Fair Value Disclosures - Schedule of Estimated Fair Values and Carrying Values of All Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	$ 164,998	$ 209,583	$ 177,615
INTEREST-BEARING DEPOSITS IN BANKS	34,511	162,764	166,820
INTEREST-BEARING TIME DEPOSITS IN BANKS	1,458	1,458	1,458
Available-for-sale securities	3,144,367	3,087,473	2,885,483
Loans Held for Investment	3,816,169	3,437,413	3,424,305
Loans Held for Sale	18,496	15,130	19,119
BORROWINGS	380,760	331,000	351,435
Carrying Value [Member] \| Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	164,998	209,583	177,615
INTEREST-BEARING DEPOSITS IN BANKS	34,511	162,764	166,820
Deposits with no stated maturities	5,694,615	5,511,706	5,232,678
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	1,458
Accrued interest receivable	28,990	36,081	26,321
Deposits with stated maturities	450,667	452,000	464,782
BORROWINGS	380,760	331,000	351,435
Accrued interest payable	303	197	179
Carrying Value [Member] \| Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	3,144,367	3,087,473	2,885,483
Carrying Value [Member] \| Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans Held for Investment	3,816,169	3,437,413	3,424,305
Loans Held for Sale	18,496	15,130	19,119
Estimated Fair Value [Member] \| Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	164,998	209,583	177,615
INTEREST-BEARING DEPOSITS IN BANKS	34,511	162,764	166,820
Deposits with no stated maturities	5,694,615	5,511,706	5,232,678
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	1,458
Accrued interest receivable	28,990	36,081	26,321
Deposits with stated maturities	450,409	451,255	465,655
BORROWINGS	380,760	331,000	351,435
Accrued interest payable	303	197	179
Estimated Fair Value [Member] \| Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	3,144,367	3,087,473	2,885,483
Estimated Fair Value [Member] \| Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans Held for Investment	3,857,059	3,455,003	3,439,484
Loans Held for Sale	$ 18,496	$ 15,130	$ 19,119

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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 investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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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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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. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

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Amount, after allowance for bad debt, of mortgage loan receivables held-for-sale that are not part of a disposal group. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.

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Amount of time deposit liabilities, including certificates of deposit.

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