Investment Securities - Reconciliation of Available-for-Sale and Held-to-Maturity Securities in Continuous Unrealized Loss Position (Detail) $ in Thousands	Mar. 31, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Investment [Line Items]
Available for Sale, Less Than 12 Months, Fair Value	$ 344,971	$ 289,390
Available for Sale, Less Than 12 Months, Unrealized Losses	6,553	2,446
Available for Sale, 12 Months or Longer, Fair Value	224,984	230,566
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 8,299	$ 5,588
Available for Sale, Number of Securities (in shares) | Security	239	185
Available for Sale, Total, Fair Value	$ 569,955	$ 519,956
Available for Sale, Total, Unrealized Losses	14,852	8,034
Held to Maturity, Less Than 12 Months, Fair Value	94,753	58,749
Held to Maturity, Less Than 12 Months, Unrealized Losses	1,748	599
Held to Maturity, 12 Months or Longer, Fair Value	30,876	31,951
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 1,485	$ 991
Held to Maturity, Number of Securities (in shares) | Security	79	54
Held to Maturity, Total, Fair Value	$ 125,629	$ 90,700
Held to Maturity, Total, Unrealized Losses	3,233	1,590
U.S. Treasury and U.S. government agencies
Investment [Line Items]
Available for Sale, Less Than 12 Months, Fair Value	79,243	80,391
Available for Sale, Less Than 12 Months, Unrealized Losses	1,393	646
Available for Sale, 12 Months or Longer, Fair Value	54,249	54,769
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 1,612	$ 1,145
Available for Sale, Number of Securities (in shares) | Security	26	27
Available for Sale, Total, Fair Value	$ 133,492	$ 135,160
Available for Sale, Total, Unrealized Losses	3,005	1,791
Held to Maturity, Less Than 12 Months, Fair Value	31,295	15,371
Held to Maturity, Less Than 12 Months, Unrealized Losses	372	95
Held to Maturity, 12 Months or Longer, Fair Value	6,605	6,720
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 421	$ 307
Held to Maturity, Number of Securities (in shares) | Security	7	4
Held to Maturity, Total, Fair Value	$ 37,900	$ 22,091
Held to Maturity, Total, Unrealized Losses	793	402
Mortgage-backed securities, residential
Investment [Line Items]
Available for Sale, Less Than 12 Months, Fair Value	236,142	199,387
Available for Sale, Less Than 12 Months, Unrealized Losses	4,725	1,723
Available for Sale, 12 Months or Longer, Fair Value	153,022	157,739
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 5,993	$ 4,040
Available for Sale, Number of Securities (in shares) | Security	147	118
Available for Sale, Total, Fair Value	$ 389,164	$ 357,126
Available for Sale, Total, Unrealized Losses	10,718	5,763
Held to Maturity, Less Than 12 Months, Fair Value	41,131	26,090
Held to Maturity, Less Than 12 Months, Unrealized Losses	1,110	426
Held to Maturity, 12 Months or Longer, Fair Value	18,323	19,203
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 874	$ 552
Held to Maturity, Number of Securities (in shares) | Security	32	25
Held to Maturity, Total, Fair Value	$ 59,454	$ 45,293
Held to Maturity, Total, Unrealized Losses	1,984	978
Mortgage-backed securities, multifamily
Investment [Line Items]
Available for Sale, Less Than 12 Months, Fair Value	7,920	0
Available for Sale, Less Than 12 Months, Unrealized Losses	144	0
Available for Sale, 12 Months or Longer, Fair Value	5,024	5,088
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 112	$ 63
Available for Sale, Number of Securities (in shares) | Security	3	1
Available for Sale, Total, Fair Value	$ 12,944	$ 5,088
Available for Sale, Total, Unrealized Losses	256	63
Held to Maturity, Less Than 12 Months, Fair Value	1,887	1,935
Held to Maturity, Less Than 12 Months, Unrealized Losses	44	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	$ 1,887	$ 1,935
Held to Maturity, Total, Unrealized Losses	44	22
Obligations of states and political subdivisions
Investment [Line Items]
Available for Sale, Less Than 12 Months, Fair Value	21,666	9,612
Available for Sale, Less Than 12 Months, Unrealized Losses	291	77
Available for Sale, 12 Months or Longer, Fair Value	12,689	12,970
Available for Sale, 12 Months or Longer, Unrealized Losses	$ 582	$ 340
Available for Sale, Number of Securities (in shares) | Security	63	39
Available for Sale, Total, Fair Value	$ 34,355	$ 22,582
Available for Sale, Total, Unrealized Losses	873	417
Held to Maturity, Less Than 12 Months, Fair Value	20,440	15,353
Held to Maturity, Less Than 12 Months, Unrealized Losses	222	56
Held to Maturity, 12 Months or Longer, Fair Value	5,948	6,028
Held to Maturity, 12 Months or Longer, Unrealized Losses	$ 190	$ 132
Held to Maturity, Number of Securities (in shares) | Security	38	23
Held to Maturity, Total, Fair Value	$ 26,388	$ 21,381
Held to Maturity, Total, Unrealized Losses	$ 412	$ 188