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Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Parenthetical) (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Accounts Notes And Loans Receivable [Line Items] |  |  |
Adjustment resulting from the change in the charge-off rate | $ 25.0 | $ 30.0 | $ 16.0
Expected future recoveries of current period defaults | 47.0 | 57.0 | 22.0
Change in balance during period | (48.0) | (55.0) | (150.0)
Sale of private education loan |  |  | 1.6
Private Education Loans [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Adjustment resulting from the change in the charge-off rate | 25.0 | 30.0 | 16.0
Beginning of period expected future recoveries on previously fully charged-off loans | 274.0 | 329.0 | 479.0
Expected future recoveries of current period defaults | 47.0 | 57.0 | 22.0
Recoveries (cash collected) | (46.0) | (56.0) | (87.0)
Charge-offs (as a result of lower recovery expectations) | (49.0) | (56.0) | (35.0)
Reduction in expected recoveries related to regulatory settlement |  |  | (50.0)
End of period expected future recoveries on previously fully charged-off loans | 226.0 | 274.0 | 329.0
Change in balance during period | (48.0) | (55.0) | (150.0)
Private Education Loans [Member] | Loans Receivable For Previously Fully Charged Off Loans [Member] |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |
Adjustment resulting from the change in the charge-off rate | $ (25.0) | $ (30.0) | $ (16.0)

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