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)