Allowance for Loan Losses - Loan Type (Detail) - FFELP Loans [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Financing Receivable Recorded Investment Past Due [Line Items]
Total loans, gross	$ 51,268	$ 57,155
Change	(5,887)
Stafford Loans [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total loans, gross	15,975	17,327
Change	(1,352)
Consolidation Loans [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total loans, gross	30,665	34,961
Change	(4,296)
Rehab Loans [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total loans, gross	4,628	$ 4,867
Change	$ (239)