Investment Securities - Reconciliation of Available-for-Sale and Held-to-Maturity Securities in Continuous Unrealized Loss Position (Detail) $ in Thousands	Sep. 30, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Investment [Line Items]
Available for Sale, Less Than 12 Months, Fair Value	$ 223,843	$ 289,390
Available for Sale, Less Than 12 Months, Unrealized Losses	4,752	2,446
Available for Sale, 12 Months or Longer, Fair Value	355,820	230,566
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 14,885	$ 5,588
Available for Sale, Number of Securities (in shares) | Security	250	185
Available for Sale, Total, Fair Value	$ 579,663	$ 519,956
Available for Sale, Total, Unrealized Losses	19,637	8,034
Held to Maturity, Less Than 12 Months, Fair Value	74,909	58,749
Held to Maturity, Less Than 12 Months, Unrealized Losses	1,659	599
Held to Maturity, 12 Months or Longer, Fair Value	57,966	31,951
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 3,124	$ 991
Held to Maturity, Number of Securities (in shares) | Security	86	54
Held to Maturity, Total, Fair Value	$ 132,875	$ 90,700
Held to Maturity, Total, Unrealized Losses	4,783	1,590
U.S. Treasury and U.S. government agencies
Investment [Line Items]
Available for Sale, Less Than 12 Months, Fair Value	58,128	80,391
Available for Sale, Less Than 12 Months, Unrealized Losses	1,279	646
Available for Sale, 12 Months or Longer, Fair Value	86,347	54,769
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 2,649	$ 1,145
Available for Sale, Number of Securities (in shares) | Security	28	27
Available for Sale, Total, Fair Value	$ 144,475	$ 135,160
Available for Sale, Total, Unrealized Losses	3,928	1,791
Held to Maturity, Less Than 12 Months, Fair Value	25,521	15,371
Held to Maturity, Less Than 12 Months, Unrealized Losses	445	95
Held to Maturity, 12 Months or Longer, Fair Value	11,678	6,720
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 636	$ 307
Held to Maturity, Number of Securities (in shares) | Security	7	4
Held to Maturity, Total, Fair Value	$ 37,199	$ 22,091
Held to Maturity, Total, Unrealized Losses	1,081	402
Mortgage-backed securities, residential
Investment [Line Items]
Available for Sale, Less Than 12 Months, Fair Value	135,669	199,387
Available for Sale, Less Than 12 Months, Unrealized Losses	2,952	1,723
Available for Sale, 12 Months or Longer, Fair Value	249,216	157,739
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 11,393	$ 4,040
Available for Sale, Number of Securities (in shares) | Security	153	118
Available for Sale, Total, Fair Value	$ 384,885	$ 357,126
Available for Sale, Total, Unrealized Losses	14,345	5,763
Held to Maturity, Less Than 12 Months, Fair Value	30,173	26,090
Held to Maturity, Less Than 12 Months, Unrealized Losses	774	426
Held to Maturity, 12 Months or Longer, Fair Value	37,234	19,203
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 2,116	$ 552
Held to Maturity, Number of Securities (in shares) | Security	37	25
Held to Maturity, Total, Fair Value	$ 67,407	$ 45,293
Held to Maturity, Total, Unrealized Losses	2,890	978
Mortgage-backed securities, multifamily
Investment [Line Items]
Available for Sale, Less Than 12 Months, Fair Value	10,871	0
Available for Sale, Less Than 12 Months, Unrealized Losses	242	0
Available for Sale, 12 Months or Longer, Fair Value	4,992	5,088
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 106	$ 63
Available for Sale, Number of Securities (in shares) | Security	4	1
Available for Sale, Total, Fair Value	$ 15,863	$ 5,088
Available for Sale, Total, Unrealized Losses	348	63
Held to Maturity, Less Than 12 Months, Fair Value	0	1,935
Held to Maturity, Less Than 12 Months, Unrealized Losses	0	22
Held to Maturity, 12 Months or Longer, Fair Value	1,820	0
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 59	$ 0
Held to Maturity, Number of Securities (in shares) | Security	2	2
Held to Maturity, Total, Fair Value	$ 1,820	$ 1,935
Held to Maturity, Total, Unrealized Losses	59	22
Obligations of states and political subdivisions
Investment [Line Items]
Available for Sale, Less Than 12 Months, Fair Value	19,175	9,612
Available for Sale, Less Than 12 Months, Unrealized Losses	279	77
Available for Sale, 12 Months or Longer, Fair Value	15,265	12,970
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 737	$ 340
Available for Sale, Number of Securities (in shares) | Security	65	39
Available for Sale, Total, Fair Value	$ 34,440	$ 22,582
Available for Sale, Total, Unrealized Losses	1,016	417
Held to Maturity, Less Than 12 Months, Fair Value	15,398	15,353
Held to Maturity, Less Than 12 Months, Unrealized Losses	257	56
Held to Maturity, 12 Months or Longer, Fair Value	7,234	6,028
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 313	$ 132
Held to Maturity, Number of Securities (in shares) | Security	39	23
Held to Maturity, Total, Fair Value	$ 22,632	$ 21,381
Held to Maturity, Total, Unrealized Losses	570	$ 188
Debt securities
Investment [Line Items]
Held to Maturity, Less Than 12 Months, Fair Value	3,817
Held to Maturity, Less Than 12 Months, Unrealized Losses	183
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	$ 3,817
Held to Maturity, Total, Unrealized Losses	$ 183