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Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 25,036	$ 27,515
Private Education Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans with cosigner	16,107	17,738
Private Education Loans without cosigner	$ 8,929	$ 9,777
Private Education Loans with cosigner in percent	64.00%	64.00%
Private Education Loans without cosigner in percent	36.00%	36.00%
Total in percent	100.00%	100.00%
School Type/FICO Scores [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 25,036	$ 27,515
School Type/FICO Scores [Member] | Private Education Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
School type FICO scores traditional recorded investment	23,006	25,280
School type FICO scores non-traditional recorded investment	$ 2,030	$ 2,235
School type FICO scores traditional recorded investment, in percent	92.00%	92.00%
School type FICO scores non-traditional recorded investment, in percent	8.00%	8.00%
Total in percent	100.00%	100.00%
Seasoning [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 25,036	$ 27,515
Seasoning [Member] | Private Education Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Seasoning - based on monthly scheduled payments due from 1-12 payments	1,348	1,776
Seasoning - based on monthly scheduled payments due from 13-24 payments	1,430	1,977
Seasoning - based on monthly scheduled payments due from 25-36 payments	2,234	2,982
Seasoning - based on monthly scheduled payments due from 37-48 payments	3,172	3,787
Seasoning - based on monthly scheduled payments due from more than 48 payments	15,313	14,953
Seasoning - based on monthly scheduled payments due from not yet in repayment	$ 1,539	$ 2,040
Seasoning based on monthly scheduled payments due from 1-12 payments, in percent	5.00%	7.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments, in percent	6.00%	7.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments, in percent	9.00%	11.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments, in percent	13.00%	14.00%
Seasoning based on monthly scheduled payments due from more than 48 payments, in percent	61.00%	54.00%
Seasoning - based on monthly scheduled payments due from not yet in repayment, in percent	6.00%	7.00%
Total in percent	100.00%	100.00%

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Loans with cosigner.

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Seasoning - based on monthly scheduled payments due from 1-12 payments.

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