Other Borrowings and Subordinated Notes and Debentures - Textual (Details) - USD ($)	1 Months Ended	9 Months Ended
Mar. 31, 2018	Sep. 30, 2021
Debt Instrument [Line Items]
FHLB advances		$ 1,310,000,000
FHLB advances outstanding		1,300,000,000
Mortgage loans and investment securities securing FHLB advances		4,400,000,000
Additional advances from FHLB		$ 3,000,000,000
Federal Home Loan Bank Owns the Options (FOTO) Advances \| Minimum
Debt Instrument [Line Items]
Debt term		10 years
Federal Home Loan Bank Owns the Options (FOTO) Advances \| Maximum
Debt Instrument [Line Items]
Debt term		15 years
Subordinated debt
Debt Instrument [Line Items]
Subordinated debentures	$ 330,000,000
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%

Advances from Federal Home Loan Bank available.

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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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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 advance from Federal Home Loan Bank (FHLBank) maturing in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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