Investment Securities - Reconciliation of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 622,818	$ 634,928
Gross Unrealized Gains	347	1,152
Gross Unrealized Losses	(16,934)	(8,034)
Fair Value	606,231	628,046
U.S. Treasury and U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	149,859	148,968
Gross Unrealized Gains	0	78
Gross Unrealized Losses	(3,514)	(1,791)
Fair Value	146,345	147,255
Mortgage-backed securities, residential
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	404,880	419,538
Gross Unrealized Gains	105	479
Gross Unrealized Losses	(12,270)	(5,763)
Fair Value	392,715	414,254
Mortgage-backed securities, multifamily
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,181	10,133
Gross Unrealized Gains	0	7
Gross Unrealized Losses	(307)	(63)
Fair Value	12,874	10,077
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,898	51,289
Gross Unrealized Gains	167	448
Gross Unrealized Losses	(843)	(417)
Fair Value	49,222	51,320
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,000	5,000
Gross Unrealized Gains	75	140
Gross Unrealized Losses	0	0
Fair Value	$ 5,075	$ 5,140