Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Fixed-Maturity Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Amortized Cost, 2018	$ 3,167
Total fixed-maturity securities, Amortized Cost	3,167	$ 3,674
Estimated Fair Value, 2018	3,153
Total fixed-maturity securities, Estimated Fair Value	$ 3,153	$ 3,660