Reconciliation of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Jan. 01, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		$ 8,514,063
Other comprehensive income (loss), net of tax:
Reclassifications from accumulated other comprehensive income		1,400	$ (36,700)
Ending Balance		8,373,400
Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		618,100	109,300
Cumulative effect of adoption of new accounting pronouncements	[1]			$ (600,500)	$ 12,900
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		(142,600)	129,600
Reclassifications from accumulated other comprehensive income		1,400	(36,700)
Total		(141,200)	92,900
Ending Balance		(123,600)	215,100
Accumulated Other Comprehensive Income | Accounting Standards Update 2016-01 |Reclassification of net unrealized gains on equity securities, net of tax
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		718,200	232,200
Cumulative effect of adoption of new accounting pronouncements	[1]			(579,000)	$ 12,900
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		(146,400)	124,900
Reclassifications from accumulated other comprehensive income		1,400	(36,700)
Total		(145,000)	88,200
Ending Balance		(5,800)	333,300
Unrealized Appreciation of Investments | Accounting Standards Update 2016-01 |Reclassification of net unrealized gains on equity securities, net of tax
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		(84,600)	(111,200)
Cumulative effect of adoption of new accounting pronouncements	[1]			(18,200)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		5,100	5,100
Total		5,100	5,100
Ending Balance		(97,700)	(106,100)
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		(15,500)	(11,700)
Cumulative effect of adoption of new accounting pronouncements	[1]			(3,300)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		(1,300)	(400)
Total		(1,300)	(400)
Ending Balance		$ (20,100)	$ (12,100)
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 regarding Alleghany's adoption of new investment accounting guidance and adoption of new guidance on certain tax effects caused by the Tax Act.