Fair Values of Assets and Liabilities - Carrying Amounts and Fair Values of Financial Instruments (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value Measurements Of Financial Instruments [Line Items]
Cash and due from banks	$ 72,276,000us-gaap_CashAndDueFromBanks	$ 74,427,000us-gaap_CashAndDueFromBanks
Interest-bearing deposits	2,262,000us-gaap_InterestBearingDepositsInBanks	3,012,000us-gaap_InterestBearingDepositsInBanks
Securities available for sale	1,309,819,000us-gaap_AvailableForSaleSecurities	1,318,365,000us-gaap_AvailableForSaleSecurities
Other investments	44,545,000us-gaap_FederalHomeLoanBankStock	35,444,000us-gaap_FederalHomeLoanBankStock
Loans held for sale	2,502,000us-gaap_LoansReceivableHeldForSaleNet	0us-gaap_LoansReceivableHeldForSaleNet
Loans	4,457,308,000	4,283,833,000	4,204,704,000
Deposits	4,315,511,000us-gaap_Deposits	4,603,863,000us-gaap_Deposits
Short-term borrowings	1,105,876,000us-gaap_ShortTermBorrowings	626,615,000us-gaap_ShortTermBorrowings	356,227,000us-gaap_ShortTermBorrowings
Long-term debt	89,459,000us-gaap_OtherLongTermDebt	144,385,000us-gaap_OtherLongTermDebt
Subordinated debt	72,167,000us-gaap_SubordinatedDebt	72,167,000us-gaap_SubordinatedDebt
Level 1 [Member]
Fair Value Measurements Of Financial Instruments [Line Items]
Securities available for sale	0	0
Other investments	0	0
Level 2 [Member]
Fair Value Measurements Of Financial Instruments [Line Items]
Securities available for sale	1,279,400,000	1,293,422,000
Other investments	44,545,000	35,444,000
Level 3 [Member]
Fair Value Measurements Of Financial Instruments [Line Items]
Securities available for sale	30,419,000	24,943,000
Other investments	0	0
Carrying Amount [Member]
Fair Value Measurements Of Financial Instruments [Line Items]
Cash and due from banks	72,276,000	74,427,000
Interest-bearing deposits	2,262,000	3,012,000
Securities available for sale	1,309,819,000	1,318,365,000
Other investments	44,545,000	35,444,000
Loans held for sale	2,502,000
Loans	4,457,308,000	4,283,833,000
Deposits	4,315,511,000	4,603,863,000
Short-term borrowings	1,105,876,000	626,615,000
Long-term debt	89,459,000	144,385,000
Subordinated debt	72,167,000	72,167,000
Estimated Fair Value [Member]
Fair Value Measurements Of Financial Instruments [Line Items]
Cash and due from banks	72,276,000	74,427,000
Interest-bearing deposits	2,262,000	3,012,000
Securities available for sale	1,309,819,000	1,318,365,000
Other investments	44,545,000	35,444,000
Loans held for sale	2,502,000
Loans	4,439,766,000	4,321,847,000
Deposits	4,319,997,000	4,531,685,000
Short-term borrowings	1,105,867,000	626,603,000
Long-term debt	90,319,000	145,477,000
Subordinated debt	62,815,000	51,706,000
Estimated Fair Value [Member] \| Level 1 [Member]
Fair Value Measurements Of Financial Instruments [Line Items]
Cash and due from banks	72,276,000	74,427,000
Interest-bearing deposits	2,262,000	3,012,000
Securities available for sale	0	0
Other investments	0	0
Loans held for sale	0
Loans	0	0
Deposits	0	0
Short-term borrowings	0	0
Long-term debt	0	0
Subordinated debt	0	0
Estimated Fair Value [Member] \| Level 2 [Member]
Fair Value Measurements Of Financial Instruments [Line Items]
Cash and due from banks	0	0
Interest-bearing deposits	0	0
Securities available for sale	1,279,400,000	1,293,422,000
Other investments	44,545,000	35,444,000
Loans held for sale	2,502,000
Loans	34,864,000	36,903,000
Deposits	4,319,997,000	4,531,685,000
Short-term borrowings	1,105,867,000	626,603,000
Long-term debt	90,319,000	145,477,000
Subordinated debt	0	0
Estimated Fair Value [Member] \| Level 3 [Member]
Fair Value Measurements Of Financial Instruments [Line Items]
Cash and due from banks	0	0
Interest-bearing deposits	0	0
Securities available for sale	30,419,000	24,943,000
Other investments	0	0
Loans held for sale	0
Loans	4,404,902,000	4,284,944,000
Deposits	0	0
Short-term borrowings	0	0
Long-term debt	0	0
Subordinated debt	$ 62,815,000	$ 51,706,000

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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For banks and other depository institutions: Includes cash on hand (currency and coin), cash items in process of collection, noninterest bearing deposits due from other financial institutions (including corporate credit unions), and noninterest bearing balances with the Federal Reserve Banks, Federal Home Loan Banks and central banks.

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The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

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Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

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For banks and other depository institutions (including Federal Reserve Banks, if applicable): Interest-bearing deposits in other financial institutions for relatively short periods of time including, for example, certificates of deposits, which are presented separately from cash on the balance sheet.

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Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

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Amount classified as loans attributable to disposal group held for sale or disposed of. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.

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Including both current and noncurrent portions, carrying amount as of the balance sheet date of other forms of debt not elsewhere specified in the taxonomy with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.

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