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Allowance for Loan Losses - Age Analysis of Past Due Loans Delinquencies (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Loans in repayment and percentage of each status:
Loans allowance for losses	$ (1,391)	$ (1,433)	$ (1,518)	$ (1,564)
FFELP Loans [Member]
Loans in repayment and percentage of each status:
Total loans in repayment	68,095		73,630
Loans allowance for losses	(64)	(67)	$ (69)	$ (78)
Loans, net	85,284	87,730
Credit Concentration Risk [Member] \| Private Education Loans - Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in-school/grace/deferment/forbearance	21,601	22,367
Loans in repayment and percentage of each status:
Loans current	18,471	19,020
Total loans in repayment	19,681	20,396
Total loans, gross	21,601	22,367
Loans unamortized premium (discount)	(381)	(402)
Total loans	21,220	21,965
Loans receivable for partially charged-off loans	518	526
Loans allowance for losses	(1,107)	(1,138)
Loans, net	20,631	21,353
Credit Concentration Risk [Member] \| Private Education Loans - Non-Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in-school/grace/deferment/forbearance	1,896	1,966
Loans in repayment and percentage of each status:
Total loans in repayment	1,686	1,754
Total loans, gross	1,896	1,966
Loans unamortized premium (discount)	(53)	(55)
Total loans	1,843	1,911
Loans receivable for partially charged-off loans	282	289
Loans allowance for losses	(204)	(213)
Loans, net	1,921	1,987
Credit Concentration Risk [Member] \| Troubled Debt Restructuring Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in-school/grace/deferment/forbearance	10,727	10,805
Loans in repayment and percentage of each status:
Total loans in repayment	9,549	9,638
Total loans, gross	10,727	10,805
Credit Concentration Risk [Member] \| School/Grace/Deferment [Member] \| Private Education Loans - Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in-school/grace/deferment/forbearance	1,218	1,271
Loans in repayment and percentage of each status:
Total loans, gross	1,218	1,271
Credit Concentration Risk [Member] \| School/Grace/Deferment [Member] \| Private Education Loans - Non-Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in-school/grace/deferment/forbearance	119	122
Loans in repayment and percentage of each status:
Total loans, gross	119	122
Credit Concentration Risk [Member] \| School/Grace/Deferment [Member] \| Troubled Debt Restructuring Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in-school/grace/deferment/forbearance	580	579
Loans in repayment and percentage of each status:
Total loans, gross	580	579
Credit Concentration Risk [Member] \| Forbearance [Member] \| Private Education Loans - Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in-school/grace/deferment/forbearance	702	700
Loans in repayment and percentage of each status:
Total loans, gross	702	700
Credit Concentration Risk [Member] \| Forbearance [Member] \| Private Education Loans - Non-Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in-school/grace/deferment/forbearance	91	90
Loans in repayment and percentage of each status:
Total loans, gross	91	90
Credit Concentration Risk [Member] \| Forbearance [Member] \| Troubled Debt Restructuring Loans [Member]
Loans in repayment and percentage of each status:
Loans in-school/grace/deferment/forbearance	598	588
Credit Concentration Risk [Member] \| Financing Receivables, Current [Member] \| Private Education Loans - Non-Traditional [Member]
Loans in repayment and percentage of each status:
Loans current	1,447	1,486
Credit Concentration Risk [Member] \| Financing Receivables, Current [Member] \| Troubled Debt Restructuring Loans [Member]
Loans in repayment and percentage of each status:
Loans current	8,309	8,273
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	362	444
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	62	78
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	346	412
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	236	269
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	43	52
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	237	267
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	612	663
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	134	138
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	657	686
Credit Concentration Risk [Member] \| FFELP Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in-school/grace/deferment/forbearance	84,513	86,918
Loans in repayment and percentage of each status:
Loans current	60,310	61,977
Total loans in repayment	68,095	70,557
Total loans, gross	84,513	86,918
Loans unamortized premium (discount)	835	879
Total loans	85,348	87,797
Loans allowance for losses	(64)	(67)
Loans, net	85,284	87,730
Credit Concentration Risk [Member] \| FFELP Loans [Member] \| School/Grace/Deferment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in-school/grace/deferment/forbearance	5,791	5,871
Loans in repayment and percentage of each status:
Total loans, gross	5,791	5,871
Credit Concentration Risk [Member] \| FFELP Loans [Member] \| Forbearance [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans in-school/grace/deferment/forbearance	10,627	10,490
Loans in repayment and percentage of each status:
Total loans, gross	10,627	10,490
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	2,300	2,820
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	1,204	1,325
Credit Concentration Risk [Member] \| FFELP Loans [Member] \| Financing Receivables, 90 Days Past Due [Member]
Loans in repayment and percentage of each status:
Loans delinquent	$ 4,281	$ 4,435
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Private Education Loans - Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Dimensions of concentration risk	100.00%	100.00%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Private Education Loans - Non-Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Dimensions of concentration risk	100.00%	100.00%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Troubled Debt Restructuring Loans [Member]
Financing Receivable, Recorded Investment, Past Due [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]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Dimensions of concentration risk	93.90%	93.30%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Current [Member] \| Private Education Loans - Non-Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Dimensions of concentration risk	85.90%	84.80%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Current [Member] \| Troubled Debt Restructuring Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Dimensions of concentration risk	87.00%	85.80%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 31 to 60 Days Past Due [Member] \| Private Education Loans - Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans delinquent, percentage	1.80%	2.20%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 31 to 60 Days Past Due [Member] \| Private Education Loans - Non-Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans delinquent, percentage	3.60%	4.50%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 31 to 60 Days Past Due [Member] \| Troubled Debt Restructuring Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans delinquent, percentage	3.60%	4.30%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 61 to 90 Days Past Due [Member] \| Private Education Loans - Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans delinquent, percentage	1.20%	1.30%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 61 to 90 Days Past Due [Member] \| Private Education Loans - Non-Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans delinquent, percentage	2.50%	2.90%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 61 to 90 Days Past Due [Member] \| Troubled Debt Restructuring Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans delinquent, percentage	2.50%	2.80%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 90 Days Past Due [Member] \| Private Education Loans - Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans delinquent, percentage	3.10%	3.20%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 90 Days Past Due [Member] \| Private Education Loans - Non-Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans delinquent, percentage	8.00%	7.80%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, 90 Days Past Due [Member] \| Troubled Debt Restructuring Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans delinquent, percentage	6.90%	7.10%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Loans In Repayment [Member] \| Private Education Loans - Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Dimensions of concentration risk	91.10%	91.20%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Loans In Repayment [Member] \| Private Education Loans - Non-Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Dimensions of concentration risk	88.90%	89.20%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Delinquent Loans in Repayment [Member] \| Private Education Loans - Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Dimensions of concentration risk	6.10%	6.70%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Delinquent Loans in Repayment [Member] \| Private Education Loans - Non-Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Dimensions of concentration risk	14.10%	15.20%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Forbearance, Loans In Repayment [Member] \| Private Education Loans - Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Dimensions of concentration risk	3.40%	3.30%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| Financing Receivables, Forbearance, Loans In Repayment [Member] \| Private Education Loans - Non-Traditional [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Dimensions of concentration risk	5.10%	4.90%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| FFELP Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Dimensions of concentration risk	100.00%	100.00%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| FFELP Loans [Member] \| Financing Receivables, Current [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Dimensions of concentration risk	88.60%	87.80%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| FFELP Loans [Member] \| Financing Receivables, 31 to 60 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans delinquent, percentage	3.40%	4.00%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| FFELP Loans [Member] \| Financing Receivables, 61 to 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans delinquent, percentage	1.80%	1.90%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| FFELP Loans [Member] \| Financing Receivables, 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans delinquent, percentage	6.20%	6.30%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| FFELP Loans [Member] \| Financing Receivables, Loans In Repayment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Dimensions of concentration risk	80.60%	81.20%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| FFELP Loans [Member] \| Financing Receivables, Delinquent Loans in Repayment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Dimensions of concentration risk	11.40%	12.20%
Credit Concentration Risk [Member] \| Accounts Receivable [Member] \| FFELP Loans [Member] \| Financing Receivables, Forbearance, Loans In Repayment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Dimensions of concentration risk	13.50%	12.90%