Investment Securities (Narrative) (Details) - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, written down to fair value	$ 0	$ 0
Writedown to fair value	0	0
Available-for-sale, allowance for credit loss	0	0
Accrued interest for AFS securities	646,000,000	634,000,000
Write-offs of accrued interest receivable on AFS securities	0	$ 0
Deposits [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value of pledged securities	1,300,000,000
Federal Home Loan Bank Advances [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value of pledged securities	51,000,000,000
Federal Reserve Bank Advances [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value of pledged securities	$ 9,900,000,000

Debt Securities, Available-for-Sale, Write-down To Fair Value

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- Definition

Amount, after allowance for credit loss, of accrued interest on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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- Definition

Amount of writeoff of accrued interest on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized by reversing interest income.

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Amount of allowance for credit loss on 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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Amount of writeoff of allowance for credit loss on 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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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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The carrying amount (fair value) as of the balance sheet date of securities held that had been pledged by counterparties under financing and lending arrangements, pursuant to which the entity has the right by agreement or custom to sell or re-pledge such securities.

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