,400	(90,600)
Unrealized Appreciation of Investments | Accounting Standards Update 2016-01
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of new accounting pronouncements	[1]					(735,600)
Unrealized Appreciation of Investments | Accounting Standards Update 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of new accounting pronouncements	[1]					156,600
Unrealized Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				(124,700)	(84,600)
Cumulative effect of adoption of new accounting pronouncements	[1]					(18,200)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications				(8,100)	(6,900)
Total				(8,100)	(6,900)
Ending Balance		(132,800)	(109,700)	(132,800)	(109,700)
Unrealized Currency Translation Adjustment | Accounting Standards Update 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of new accounting pronouncements	[1]					(18,200)
Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance				(15,700)	(15,500)
Cumulative effect of adoption of new accounting pronouncements	[1]					(3,300)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications				1,400	(1,700)
Total				1,400	(1,700)
Ending Balance		$ (14,300)	$ (20,500)	$ (14,300)	$ (20,500)
Retirement Plans | Accounting Standards Update 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of new accounting pronouncements	[1]					$ (3,300)

[1] See Note 1(c) of this Form 10-Q for additional information on Alleghany’s adoption of new investment accounting guidance and new guidance on certain tax effects caused by the Tax Act.