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Amount of subordinated debentures, assumed at the acquisition date.

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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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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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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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Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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

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Amount of advances from Federal Home Loan Bank (FHLBank) maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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

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Other Borrowings and Subordinated Debentures (Details) - USD ($)	1 Months Ended			12 Months Ended
	Oct. 31, 2018	Sep. 30, 2018	Oct. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Oct. 31, 2019	Oct. 30, 2019	Mar. 31, 2018	May 15, 2017	May 14, 2017
Debt Instrument [Line Items]
Commitment fee on unused portion of credit facility (as percent)			0.25%
Balance due				$ 1,685,859,000	$ 1,699,400,000
Repayments of subordinated debentures				0	113,990,000	$ 3,000,000
FHLB advances outstanding				1,250,000,000
Advances from Federal Home Loan Banks				1,260,000,000
Additional advances from Federal Home Loan Bank				3,200,000,000
Mortgage loans and investment securities securing FHLB advances				5,900,000,000
Assets				$ 21,259,143,000	$ 16,543,337,000	$ 15,000,000,000
Federal Home Loan Bank Owns the Options (FOTO) Advances \| Minimum
Debt Instrument [Line Items]
Term of debt instrument				10 years
Federal Home Loan Bank Owns the Options (FOTO) Advances \| Maximum
Debt Instrument [Line Items]
Term of debt instrument				15 years
Landrum Company
Debt Instrument [Line Items]
Subordinated debentures							$ 33,917,000	$ 34,794,000
Hardeman County Investment Company, Inc.
Debt Instrument [Line Items]
Subordinated debentures										$ 6,702,000	$ 6,702,000
OKSB and First Texas Acquisitions
Debt Instrument [Line Items]
Repayments of subordinated debentures		$ 77,300,000
U.S. Bank National Association \| Revolving Credit Facility
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	$ 50,000,000.0		$ 75,000,000.0
Commitment fee on unused portion of credit facility (as percent)	0.30%
Balance due		$ 0
Subordinated Debt
Debt Instrument [Line Items]
Face amount of debt									$ 330,000,000.0
Debt instrument, interest rate				5.00%					5.00%
Debt issuance costs									$ 3,600,000