Investment Securities - Reconciliation of Held-to-Maturity Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 152,127	$ 139,685
Gross Unrealized Gains	319	593
Gross Unrealized Losses	(3,233)	(1,590)
Fair Value	149,213	138,688
U.S. government agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	38,694	33,415
Gross Unrealized Gains	0	24
Gross Unrealized Losses	(793)	(402)
Fair Value	37,901	33,037
Mortgage-backed securities, residential
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	65,614	54,991
Gross Unrealized Gains	164	249
Gross Unrealized Losses	(1,984)	(978)
Fair Value	63,794	54,262
Mortgage-backed securities, multifamily
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,931	1,957
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(44)	(22)
Fair Value	1,887	1,935
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	39,886	43,318
Gross Unrealized Gains	133	306
Gross Unrealized Losses	(412)	(188)
Fair Value	39,607	43,436
Debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	6,002	6,004
Gross Unrealized Gains	22	14
Gross Unrealized Losses	0	0
Fair Value	$ 6,024	$ 6,018