Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) - USD ($) $ in Millions		12 Months Ended
	Jan. 01, 2020	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Allowance for Loan Losses
Allowance at beginning of period	$ 1,112	$ 1,271	$ 1,377	$ 1,112
Provision:
Reversal of allowance related to loan sales			(107)
Remaining provision			46
Total provision		79	(61)	155
Charge-offs:
Gross charge-offs		(410)	(201)	(265)
Expected future recoveries on current period gross charge-offs		57	22	32
Total		(353)	(179)	(233)
Adjustment resulting from the change in charge-off rate		(30)	(16)	(23)
Net charge-offs		(383)	(195)	(256)
Decrease in expected future recoveries on previously fully charged-off loans		55	150	109
Allowance at end of period		1,022	1,271	1,377
ASU 2016-13 [Member]
Charge-offs:
Allowance at end of period	1,369
ASU 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjustment
Allowance for Loan Losses
Allowance at beginning of period	257			257
ASU 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Allowance for Loan Losses
Allowance at beginning of period	1,369			1,369
FFELP Loans [Member]
Allowance for Loan Losses
Allowance at beginning of period	64	262	288	64
Provision:
Total provision				13
Charge-offs:
Gross charge-offs		(40)	(26)	(49)
Total		(40)	(26)	(49)
Net charge-offs		(40)	(26)	(49)
Allowance at end of period		$ 222	$ 262	$ 288
Net charge-offs as a percentage of average loans in repayment, excluding the net adjustment resulting from the change in charge-off rate		0.10%	0.06%	0.10%
Net charge-offs as a percentage of average loans in repayment		0.10%	0.06%	0.10%
Ending total loans		$ 43,747	$ 52,903	$ 58,572
Average loans in repayment		40,332	45,781	48,130
Ending loans in repayment		34,372	44,390	48,057
FFELP Loans [Member] | ASU 2016-13 [Member]
Charge-offs:
Allowance at end of period	324
FFELP Loans [Member] | ASU 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjustment
Allowance for Loan Losses
Allowance at beginning of period	260			260
FFELP Loans [Member] | ASU 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Allowance for Loan Losses
Allowance at beginning of period	324			324
Private Education Loans [Member]
Allowance for Loan Losses
Allowance at beginning of period	1,048	1,009	1,089	1,048
Provision:
Reversal of allowance related to loan sales			(107)
Remaining provision			46
Total provision		79	(61)	142
Charge-offs:
Gross charge-offs		(370)	(175)	(216)
Expected future recoveries on current period gross charge-offs		57	22	32
Total		(313)	(153)	(184)
Adjustment resulting from the change in charge-off rate		(30)	(16)	(23)
Net charge-offs		(343)	(169)	(207)
Decrease in expected future recoveries on previously fully charged-off loans		55	150	109
Allowance at end of period		$ 800	$ 1,009	$ 1,089
Net charge-offs as a percentage of average loans in repayment, excluding the net adjustment resulting from the change in charge-off rate		1.59%	0.76%	0.88%
Net adjustment resulting from the change in charge-off rate as a percentage of average loans in repayment		0.15%	0.08%	0.11%
Net charge-offs as a percentage of average loans in repayment		1.74%	0.84%	0.99%
Ending total loans		$ 19,525	$ 21,180	$ 22,168
Average loans in repayment		19,796	20,150	20,790
Ending loans in repayment		$ 18,770	$ 20,284	20,841
Private Education Loans [Member] | ASU 2016-13 [Member]
Charge-offs:
Allowance at end of period	1,045
Private Education Loans [Member] | ASU 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjustment
Allowance for Loan Losses
Allowance at beginning of period	(3)			(3)
Private Education Loans [Member] | ASU 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Allowance for Loan Losses
Allowance at beginning of period	$ 1,045			$ 1,045