Allowance for Loan Losses - Accrued Interest Receivable (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Accounts Notes And Loans Receivable [Line Items]
Accrued Interest Receivable	$ 1,994	$ 1,999
Private Education Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Accrued Interest Receivable	319	354
Accrued Interest Receivable, Allowance for Uncollectible Interest	28	27
Private Education Loans [Member] | TDR [Member]
Accounts Notes And Loans Receivable [Line Items]
Accrued Interest Receivable	191	205
Accrued Interest Receivable, Allowance for Uncollectible Interest	22	23
Private Education Loans [Member] | Non-TDR [Member]
Accounts Notes And Loans Receivable [Line Items]
Accrued Interest Receivable	128	149
Accrued Interest Receivable, Allowance for Uncollectible Interest	6	4
Accrued Interest Receivable, Greater Than 90 Days Past Due [Member] | Private Education Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Accrued Interest Receivable	29	29
Accrued Interest Receivable, Greater Than 90 Days Past Due [Member] | Private Education Loans [Member] | TDR [Member]
Accounts Notes And Loans Receivable [Line Items]
Accrued Interest Receivable	26	26
Accrued Interest Receivable, Greater Than 90 Days Past Due [Member] | Private Education Loans [Member] | Non-TDR [Member]
Accounts Notes And Loans Receivable [Line Items]
Accrued Interest Receivable	$ 3	$ 3