Securities Held-to-Maturity - Schedule of Unrealized Gains, Losses and Fair Value of Securities Held-to-Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity cost or amortized cost	$ 21,299	$ 22,496
Held-to-maturity gross unrealized gains	9	13
Held-to-maturity gross unrealized losses	(1,257)	(1,353)
Held-to-maturity fair value	$ 20,051	$ 21,156