Investment Securities - Reconciliation of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 649,741	$ 634,928
Gross Unrealized Gains	1,078	1,152
Gross Unrealized Losses	(12,201)	(8,034)
Fair Value	638,618	628,046
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,000	5,000
Gross Unrealized Gains	92	140
Gross Unrealized Losses	0	0
Fair Value	5,092	5,140
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	45,951	51,289
Gross Unrealized Gains	140	448
Gross Unrealized Losses	(586)	(417)
Fair Value	45,505	51,320
Mortgage-backed securities, multifamily
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,087	10,133
Gross Unrealized Gains	67	7
Gross Unrealized Losses	(204)	(63)
Fair Value	20,950	10,077
Mortgage-backed securities, residential
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	434,208	419,538
Gross Unrealized Gains	779	479
Gross Unrealized Losses	(8,843)	(5,763)
Fair Value	426,144	414,254
U.S. Treasury and U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	143,495	148,968
Gross Unrealized Gains	0	78
Gross Unrealized Losses	(2,568)	(1,791)
Fair Value	$ 140,927	$ 147,255