Investment Securities - Reconciliation of Held-to-Maturity Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 158,832	$ 139,685
Gross Unrealized Gains	260	593
Gross Unrealized Losses	(3,776)	(1,590)
Fair Value	155,316	138,688
U.S. government agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	38,437	33,415
Gross Unrealized Gains	0	24
Gross Unrealized Losses	(943)	(402)
Fair Value	37,494	33,037
Mortgage-backed securities, residential
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	73,538	54,991
Gross Unrealized Gains	127	249
Gross Unrealized Losses	(2,355)	(978)
Fair Value	71,310	54,262
Mortgage-backed securities, multifamily
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,905	1,957
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(56)	(22)
Fair Value	1,849	1,935
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	39,452	43,318
Gross Unrealized Gains	133	306
Gross Unrealized Losses	(421)	(188)
Fair Value	39,164	43,436
Debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,500	6,004
Gross Unrealized Gains	0	14
Gross Unrealized Losses	(1)	0
Fair Value	$ 5,499	$ 6,018