Investment Securities - Reconciliation of Held-to-Maturity Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 139,685	$ 147,614
Gross Unrealized Gains	593	833
Gross Unrealized Losses	(1,590)	(1,457)
Fair Value	138,688	146,990
U.S. government agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	33,415	33,553
Gross Unrealized Gains	24	144
Gross Unrealized Losses	(402)	(430)
Fair Value	33,037	33,267
Mortgage-backed securities, residential
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	54,991	38,706
Gross Unrealized Gains	249	369
Gross Unrealized Losses	(978)	(598)
Fair Value	54,262	38,477
Mortgage-backed securities, multifamily
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,957	2,059
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(22)	(44)
Fair Value	1,935	2,015
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	43,318	71,284
Gross Unrealized Gains	306	269
Gross Unrealized Losses	(188)	(385)
Fair Value	43,436	71,168
Debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	6,004	2,012
Gross Unrealized Gains	14	51
Gross Unrealized Losses	0	0
Fair Value	$ 6,018	$ 2,063