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	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
Unrealized gains	$ 3,364	$ 2,446