Reclassifications of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized capital gains		$ 3,241	$ (107,222)	$ (63,205)
Other than temporary impairment losses		1,328
Other than temporary impairment losses			16,871	45,165
Income taxes		(15,062)	(63,802)	$ 187,141
Unrealized Appreciation of Investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications		11,700
Accumulated Unrealized Gain (Loss) on Investments in Available-For-Sale Debt and Equity Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications		11,700	(47,900)
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Appreciation of Investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized capital gains	[1]	13,500
Other than temporary impairment losses		1,300
Income taxes		$ (3,100)
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Unrealized Gain (Loss) on Investments in Available-For-Sale Debt and Equity Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized capital gains	[1]		(90,600)
Other than temporary impairment losses			16,900
Income taxes			$ 25,800

[1] For 2018, excludes a $45.7 million pre-tax gain from AIHL’s conversion of its limited partnership interests in certain subsidiaries of Ares into Ares common units, and a $35.4 million capital loss due to an impairment charge from the write-down of certain SORC assets arising from a decline in energy prices as of December 31, 2018. See Note 4(i) for additional information. For 2017, excludes an $8.4 million pre-tax gain from the sale of PacificComp, a $20.9 million realized gain on bulk settlement of certain contingent liabilities by Alleghany Capital, a ($4.8) million loss realized from the sale of a SORC legacy oil field on December 29, 2017 and a ($7.9) million write-down of certain Reinsurance Segment assets.