Summary of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Measurements, Recurring
Financial Assets:
Trading account assets	$ 2,259	$ 2,244
Financial Liabilities:
Interest rate swap agreements	12,718	11,387
Fair Value, Measurements, Recurring \| Obligations of states and political subdivisions
Financial Assets:
Securities available-for-sale	36,317	39,758
Fair Value, Measurements, Recurring \| Mortgage-backed securities issued or guaranteed by U.S. government sponsored enterprises
Financial Assets:
Securities available-for-sale	386,947	376,934
Fair Value, Measurements, Recurring \| Collateralized mortgage obligations issued or guaranteed by U.S. government sponsored enterprises
Financial Assets:
Securities available-for-sale	296,982	128,450
Fair Value, Measurements, Recurring \| Private issue collateralized mortgage obligations
Financial Assets:
Securities available-for-sale	10,384	10,641
Fair Value, Measurements, Recurring \| Obligations of U.S. government sponsored enterprises
Financial Assets:
Securities available-for-sale		30,107
Fair Value, Measurements, Recurring \| Equity securities
Financial Assets:
Securities available-for-sale		4,146
Readily Available Market Prices (Level 1) \| Fair Value, Measurements, Recurring
Financial Assets:
Trading account assets	2,259	2,244
Observable Market Data (Level 2)
Financial Assets:
Securities available-for-sale	730,630
Financial Liabilities:
Interest rate swap agreements	12,718
Observable Market Data (Level 2) \| Fair Value, Measurements, Recurring
Financial Liabilities:
Interest rate swap agreements	12,718	11,387
Observable Market Data (Level 2) \| Fair Value, Measurements, Recurring \| Obligations of states and political subdivisions
Financial Assets:
Securities available-for-sale	36,317	39,758
Observable Market Data (Level 2) \| Fair Value, Measurements, Recurring \| Mortgage-backed securities issued or guaranteed by U.S. government sponsored enterprises
Financial Assets:
Securities available-for-sale	386,947	376,934
Observable Market Data (Level 2) \| Fair Value, Measurements, Recurring \| Collateralized mortgage obligations issued or guaranteed by U.S. government sponsored enterprises
Financial Assets:
Securities available-for-sale	296,982	128,450
Observable Market Data (Level 2) \| Fair Value, Measurements, Recurring \| Private issue collateralized mortgage obligations
Financial Assets:
Securities available-for-sale	10,384	10,641
Observable Market Data (Level 2) \| Fair Value, Measurements, Recurring \| Obligations of U.S. government sponsored enterprises
Financial Assets:
Securities available-for-sale		30,107
Observable Market Data (Level 2) \| Fair Value, Measurements, Recurring \| Equity securities
Financial Assets:
Securities available-for-sale		$ 4,146

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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Derivative instrument obligations meeting the definition of a liability which the Company is a party to as of the balance sheet date.

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- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Trading Securities which consist of all investments in certain debt and equity securities that are bought and held principally for the purpose of selling them in the near term (thus held for only a short period of time) or for which an entity has elected the fair value option. Trading generally reflects active and frequent buying and selling, and trading securities are generally used with the objective of generating profits on short-term differences in price, but this category of investments may also include items which are not intended to be sold in the near term or for which the fair value option has been elected.

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