Securities Held-to-Maturity - Schedule of Unrealized Gains, Losses and Fair Value of Securities Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity cost or amortized cost	$ 21,264	$ 21,299
Held-to-maturity gross unrealized gains	7	9
Held-to-maturity gross unrealized losses	(1,770)	(1,257)
Held-to-maturity fair value	$ 19,501	$ 20,051