Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 37,489
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	512
Held to Maturity, More than 12 Months, Estimated Fair Value	154,689
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,185
Held to Maturity, Total Estimated Fair Value	192,178
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	3,697	$ 6,166
Available for Sale, Less than 12 Months, Estimated Fair Value	10,479
Available for Sale, Less than 12 Months, Unrealized Losses	10
Available for Sale, More than 12 Months, Estimated Fair Value	16,844
Available for Sale, More than 12 Months, Unrealized Losses	110
Available for Sale, Total Estimated Fair Value	27,323
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	120	10,700
Residential mortgage-backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0
Held to Maturity, More than 12 Months, Estimated Fair Value	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Held to Maturity, Total Estimated Fair Value	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	4
Available for Sale, Less than 12 Months, Estimated Fair Value	139
Available for Sale, Less than 12 Months, Unrealized Losses	3
Available for Sale, More than 12 Months, Estimated Fair Value	16,844
Available for Sale, More than 12 Months, Unrealized Losses	110
Available for Sale, Total Estimated Fair Value	16,983
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	113	734
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	37,489
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	512
Held to Maturity, More than 12 Months, Estimated Fair Value	154,689
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,185
Held to Maturity, Total Estimated Fair Value	192,178
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	3,697	6,149
Available for Sale, Less than 12 Months, Estimated Fair Value	10,340
Available for Sale, Less than 12 Months, Unrealized Losses	7
Available for Sale, More than 12 Months, Estimated Fair Value	0
Available for Sale, More than 12 Months, Unrealized Losses	0
Available for Sale, Total Estimated Fair Value	10,340
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 7	$ 1,459