Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 3,084
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1
Held to Maturity, More than 12 Months, Estimated Fair Value	186,220
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,808
Held to Maturity, Total Estimated Fair Value	189,304
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	3,809	$ 6,166
Available for Sale, Less than 12 Months, Estimated Fair Value	573,705
Available for Sale, Less than 12 Months, Unrealized Losses	5,822
Available for Sale, More than 12 Months, Estimated Fair Value	7,050
Available for Sale, More than 12 Months, Unrealized Losses	44
Available for Sale, Total Estimated Fair Value	580,755
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	5,866	10,700
Residential mortgage-backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	4
Available for Sale, Less than 12 Months, Estimated Fair Value	8,539
Available for Sale, Less than 12 Months, Unrealized Losses	28
Available for Sale, More than 12 Months, Estimated Fair Value	7,050
Available for Sale, More than 12 Months, Unrealized Losses	44
Available for Sale, Total Estimated Fair Value	15,589
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	72	734
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	3,084
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1
Held to Maturity, More than 12 Months, Estimated Fair Value	186,220
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,808
Held to Maturity, Total Estimated Fair Value	189,304
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	3,809	6,149
Available for Sale, Less than 12 Months, Estimated Fair Value	565,166
Available for Sale, Less than 12 Months, Unrealized Losses	5,794
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	565,166
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 5,794	$ 1,459

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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