Fair Value Disclosures - Schedule of Estimated Fair Values and Carrying Values of All Financial Instruments (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	$ 163,435	$ 204,782	$ 135,092
FEDERAL FUNDS SOLD	3,740	3,130	2,960
INTEREST-BEARING DEPOSITS IN BANKS	53,336	48,574	67,746
INTEREST-BEARING TIME DEPOSITS IN BANKS	1,458	1,707	2,427
Available-for-sale Securities	2,964,513	2,860,837	2,795,356
Held-to-maturity securities	105	121	137
Loans	3,410,269	3,338,426	3,264,328
BORROWINGS	379,324	445,770	556,924
Carrying Value [Member] \| Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	163,435	204,782	135,092
FEDERAL FUNDS SOLD	3,740	3,130	2,960
INTEREST-BEARING DEPOSITS IN BANKS	53,336	48,574	67,746
Deposits with no stated maturities	5,149,168	4,969,543	4,501,536
Carrying Value [Member] \| Level 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	1,458	1,707	2,427
Held-to-maturity securities	105	121	137
Accrued interest receivable	35,600	36,469	33,516
Deposits with stated maturities	477,441	508,996	554,753
BORROWINGS	379,324	445,770	556,924
Accrued interest Payable	164	225	283
Carrying Value [Member] \| Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	2,964,513	2,860,837	2,795,356
Carrying Value [Member] \| Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,410,269	3,338,426	3,264,328
Estimated Fair Value [Member] \| Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	163,435	204,782	135,092
FEDERAL FUNDS SOLD	3,740	3,130	2,960
INTEREST-BEARING DEPOSITS IN BANKS	53,336	48,574	67,746
Deposits with no stated maturities	5,149,168	4,969,543	4,501,536
Estimated Fair Value [Member] \| Level 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	1,458	1,709	2,429
Held-to-maturity securities	107	124	141
Accrued interest receivable	35,600	36,469	33,516
Deposits with stated maturities	478,393	510,304	556,224
BORROWINGS	379,324	445,770	556,924
Accrued interest Payable	164	225	283
Estimated Fair Value [Member] \| Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	2,964,513	2,860,837	2,795,356
Estimated Fair Value [Member] \| Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	$ 3,466,431	$ 3,361,735	$ 3,269,221

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Deposits with no stated maturities.

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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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