Advances from Federal Home Loan Bank available.

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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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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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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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Other Borrowings and Subordinated Notes and Debentures - Textual (Details) - USD ($)	1 Months Ended	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2020	Dec. 31, 2019	Oct. 31, 2019	Oct. 30, 2019	Dec. 31, 2017
Short-term Debt [Line Items]
FHLB advances outstanding		$ 1,350,000,000
FHLB advances		1,360,000,000
Additional advances from FHLB		2,100,000,000
Debt Instrument [Line Items]
Mortgage loans and investment securities securing FHLB advances		5,400,000,000
Assets		$ 20,841,352,000	$ 21,259,143,000			$ 15,000,000,000
Subordinated debt
Debt Instrument [Line Items]
Subordinated debentures	$ 330,000,000.0
Fixed rate (as percent)	5.00%	5.00%
Debt issuance costs	$ 3,600,000
Subordinated debt \| London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Floating rate (as percent)	2.15%
Federal Home Loan Bank Owns the Options (FOTO) Advances
Short-term Debt [Line Items]
FHLB advances outstanding		$ 1,300,000,000
Federal Home Loan Bank Owns the Options (FOTO) Advances \| Minimum
Short-term Debt [Line Items]
Debt term		10 years
Federal Home Loan Bank Owns the Options (FOTO) Advances \| Maximum
Short-term Debt [Line Items]
Debt term		15 years
Federal Home Loan Bank Advances
Short-term Debt [Line Items]
Debt term		1 year
Landrum Company
Debt Instrument [Line Items]
Subordinated debentures				$ 33,917,000	$ 34,794,000