Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) SecurityLoan	Jun. 30, 2018 USD ($) SecurityLoan	Jun. 30, 2019 USD ($) SecurityLoan	Jun. 30, 2018 USD ($) SecurityLoan
Allowance for Loan Losses
Beginning balance	$ 1,252	$ 1,367	$ 1,286	$ 1,367
Total provision	68	112	144	199
Charge-offs	(95)	(92)	(207)	(181)
Reclassification of interest reserve	1	2	4	4
Loan sales	(8)		(9)
Ending balance	1,218	1,389	1,218	1,389
Allowance Ending Balance:
Ending balance: individually evaluated for impairment - TDR	1,040	1,151	1,040	1,151
Collectively evaluated for impairment:
Excluding Purchased Non-Credit Impaired Loans acquired at a discount and Purchased Credit Impaired Loans	178	238	178	238
Ending balance	1,218	1,389	1,218	1,389
Loans Ending Balance:
Ending balance: individually evaluated for impairment - TDR	10,000	10,707	10,000	10,707
Collectively evaluated for impairment:
Excluding Purchased Non-Credit Impaired Loans acquired at a discount and Purchased Credit Impaired Loans	76,002	84,480	76,002	84,480
Purchased Non-Credit Impaired Loans acquired at a discount	4,648	5,420	4,648	5,420
Purchased Credit Impaired Loans	214	236	214	236
Ending total loans	90,864	100,843	90,864	100,843
FFELP Loans [Member]
Allowance for Loan Losses
Beginning balance	67	59	76	60
Total provision	8	40	15	50
Charge-offs	(7)	(17)	(24)	(28)
Ending balance	67	82	67	82
Collectively evaluated for impairment:
Excluding Purchased Non-Credit Impaired Loans acquired at a discount and Purchased Credit Impaired Loans	67	82	67	82
Ending balance	67	82	67	82
Collectively evaluated for impairment:
Excluding Purchased Non-Credit Impaired Loans acquired at a discount and Purchased Credit Impaired Loans	64,783	73,018	64,783	73,018
Purchased Non-Credit Impaired Loans acquired at a discount	2,671	3,034	2,671	3,034
Ending total loans	$ 67,454	$ 76,052	$ 67,454	$ 76,052
Charge-offs as a percentage of average loans in repayment	0.05%	0.11%	0.08%	0.09%
Allowance coverage of charge-offs | SecurityLoan	2.2	1.2	1.4	1.5
Allowance as a percentage of the ending total loan balance	0.10%	0.11%	0.10%	0.11%
Allowance as a percentage of the ending loans in repayment	0.12%	0.13%	0.12%	0.13%
Average loans in repayment	$ 56,657	$ 64,238	$ 57,435	$ 64,940
Ending loans in repayment	55,684	62,952	55,684	62,952
Private Education Loans [Member]
Allowance for Loan Losses
Beginning balance	1,178	1,298	1,201	1,297
Total provision	60	72	128	149
Charge-offs	(87)	(75)	(181)	(153)
Reclassification of interest reserve	1	2	4	4
Loan sales	(1)		(1)
Ending balance	1,151	1,297	1,151	1,297
Allowance Ending Balance:
Ending balance: individually evaluated for impairment - TDR	1,040	1,142	1,040	1,142
Collectively evaluated for impairment:
Excluding Purchased Non-Credit Impaired Loans acquired at a discount and Purchased Credit Impaired Loans	111	155	111	155
Ending balance	1,151	1,297	1,151	1,297
Loans Ending Balance:
Ending balance: individually evaluated for impairment - TDR	10,000	10,679	10,000	10,679
Collectively evaluated for impairment:
Excluding Purchased Non-Credit Impaired Loans acquired at a discount and Purchased Credit Impaired Loans	11,215	11,411	11,215	11,411
Purchased Non-Credit Impaired Loans acquired at a discount	1,977	2,386	1,977	2,386
Purchased Credit Impaired Loans	214	236	214	236
Ending total loans	$ 23,406	$ 24,712	$ 23,406	$ 24,712
Charge-offs as a percentage of average loans in repayment	1.59%	1.34%	1.66%	1.37%
Allowance coverage of charge-offs | SecurityLoan	3.3	4.3	3.2	4.2
Allowance as a percentage of the ending total loan balance	4.92%	5.25%	4.92%	5.25%
Allowance as a percentage of the ending loans in repayment	5.37%	5.85%	5.37%	5.85%
Average loans in repayment	$ 21,854	$ 22,289	$ 21,957	$ 22,474
Ending loans in repayment	21,439	22,174	21,439	22,174
Other Loans [Member]
Allowance for Loan Losses
Beginning balance	7	10	9	10
Total provision			1
Charge-offs	(1)		(2)
Loan sales	(7)		(8)
Ending balance		10		10
Allowance Ending Balance:
Ending balance: individually evaluated for impairment - TDR		9		9
Collectively evaluated for impairment:
Excluding Purchased Non-Credit Impaired Loans acquired at a discount and Purchased Credit Impaired Loans		1		1
Ending balance		10		10
Loans Ending Balance:
Ending balance: individually evaluated for impairment - TDR		28		28
Collectively evaluated for impairment:
Excluding Purchased Non-Credit Impaired Loans acquired at a discount and Purchased Credit Impaired Loans	4	51	4	51
Ending total loans	$ 4	$ 79	$ 4	$ 79
Charge-offs as a percentage of average loans in repayment	10.19%	2.63%	7.78%	1.13%
Allowance coverage of charge-offs | SecurityLoan		5.1		11.9
Allowance as a percentage of the ending total loan balance		12.54%		12.21%
Allowance as a percentage of the ending loans in repayment		12.54%		12.21%
Average loans in repayment	$ 28	$ 73	$ 52	$ 72
Ending loans in repayment	$ 4	$ 79	$ 4	$ 79