Fair Value Disclosures - Schedule of Estimated Fair Values and Carrying Values of All Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	$ 166,981	$ 179,140	$ 133,340
FEDERAL FUNDS SOLD	3,400	3,810	2,790
INTEREST-BEARING DEPOSITS IN BANKS	117,334	89,936	4,268
INTEREST-BEARING TIME DEPOSITS IN BANKS	1,707	3,495	4,491
Available-for-sale Securities	2,729,030	2,733,899	2,737,353
Held-to-maturity securities	129	278	286
Loans	3,324,086	3,308,716	3,248,002
Carrying Value [Member] \| Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	166,981	179,140	133,340
FEDERAL FUNDS SOLD	3,400	3,810	2,790
INTEREST-BEARING DEPOSITS IN BANKS	117,334	89,936	4,268
Deposits with no stated maturities	4,699,671	4,569,317	4,444,364
Carrying Value [Member] \| Level 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	1,707	3,495	4,491
Held-to-maturity securities	129	278	286
Accrued interest receivable	26,209	34,697	26,888
Deposits with stated maturities	535,793	620,852	652,919
Borrowings	513,759	615,675	500,903
Accrued interest Payable	230	240	261
Carrying Value [Member] \| Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	2,729,030	2,733,899	2,737,353
Carrying Value [Member] \| Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,324,086	3,308,716	3,248,002
Estimated Fair Value [Member] \| Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	166,981	179,140	133,340
FEDERAL FUNDS SOLD	3,400	3,810	2,790
INTEREST-BEARING DEPOSITS IN BANKS	117,334	89,936	4,268
Deposits with no stated maturities	4,699,671	4,569,317	4,444,364
Estimated Fair Value [Member] \| Level 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	1,709	3,500	4,498
Held-to-maturity securities	133	283	291
Accrued interest receivable	26,209	34,697	26,888
Deposits with stated maturities	537,167	622,572	654,705
Borrowings	513,759	615,675	500,903
Accrued interest Payable	230	240	261
Estimated Fair Value [Member] \| Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	2,729,030	2,733,899	2,737,353
Estimated Fair Value [Member] \| Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	$ 3,334,965	$ 3,316,243	$ 3,249,558

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Deposits with no stated maturities.

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Deposits with stated maturities.

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Interest-bearing time deposits in banks.

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Amount of debt 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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Represents the aggregate of total long-term debt, including current maturities and short-term debt.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The amount outstanding of funds lent to other depository institutions, securities brokers, or securities dealers in the form of Federal Funds sold; for example, immediately available funds lent under agreements or contracts that mature in one business day or roll over under a continuing contract, regardless of the nature of the transaction or the collateral involved, excluding overnight lending for commercial and industrial purposes. Also include Federal Funds sold under agreements to resell on a gross basis, excluding (1) sales of term Federal Funds, (2) due bills representing purchases of securities or other assets by the reporting bank that have not yet been delivered and similar instruments, (3) resale agreements that mature in more than one business day involving assets other than securities, and (4) yield maintenance dollar repurchase agreements.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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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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Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

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Amount after allowance and 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. Includes loans held for sale. Excludes loans and leases covered under loss sharing agreements.

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