Investment Securities - Reconciliation of Held-to-Maturity Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 158,576	$ 139,685
Gross Unrealized Gains	182	593
Gross Unrealized Losses	(4,783)	(1,590)
Fair Value	153,975	138,688
U.S. government agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	38,278	33,415
Gross Unrealized Gains	0	24
Gross Unrealized Losses	(1,081)	(402)
Fair Value	37,197	33,037
Mortgage-backed securities, residential
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	75,825	54,991
Gross Unrealized Gains	105	249
Gross Unrealized Losses	(2,890)	(978)
Fair Value	73,040	54,262
Mortgage-backed securities, multifamily
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,879	1,957
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(59)	(22)
Fair Value	1,820	1,935
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	37,594	43,318
Gross Unrealized Gains	77	306
Gross Unrealized Losses	(570)	(188)
Fair Value	37,101	43,436
Debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,000	6,004
Gross Unrealized Gains	0	14
Gross Unrealized Losses	(183)	0
Fair Value	$ 4,817	$ 6,018