Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 62,407
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,018
Held to Maturity, More than 12 Months, Estimated Fair Value	142,187
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	4,072
Held to Maturity, Total Estimated Fair Value	204,594
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	5,090	$ 6,166
Available for Sale, Less than 12 Months, Estimated Fair Value	164,507
Available for Sale, Less than 12 Months, Unrealized Losses	451
Available for Sale, More than 12 Months, Estimated Fair Value	91,930
Available for Sale, More than 12 Months, Unrealized Losses	641
Available for Sale, Total Estimated Fair Value	256,437
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1,092	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	2,352
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2
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	2,352
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	2	4
Available for Sale, Less than 12 Months, Estimated Fair Value	227
Available for Sale, Less than 12 Months, Unrealized Losses	2
Available for Sale, More than 12 Months, Estimated Fair Value	24,321
Available for Sale, More than 12 Months, Unrealized Losses	214
Available for Sale, Total Estimated Fair Value	24,548
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	216	734
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	60,055
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,016
Held to Maturity, More than 12 Months, Estimated Fair Value	142,187
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	4,072
Held to Maturity, Total Estimated Fair Value	202,242
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	5,088	6,149
Available for Sale, Less than 12 Months, Estimated Fair Value	164,280
Available for Sale, Less than 12 Months, Unrealized Losses	449
Available for Sale, More than 12 Months, Estimated Fair Value	67,609
Available for Sale, More than 12 Months, Unrealized Losses	427
Available for Sale, Total Estimated Fair Value	231,889
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 876	$ 1,459