Investment Securities - Reconciliation of Available-for-Sale and Held-to-Maturity Securities in Continuous Unrealized Loss Position (Detail) $ in Thousands	Dec. 31, 2017 USD ($) Security	Dec. 31, 2016 USD ($) Security
Investment [Line Items]
Available for Sale, Less than 12 Months, Fair Value	$ 289,390	$ 428,488
Available for Sale, Less than 12 Months, Unrealized Losses	2,446	6,421
Available for Sale, 12 Months or Longer, Fair Value	240,223	20,173
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 6,044	$ 747
Available for Sale, Number of Securities | Security	187	167
Available for Sale, Total, Fair Value	$ 529,613	$ 448,661
Available for Sale, Total, Unrealized Losses	8,490	7,168
Held to Maturity, Less than 12 Months, Fair Value	58,749	97,373
Held to Maturity, Less than 12 Months, Unrealized Losses	599	1,393
Held to Maturity, 12 Months or Longer, Fair Value	31,951	1,462
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 991	$ 64
Held to Maturity, Number of Securities | Security	54	63
Held to Maturity, Total, Fair Value	$ 90,700	$ 98,835
Held to Maturity, Total, Unrealized Losses	1,590	1,457
U.S. Treasury and U.S. government agencies
Investment [Line Items]
Available for Sale, Less than 12 Months, Fair Value	80,391	94,153
Available for Sale, Less than 12 Months, Unrealized Losses	646	1,280
Available for Sale, 12 Months or Longer, Fair Value	54,769	0
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 1,145	$ 0
Available for Sale, Number of Securities | Security	27	18
Available for Sale, Total, Fair Value	$ 135,160	$ 94,153
Available for Sale, Total, Unrealized Losses	1,791	1,280
Held to Maturity, Less than 12 Months, Fair Value	15,371	17,147
Held to Maturity, Less than 12 Months, Unrealized Losses	95	430
Held to Maturity, 12 Months or Longer, Fair Value	6,720	0
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 307	$ 0
Held to Maturity, Number of Securities | Security	4	3
Held to Maturity, Total, Fair Value	$ 22,091	$ 17,147
Held to Maturity, Total, Unrealized Losses	402	430
Mortgage-backed securities, residential
Investment [Line Items]
Available for Sale, Less than 12 Months, Fair Value	199,387	292,873
Available for Sale, Less than 12 Months, Unrealized Losses	1,723	4,078
Available for Sale, 12 Months or Longer, Fair Value	157,739	15,453
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 4,040	$ 409
Available for Sale, Number of Securities | Security	118	91
Available for Sale, Total, Fair Value	$ 357,126	$ 308,326
Available for Sale, Total, Unrealized Losses	5,763	4,487
Held to Maturity, Less than 12 Months, Fair Value	26,090	27,909
Held to Maturity, Less than 12 Months, Unrealized Losses	426	535
Held to Maturity, 12 Months or Longer, Fair Value	19,203	1,061
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 552	$ 63
Held to Maturity, Number of Securities | Security	25	15
Held to Maturity, Total, Fair Value	$ 45,293	$ 28,970
Held to Maturity, Total, Unrealized Losses	978	598
Mortgage-backed securities, multifamily
Investment [Line Items]
Available for Sale, Less than 12 Months, Fair Value	0	5,178
Available for Sale, Less than 12 Months, Unrealized Losses	0	35
Available for Sale, 12 Months or Longer, Fair Value	5,088	0
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 63	$ 0
Available for Sale, Number of Securities | Security	1	1
Available for Sale, Total, Fair Value	$ 5,088	$ 5,178
Available for Sale, Total, Unrealized Losses	63	35
Held to Maturity, Less than 12 Months, Fair Value	1,935	2,015
Held to Maturity, Less than 12 Months, Unrealized Losses	22	44
Held to Maturity, 12 Months or Longer, Fair Value	0	0
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 0	$ 0
Held to Maturity, Number of Securities | Security	2	2
Held to Maturity, Total, Fair Value	$ 1,935	$ 2,015
Held to Maturity, Total, Unrealized Losses	22	44
Obligations of states and political subdivisions
Investment [Line Items]
Available for Sale, Less than 12 Months, Fair Value	9,612	29,904
Available for Sale, Less than 12 Months, Unrealized Losses	77	933
Available for Sale, 12 Months or Longer, Fair Value	12,970	0
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 340	$ 0
Available for Sale, Number of Securities | Security	39	54
Available for Sale, Total, Fair Value	$ 22,582	$ 29,904
Available for Sale, Total, Unrealized Losses	417	933
Held to Maturity, Less than 12 Months, Fair Value	15,353	50,302
Held to Maturity, Less than 12 Months, Unrealized Losses	56	384
Held to Maturity, 12 Months or Longer, Fair Value	6,028	401
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 132	$ 1
Held to Maturity, Number of Securities | Security	23	43
Held to Maturity, Total, Fair Value	$ 21,381	$ 50,703
Held to Maturity, Total, Unrealized Losses	188	385
Debt securities
Investment [Line Items]
Available for Sale, Less than 12 Months, Fair Value		350
Available for Sale, Less than 12 Months, Unrealized Losses		1
Available for Sale, 12 Months or Longer, Fair Value		0
Available for Sale, 12 Months or Longer, Unrealized Losses		$ 0
Available for Sale, Number of Securities | Security		1
Available for Sale, Total, Fair Value		$ 350
Available for Sale, Total, Unrealized Losses		1
Held to Maturity, Total, Unrealized Losses	0	0
Equity securities
Investment [Line Items]
Available for Sale, Less than 12 Months, Fair Value	0	6,030
Available for Sale, Less than 12 Months, Unrealized Losses	0	94
Available for Sale, 12 Months or Longer, Fair Value	9,657	4,720
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 456	$ 338
Available for Sale, Number of Securities | Security	2	2
Available for Sale, Total, Fair Value	$ 9,657	$ 10,750
Available for Sale, Total, Unrealized Losses	$ 456	$ 432