Schedule V - Supplemental Information Concerning Property and Casualty Insurance Operations (Parenthetical) (Detail)	Dec. 31, 2017	Dec. 31, 2016
Minimum [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Discount deducted from claim and claim adjustment expense reserves interest rates	3.50%	3.50%
Maximum [Member]
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Discount deducted from claim and claim adjustment expense reserves interest rates	8.00%	8.00%