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Call option period.

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Irrevocable letter of credit.

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Letter of credit facility expiration period.

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Letter of credit facility extended expiration period.

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Subordinated debt maturity period.

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Unregistered debt transactions.

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Unregistered multi tranche debt transactions.

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Variable letter of credit.

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Amount of borrowings as of the balance sheet date from the Federal Home Loan Bank, which are primarily used to cover shortages in the required reserve balance and liquidity shortages.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Amount of the collateral pledged in support of federal home loan bank advances outstanding.

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Amount of maximum advances or credit lines available from the Federal Home Loan Bank (FHLB).

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

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Carrying amount as of the balance sheet date of loans that have been pledged as collateral for borrowings. If the secured party has the right by contract or custom to sell or re-pledge the collateral, then the debtor reclassifies that asset and report that asset in its statement of financial position separately from other assets not so encumbered.

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The cash inflow from a borrowing where a lender is placed in a lien position behind debt having a higher priority of repayment (senior) in liquidation of the entity's assets or underlying collateral.

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Borrowed Funds - Additional Information (Details) - USD ($)		1 Months Ended			3 Months Ended	12 Months Ended
	Mar. 29, 2020	Jun. 30, 2019	Mar. 31, 2015	Jun. 30, 2014	Mar. 31, 2020	Dec. 31, 2019	Mar. 29, 2019
Borrowed Funds [Line Items]
Unregistered multi tranche debt Transactions				$ 245,000,000
Unregistered debt transactions			$ 50,000,000
Subordinated debt					$ 183,032,000	$ 182,712,000
Subordinate debt capital treatment achievement period					10 years
FHLB advances					$ 100,000,000	100,000,000
FHLB borrowing availability					1,300,000,000
Irrevocable letter of credit					566,000,000	391,000,000
Notes payable					$ 2,102,000	2,078,000
Holding Company Revolving Loan Facility \| Revolving Credit Facility
Borrowed Funds [Line Items]
Debt instrument, maturity date	Mar. 29, 2021				Mar. 29, 2020
Borrowings					$ 0
Line of credit facility, borrowing capacity							$ 100,000,000
Linscomb & Williams, Inc. \| Wealth & Pension Services Group, Inc.
Borrowed Funds [Line Items]
Notes payable					2,100,000
FRB
Borrowed Funds [Line Items]
Borrowings					0	$ 0
Public Fund Treasury Management Deposit
Borrowed Funds [Line Items]
Variable letter of credit					$ 500,000,000
Municipal Deposits
Borrowed Funds [Line Items]
Letter of credit expiration date					Jul. 20, 2020
Commercial Loans \| FRB
Borrowed Funds [Line Items]
Collateralized borrowings from FRB					$ 805,700,000
Commercial and Residential Real Estate Loan
Borrowed Funds [Line Items]
FHLB advances collateral amount					$ 1,900,000,000
4.75% Fixed to Floating Rate Subordinated Notes Due 2029
Borrowed Funds [Line Items]
Aggregate principle amount		$ 85,000,000
Debt instrument, interest rate		4.75%
4.875% Senior Notes, Due June 28, 2019
Borrowed Funds [Line Items]
Debt instrument, interest rate		4.875%
Debt instrument, maturity date					Jun. 28, 2019
Subordinated debt maturity period					4 years
5.375% Senior Notes, Due June 28, 2021
Borrowed Funds [Line Items]
Debt instrument, interest rate					5.375%	5.375%
Debt instrument, maturity date					Jun. 28, 2021	Jun. 28, 2021
Subordinated debt maturity period					7 years
7.250% Subordinated Notes, Due June 28, 2029, Callable in 2024
Borrowed Funds [Line Items]
Debt instrument, interest rate					7.25%	7.25%
Debt instrument, maturity date					Jun. 28, 2029	Jun. 28, 2029
Subordinated debt maturity period					15 years
Subordinated debt					$ 35,000,000
Call option period					10 years
6.250% Subordinated Notes, Due June 28, 2029, Callable in 2024
Borrowed Funds [Line Items]
Debt instrument, interest rate					6.25%	6.25%
Debt instrument, maturity date					Jun. 28, 2029	Jun. 28, 2029
Subordinated debt maturity period					15 years
Subordinated debt					$ 25,000,000
Call option period					10 years
4.750% Subordinated Notes, Due June 2029, Callable in 2024
Borrowed Funds [Line Items]
Debt instrument, interest rate					4.75%	4.75%
Debt instrument, maturity date					Jun. 30, 2029	Jun. 30, 2029
Subordinated debt maturity period					10 years
Subordinated debt					$ 85,000,000
Call option period					5 years
6.500% Subordinated Notes, Due March 2025, Callable in 2020
Borrowed Funds [Line Items]
Debt instrument, interest rate					6.50%	6.50%
Debt instrument, maturity date					Mar. 31, 2025	Mar. 31, 2025
Subordinated debt					$ 40,000,000
Call option period					5 years
Expiration on June 02, 2020 \| Harris County Deposits
Borrowed Funds [Line Items]
Irrevocable letter of credit					$ 60,000,000
Letter of credit expiration date					Jun. 02, 2020
Expiration on April 16, 2020 \| Public Fund Treasury Management Deposit
Borrowed Funds [Line Items]
Letter of credit expiration date					Apr. 16, 2020
Letter of credit expiration period					45 days
Letter of credit extended expiration term					1 year
Letter of Credit \| Municipal Deposits
Borrowed Funds [Line Items]
Borrowings					$ 6,000,000