Securities Held-to-Maturity - Schedule of Maturity Profile of Securities Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity due within one year		$ 1,000
Held-to-maturity due one year through five years	$ 6	11
Held-to-maturity due five years through ten years	6,025	6,027
Held-to-maturity due over ten years	14,198	14,261
Held-to-maturity cost or amortized cost	$ 20,229	$ 21,299