Amortized Cost/Cost, Gross Unrealized Gains and Losses, Estimated Fair Values and ("OTTI") Reflected in ("AOCI") of Investments in Fixed Maturity and Equity Available-for-Sale Securities (Parenthetical) (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Unrealized gains	$ 3,888	$ 3,364