Fair Value Disclosures - Schedule of Estimated Fair Values and Carrying Values of All Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	$ 163,674	$ 204,782	$ 139,995
FEDERAL FUNDS SOLD	3,840	3,130	2,660
INTEREST-BEARING DEPOSITS IN BANKS	55,165	48,574	22,993
INTEREST-BEARING TIME DEPOSITS IN BANKS	1,707	1,707	2,427
Available-for-sale Securities	3,018,279	2,860,837	2,763,039
Held-to-maturity securities	114	121	146
Loans	3,339,949	3,338,426	3,255,135
BORROWINGS	360,264	445,770	525,340
Carrying Value [Member] \| Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	163,674	204,782	139,995
FEDERAL FUNDS SOLD	3,840	3,130	2,660
INTEREST-BEARING DEPOSITS IN BANKS	55,165	48,574	22,993
Deposits with no stated maturities	5,166,248	4,969,543	4,473,909
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	1,707	2,427
Held-to-maturity securities	114	121	146
Accrued interest receivable	26,639	36,469	26,166
Deposits with stated maturities	495,720	508,996	589,898
BORROWINGS	360,264	445,770	525,340
Accrued interest Payable	166	225	254
Carrying Value [Member] \| Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	3,018,279	2,860,837	2,763,039
Carrying Value [Member] \| Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,339,949	3,338,426	3,255,135
Estimated Fair Value [Member] \| Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	163,674	204,782	139,995
FEDERAL FUNDS SOLD	3,840	3,130	2,660
INTEREST-BEARING DEPOSITS IN BANKS	55,165	48,574	22,993
Deposits with no stated maturities	5,166,248	4,969,543	4,473,909
Estimated Fair Value [Member] \| Level 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	1,708	1,709	2,430
Held-to-maturity securities	116	124	150
Accrued interest receivable	26,639	36,469	26,166
Deposits with stated maturities	496,782	510,304	591,675
BORROWINGS	360,264	445,770	525,340
Accrued interest Payable	166	225	254
Estimated Fair Value [Member] \| Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	3,018,279	2,860,837	2,763,039
Estimated Fair Value [Member] \| Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	$ 3,368,084	$ 3,361,735	$ 3,260,544

Accrued interest payable.

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Deposits with no 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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Amount of time deposit liabilities, including certificates of deposit.

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