Investment Securities - Reconciliation of Available-for-Sale and Held-to-Maturity Securities in Continuous Unrealized Loss Position (Detail) $ in Thousands	Jun. 30, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Investment [Line Items]
Available for Sale, Less Than 12 Months, Fair Value	$ 333,907	$ 289,390
Available for Sale, Less Than 12 Months, Unrealized Losses	7,531	2,446
Available for Sale, 12 Months or Longer, Fair Value	232,842	230,566
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 9,403	$ 5,588
Available for Sale, Number of Securities (in shares) | Security	233	185
Available for Sale, Total, Fair Value	$ 566,749	$ 519,956
Available for Sale, Total, Unrealized Losses	16,934	8,034
Held to Maturity, Less Than 12 Months, Fair Value	97,724	58,749
Held to Maturity, Less Than 12 Months, Unrealized Losses	2,022	599
Held to Maturity, 12 Months or Longer, Fair Value	32,334	31,951
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 1,754	$ 991
Held to Maturity, Number of Securities (in shares) | Security	78	54
Held to Maturity, Total, Fair Value	$ 130,058	$ 90,700
Held to Maturity, Total, Unrealized Losses	3,776	1,590
U.S. Treasury and U.S. government agencies
Investment [Line Items]
Available for Sale, Less Than 12 Months, Fair Value	92,272	80,391
Available for Sale, Less Than 12 Months, Unrealized Losses	1,781	646
Available for Sale, 12 Months or Longer, Fair Value	54,073	54,769
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 1,733	$ 1,145
Available for Sale, Number of Securities (in shares) | Security	28	27
Available for Sale, Total, Fair Value	$ 146,345	$ 135,160
Available for Sale, Total, Unrealized Losses	3,514	1,791
Held to Maturity, Less Than 12 Months, Fair Value	30,948	15,371
Held to Maturity, Less Than 12 Months, Unrealized Losses	464	95
Held to Maturity, 12 Months or Longer, Fair Value	6,546	6,720
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 479	$ 307
Held to Maturity, Number of Securities (in shares) | Security	7	4
Held to Maturity, Total, Fair Value	$ 37,494	$ 22,091
Held to Maturity, Total, Unrealized Losses	943	402
Mortgage-backed securities, residential
Investment [Line Items]
Available for Sale, Less Than 12 Months, Fair Value	213,396	199,387
Available for Sale, Less Than 12 Months, Unrealized Losses	5,296	1,723
Available for Sale, 12 Months or Longer, Fair Value	161,111	157,739
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 6,974	$ 4,040
Available for Sale, Number of Securities (in shares) | Security	142	118
Available for Sale, Total, Fair Value	$ 374,507	$ 357,126
Available for Sale, Total, Unrealized Losses	12,270	5,763
Held to Maturity, Less Than 12 Months, Fair Value	45,533	26,090
Held to Maturity, Less Than 12 Months, Unrealized Losses	1,302	426
Held to Maturity, 12 Months or Longer, Fair Value	19,894	19,203
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 1,053	$ 552
Held to Maturity, Number of Securities (in shares) | Security	35	25
Held to Maturity, Total, Fair Value	$ 65,427	$ 45,293
Held to Maturity, Total, Unrealized Losses	2,355	978
Mortgage-backed securities, multifamily
Investment [Line Items]
Available for Sale, Less Than 12 Months, Fair Value	7,872	0
Available for Sale, Less Than 12 Months, Unrealized Losses	189	0
Available for Sale, 12 Months or Longer, Fair Value	5,002	5,088
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 118	$ 63
Available for Sale, Number of Securities (in shares) | Security	3	1
Available for Sale, Total, Fair Value	$ 12,874	$ 5,088
Available for Sale, Total, Unrealized Losses	307	63
Held to Maturity, Less Than 12 Months, Fair Value	1,849	1,935
Held to Maturity, Less Than 12 Months, Unrealized Losses	56	22
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 (in shares) | Security	2	2
Held to Maturity, Total, Fair Value	$ 1,849	$ 1,935
Held to Maturity, Total, Unrealized Losses	56	22
Obligations of states and political subdivisions
Investment [Line Items]
Available for Sale, Less Than 12 Months, Fair Value	20,367	9,612
Available for Sale, Less Than 12 Months, Unrealized Losses	265	77
Available for Sale, 12 Months or Longer, Fair Value	12,656	12,970
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 578	$ 340
Available for Sale, Number of Securities (in shares) | Security	60	39
Available for Sale, Total, Fair Value	$ 33,023	$ 22,582
Available for Sale, Total, Unrealized Losses	843	417
Held to Maturity, Less Than 12 Months, Fair Value	15,394	15,353
Held to Maturity, Less Than 12 Months, Unrealized Losses	199	56
Held to Maturity, 12 Months or Longer, Fair Value	5,894	6,028
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 222	$ 132
Held to Maturity, Number of Securities (in shares) | Security	33	23
Held to Maturity, Total, Fair Value	$ 21,288	$ 21,381
Held to Maturity, Total, Unrealized Losses	421	$ 188
Debt securities
Investment [Line Items]
Held to Maturity, Less Than 12 Months, Fair Value	4,000
Held to Maturity, Less Than 12 Months, Unrealized Losses	1
Held to Maturity, 12 Months or Longer, Fair Value	0
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 0
Held to Maturity, Number of Securities (in shares) | Security	1
Held to Maturity, Total, Fair Value	$ 4,000
Held to Maturity, Total, Unrealized Losses	$ 1