Reconciliation of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		$ 7,692,710	$ 8,514,063
Other comprehensive income (loss), net of tax:
Ending Balance		8,776,734	7,692,710
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		403,000	(212,500)
Reclassifications from accumulated other comprehensive income		(20,400)	11,700
Total		382,600	(200,800)
Ending Balance		321,000	(61,600)
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		2,900	(21,900)
Total		2,900	(21,900)
Ending Balance		(121,800)	(124,700)
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		(12,200)	3,100
Total		(12,200)	3,100
Ending Balance		(27,900)	(15,700)
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)
Accumulated Other Comprehensive Income (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(202,003)	618,118
Cumulative effect of adoption of new accounting pronouncements				(600,508)	[1]	$ 12,899
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		393,700	(231,300)
Reclassifications from accumulated other comprehensive income		(20,400)	11,700
Total		373,300	(219,600)
Ending Balance		$ 171,350	$ (202,003)
Accumulated Other Comprehensive Income (loss) | 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 (loss) | Accounting Standards Update 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of new accounting pronouncements	[1]			$ 135,100

[1] See Note 1(r) for additional information regarding Alleghany’s adoption of new investment accounting guidance and new guidance on certain tax effects caused by the Tax Act.