Reconciliation of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 8,595,373	$ 8,695,881	$ 9,186,882	$ 8,755,720
Other comprehensive (loss) income, net of tax:
Ending Balance	7,906,670	9,142,031	7,906,670	9,142,031
Unrealized Appreciation of Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			259,100	564,900
Other comprehensive (loss) income, net of tax:
Other comprehensive (loss) income before reclassifications			(1,085,600)	(126,400)
Reclassifications from accumulated other comprehensive income	(2,800)	(10,400)	5,500	(22,200)
Total			(1,080,100)	(148,600)
Ending Balance	(821,000)	416,300	(821,000)	416,300
Unrealized Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(103,700)	(99,400)
Other comprehensive (loss) income, net of tax:
Other comprehensive (loss) income before reclassifications			(26,200)	200
Reclassifications from accumulated other comprehensive income			0	0
Total			(26,200)	200
Ending Balance	(129,900)	(99,200)	(129,900)	(99,200)
Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(13,600)	(13,100)
Other comprehensive (loss) income, net of tax:
Other comprehensive (loss) income before reclassifications			200	(800)
Reclassifications from accumulated other comprehensive income			0	0
Total			200	(800)
Ending Balance	(13,400)	(13,900)	(13,400)	(13,900)
Accumulated Other Comprehensive Income (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(464,460)	224,190	141,822	452,402
Other comprehensive (loss) income, net of tax:
Other comprehensive (loss) income before reclassifications			(1,111,600)	(127,000)
Reclassifications from accumulated other comprehensive income			5,500	(22,200)
Total			(1,106,100)	(149,200)
Ending Balance	$ (964,281)	$ 303,196	$ (964,281)	$ 303,196

ALLEGHANY CORP /DE 10-Q