Fair Value Disclosures - Schedule of Estimated Fair Values and Carrying Values of All Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	$ 130,979	$ 209,583	$ 163,674
FEDERAL FUNDS SOLD			3,840
INTEREST-BEARING DEPOSITS IN BANKS	67,060	162,764	55,165
INTEREST-BEARING TIME DEPOSITS IN BANKS	1,458	1,458	1,707
Available-for-sale Securities	3,276,193	3,087,473	3,018,279
Held-to-maturity securities	0	0	114
Loans	3,697,582	3,452,543	3,339,949
BORROWINGS	372,155	331,000	360,264
Carrying Value [Member] \| Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	130,979	209,583	163,674
FEDERAL FUNDS SOLD			3,840
INTEREST-BEARING DEPOSITS IN BANKS	67,060	162,764	55,165
Deposits with no stated maturities	5,722,685	5,511,706	5,166,248
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	1,458	1,707
Held-to-maturity securities			114
Accrued interest receivable	28,631	36,081	26,639
Deposits with stated maturities	468,078	451,255	495,720
BORROWINGS	372,155	331,000	360,264
Accrued interest Payable	195	197	166
Carrying Value [Member] \| Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	3,276,193	3,087,473	3,018,279
Carrying Value [Member] \| Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,697,582	3,452,543	3,339,949
Estimated Fair Value [Member] \| Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	130,979	209,583	163,674
FEDERAL FUNDS SOLD			3,840
INTEREST-BEARING DEPOSITS IN BANKS	67,060	162,764	55,165
Deposits with no stated maturities	5,722,685	5,511,706	5,166,248
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	1,458	1,708
Held-to-maturity securities			116
Accrued interest receivable	28,631	36,081	26,639
Deposits with stated maturities	469,000	451,255	496,782
BORROWINGS	372,155	331,000	360,264
Accrued interest Payable	195	197	166
Estimated Fair Value [Member] \| Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	3,276,193	3,087,473	3,018,279
Estimated Fair Value [Member] \| Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	$ 3,718,987	$ 3,470,133	$ 3,368,084

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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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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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