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Allowance for Loan Losses - Age Analysis of Past Due Loans Delinquencies (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Jun. 30, 2016	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Loans in repayment and percentage of each status:
Loans allowance for losses	$ (1,465)	$ (1,564)	$ (1,487)	$ (1,608)	$ (1,643)	$ (2,033)
FFELP Loans [Member]
Loans in repayment and percentage of each status:
Total loans in repayment	72,269			75,294
Loans allowance for losses	(58)	(78)	$ (62)	$ (84)	$ (89)	$ (93)
Loans, net	90,146	96,498
Credit Concentration Risk [Member] \| Private Education Loans - Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in-school/grace/deferment	1,405	1,859
Loans in forbearance	833	863
Loans in repayment and percentage of each status:
Loans current	19,471	21,085
Total loans in repayment	20,768	22,558
Total loans, gross	23,006	25,280
Loans unamortized premium (discount)	(407)	(470)
Total loans	22,599	24,810
Loans receivable for partially charged-off loans	532	560
Loans allowance for losses	(1,176)	(1,236)
Loans, net	21,955	24,134
Credit Concentration Risk [Member] \| Private Education Loans - Non-Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in-school/grace/deferment	134	181
Loans in forbearance	108	110
Loans in repayment and percentage of each status:
Total loans in repayment	1,788	1,944
Total loans, gross	2,030	2,235
Loans unamortized premium (discount)	(55)	(61)
Total loans	1,975	2,174
Loans receivable for partially charged-off loans	296	321
Loans allowance for losses	(216)	(235)
Loans, net	2,055	2,260
Credit Concentration Risk [Member] \| Troubled Debt Restructuring Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in-school/grace/deferment	605	706
Loans in forbearance	669	695
Loans in repayment and percentage of each status:
Total loans in repayment	9,558	9,241
Total loans, gross	10,832	10,642
Credit Concentration Risk [Member] \| Financing Receivables, Current [Member] \| Private Education Loans - Non-Traditional [Member]
Loans in repayment and percentage of each status:
Loans current	1,539	1,646
Credit Concentration Risk [Member] \| Financing Receivables, Current [Member] \| Troubled Debt Restructuring Loans [Member]
Loans in repayment and percentage of each status:
Loans current	8,299	7,885
Credit Concentration Risk [Member] \| Financing Receivables, 31 to 60 Days Past Due [Member] \| Private Education Loans - Traditional [Member]
Loans in repayment and percentage of each status:
Loans delinquent	434	491
Credit Concentration Risk [Member] \| Financing Receivables, 31 to 60 Days Past Due [Member] \| Private Education Loans - Non-Traditional [Member]
Loans in repayment and percentage of each status:
Loans delinquent	73	86
Credit Concentration Risk [Member] \| Financing Receivables, 31 to 60 Days Past Due [Member] \| Troubled Debt Restructuring Loans [Member]
Loans in repayment and percentage of each status:
Loans delinquent	394	414
Credit Concentration Risk [Member] \| Financing Receivables, 61 to 90 Days Past Due [Member] \| Private Education Loans - Traditional [Member]
Loans in repayment and percentage of each status:
Loans delinquent	262	292
Credit Concentration Risk [Member] \| Financing Receivables, 61 to 90 Days Past Due [Member] \| Private Education Loans - Non-Traditional [Member]
Loans in repayment and percentage of each status:
Loans delinquent	52	56
Credit Concentration Risk [Member] \| Financing Receivables, 61 to 90 Days Past Due [Member] \| Troubled Debt Restructuring Loans [Member]
Loans in repayment and percentage of each status:
Loans delinquent	254	263
Credit Concentration Risk [Member] \| Financing Receivables, 90 Days Past Due [Member] \| Private Education Loans - Traditional [Member]
Loans in repayment and percentage of each status:
Loans delinquent	601	690
Credit Concentration Risk [Member] \| Financing Receivables, 90 Days Past Due [Member] \| Private Education Loans - Non-Traditional [Member]
Loans in repayment and percentage of each status:
Loans delinquent	124	156
Credit Concentration Risk [Member] \| Financing Receivables, 90 Days Past Due [Member] \| Troubled Debt Restructuring Loans [Member]
Loans in repayment and percentage of each status:
Loans delinquent	611	679
Credit Concentration Risk [Member] \| FFELP Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in-school/grace/deferment	6,492	8,257
Loans in forbearance	10,530	13,298
Loans in repayment and percentage of each status:
Loans current	64,078	62,651
Total loans in repayment	72,269	73,934
Total loans, gross	89,291	95,489
Loans unamortized premium (discount)	913	1,087
Total loans	90,204	96,576
Loans allowance for losses	(58)	(78)
Loans, net	90,146	96,498
Credit Concentration Risk [Member] \| FFELP Loans [Member] \| Financing Receivables, 31 to 60 Days Past Due [Member]
Loans in repayment and percentage of each status:
Loans delinquent	2,462	3,285
Credit Concentration Risk [Member] \| FFELP Loans [Member] \| Financing Receivables, 61 to 90 Days Past Due [Member]
Loans in repayment and percentage of each status:
Loans delinquent	818	1,856
Credit Concentration Risk [Member] \| FFELP Loans [Member] \| Financing Receivables, 90 Days Past Due [Member]
Loans in repayment and percentage of each status:
Loans delinquent	$ 4,911	$ 6,142
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Private Education Loans - Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	100.00%	100.00%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Private Education Loans - Non-Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	100.00%	100.00%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Troubled Debt Restructuring Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	100.00%	100.00%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Current [Member] \| Private Education Loans - Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	93.80%	93.50%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Current [Member] \| Private Education Loans - Non-Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	86.10%	84.70%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Current [Member] \| Troubled Debt Restructuring Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	86.80%	85.30%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 31 to 60 Days Past Due [Member] \| Private Education Loans - Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans delinquent, percentage	2.10%	2.20%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 31 to 60 Days Past Due [Member] \| Private Education Loans - Non-Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans delinquent, percentage	4.10%	4.40%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 31 to 60 Days Past Due [Member] \| Troubled Debt Restructuring Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans delinquent, percentage	4.10%	4.50%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 61 to 90 Days Past Due [Member] \| Private Education Loans - Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans delinquent, percentage	1.30%	1.30%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 61 to 90 Days Past Due [Member] \| Private Education Loans - Non-Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans delinquent, percentage	2.90%	2.90%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 61 to 90 Days Past Due [Member] \| Troubled Debt Restructuring Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans delinquent, percentage	2.70%	2.80%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 90 Days Past Due [Member] \| Private Education Loans - Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans delinquent, percentage	2.80%	3.00%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 90 Days Past Due [Member] \| Private Education Loans - Non-Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans delinquent, percentage	6.90%	8.00%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 90 Days Past Due [Member] \| Troubled Debt Restructuring Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans delinquent, percentage	6.40%	7.40%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Loans In Repayment [Member] \| Private Education Loans - Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	90.30%	89.20%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Loans In Repayment [Member] \| Private Education Loans - Non-Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	88.10%	87.00%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Delinquent Loans in Repayment [Member] \| Private Education Loans - Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	6.20%	6.50%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Delinquent Loans in Repayment [Member] \| Private Education Loans - Non-Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	13.90%	15.30%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Forbearance, Loans In Repayment [Member] \| Private Education Loans - Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	3.90%	3.70%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Forbearance, Loans In Repayment [Member] \| Private Education Loans - Non-Traditional [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	5.70%	5.40%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| FFELP Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	100.00%	100.00%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| FFELP Loans [Member] \| Financing Receivables, Current [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	88.70%	84.70%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| FFELP Loans [Member] \| Financing Receivables, 31 to 60 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans delinquent, percentage	3.40%	4.50%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| FFELP Loans [Member] \| Financing Receivables, 61 to 90 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans delinquent, percentage	1.10%	2.50%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| FFELP Loans [Member] \| Financing Receivables, 90 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans delinquent, percentage	6.80%	8.30%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| FFELP Loans [Member] \| Financing Receivables, Loans In Repayment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	80.90%	77.40%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| FFELP Loans [Member] \| Financing Receivables, Delinquent Loans in Repayment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	11.30%	15.30%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| FFELP Loans [Member] \| Financing Receivables, Forbearance, Loans In Repayment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dimensions of concentration risk	12.70%	15.20%