Reconciliation of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		$ 7,939,945	$ 7,554,707
Other comprehensive income (loss), net of tax:
Reclassifications from accumulated other comprehensive income		(47,900)	(67,400)
Ending Balance		8,514,063	7,939,945
Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		109,284	116,273
Cumulative effect of adoption of new accounting pronouncements	[1]	12,899
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		543,800	60,400
Reclassifications from accumulated other comprehensive income		(47,900)	(67,400)
Total		495,900	(7,000)
Ending Balance		618,118	109,284
Unrealized Appreciation of Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		232,200	231,900
Cumulative effect of adoption of new accounting pronouncements	[1]	12,900
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		521,000	67,700
Reclassifications from accumulated other comprehensive income		(47,900)	(67,400)
Total		473,100	300
Ending Balance		718,200	232,200
Unrealized Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(111,200)	(104,000)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		26,600	(7,200)
Total		26,600	(7,200)
Ending Balance		(84,600)	(111,200)
Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(11,700)	(11,600)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		(3,800)	(100)
Total		(3,800)	(100)
Ending Balance		$ (15,500)	$ (11,700)

[1] See Note 1(r) for additional information regarding Alleghany's adoption of new investment accounting guidance in 2017.