Investments - Amortized Cost and Fair Value of Fixed Maturity Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 16,934	$ 5,551
Due after one year through five years	13,876	22,757
Due after five years through ten years	232	330
Amortized Cost, Total	31,042	28,638
Fair Value
Due in one year or less	16,899	5,554
Due after one year through five years	13,708	22,708
Due after five years through ten years	241	340
Fair Value, Total	$ 30,848	$ 28,602