Investment Securities - Reconciliation of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 637,626	$ 634,928
Gross Unrealized Gains	468	1,152
Gross Unrealized Losses	(14,852)	(8,034)
Fair Value	623,242	628,046
U.S. Treasury and U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	147,678	148,968
Gross Unrealized Gains	27	78
Gross Unrealized Losses	(3,005)	(1,791)
Fair Value	144,700	147,255
Mortgage-backed securities, residential
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	421,077	419,538
Gross Unrealized Gains	158	479
Gross Unrealized Losses	(10,718)	(5,763)
Fair Value	410,517	414,254
Mortgage-backed securities, multifamily
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,200	10,133
Gross Unrealized Gains	0	7
Gross Unrealized Losses	(256)	(63)
Fair Value	12,944	10,077
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	50,671	51,289
Gross Unrealized Gains	189	448
Gross Unrealized Losses	(873)	(417)
Fair Value	49,987	51,320
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,000	5,000
Gross Unrealized Gains	94	140
Gross Unrealized Losses	0	0
Fair Value	$ 5,094	$ 5,140