Fair Value Disclosures - Schedule of Estimated Fair Values and Carrying Values of All Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	$ 177,615	$ 204,782	$ 166,981
FEDERAL FUNDS SOLD		3,130	3,400
INTEREST-BEARING DEPOSITS IN BANKS	166,820	48,574	117,334
INTEREST-BEARING TIME DEPOSITS IN BANKS	1,458	1,707	1,707
Available-for-sale Securities	2,885,483	2,860,837	2,729,030
Held-to-maturity securities	0	121	129
Loans	3,443,424	3,338,426	3,324,086
BORROWINGS	351,435	445,770	513,759
Carrying Value [Member] \| Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	177,615	204,782	166,981
FEDERAL FUNDS SOLD		3,130	3,400
INTEREST-BEARING DEPOSITS IN BANKS	166,820	48,574	117,334
Deposits with no stated maturities	5,232,678	4,969,543	4,699,671
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	1,707
Held-to-maturity securities		121	129
Accrued interest receivable	26,321	36,469	26,209
Deposits with stated maturities	464,782	508,996	535,793
BORROWINGS	351,435	445,770	513,759
Accrued interest Payable	179	225	230
Carrying Value [Member] \| Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	2,885,483	2,860,837	2,729,030
Carrying Value [Member] \| Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,443,424	3,338,426	3,324,086
Estimated Fair Value [Member] \| Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	177,615	204,782	166,981
FEDERAL FUNDS SOLD		3,130	3,400
INTEREST-BEARING DEPOSITS IN BANKS	166,820	48,574	117,334
Deposits with no stated maturities	5,232,678	4,969,543	4,699,671
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	1,709
Held-to-maturity securities		124	133
Accrued interest receivable	26,321	36,469	26,209
Deposits with stated maturities	465,655	510,304	537,167
BORROWINGS	351,435	445,770	513,759
Accrued interest Payable	179	225	230
Estimated Fair Value [Member] \| Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	2,885,483	2,860,837	2,729,030
Estimated Fair Value [Member] \| Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	$ 3,458,603	$ 3,361,735	$ 3,334,965

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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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