Investment Securities - Reconciliation of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 632,646	$ 634,928
Gross Unrealized Gains	234	1,152
Gross Unrealized Losses	(19,637)	(8,034)
Fair Value	613,243	628,046
U.S. Treasury and U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	148,402	148,968
Gross Unrealized Gains	0	78
Gross Unrealized Losses	(3,928)	(1,791)
Fair Value	144,474	147,255
Mortgage-backed securities, residential
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	412,979	419,538
Gross Unrealized Gains	77	479
Gross Unrealized Losses	(14,345)	(5,763)
Fair Value	398,711	414,254
Mortgage-backed securities, multifamily
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,166	10,133
Gross Unrealized Gains	18	7
Gross Unrealized Losses	(348)	(63)
Fair Value	18,836	10,077
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	47,099	51,289
Gross Unrealized Gains	98	448
Gross Unrealized Losses	(1,016)	(417)
Fair Value	46,181	51,320
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,000	5,000
Gross Unrealized Gains	41	140
Gross Unrealized Losses	0	0
Fair Value	$ 5,041	$ 5,140