Investment Securities - Reconciliation of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 650,473	$ 607,112
Gross Unrealized Gains	4,152	6,760
Gross Unrealized Losses	(8,490)	(7,168)
Fair Value	646,135	606,704
U.S. Treasury and U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	148,968	118,537
Gross Unrealized Gains	78	102
Gross Unrealized Losses	(1,791)	(1,280)
Fair Value	147,255	117,359
Mortgage-backed securities, residential
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	419,538	406,851
Gross Unrealized Gains	479	1,174
Gross Unrealized Losses	(5,763)	(4,487)
Fair Value	414,254	403,538
Mortgage-backed securities, multifamily
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,133	10,192
Gross Unrealized Gains	7	30
Gross Unrealized Losses	(63)	(35)
Fair Value	10,077	10,187
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	51,289	48,868
Gross Unrealized Gains	448	391
Gross Unrealized Losses	(417)	(933)
Fair Value	51,320	48,326
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,000	5,350
Gross Unrealized Gains	140	63
Gross Unrealized Losses	0	(1)
Fair Value	5,140	5,412
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,545	17,314
Gross Unrealized Gains	3,000	5,000
Gross Unrealized Losses	(456)	(432)
Fair Value	$ 18,089	$ 21,882