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Allowance for Loan Losses - Age Analysis of Past Due Loans Delinquencies (Detail) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2016 | Dec. 31, 2015 | Mar. 31, 2016 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014
Loans in repayment and percentage of each status: |  |  |  |  |  |
Loans allowance for losses | $ (1,487) | $ (1,564) | $ (1,519) | $ (1,643) | $ (1,963) | $ (2,033)
FFELP Loans [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Total loans in repayment | 72,058 |  |  | 75,244 |  |
Loans allowance for losses | (62) | (78) | $ (70) | $ (89) | $ (91) | $ (93)
Loans, net | 92,618 | 96,498 |  |  |  |
Credit Concentration Risk [Member] | Private Education Loans - Traditional [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Loans in-school/grace/deferment | 1,489 | 1,859 |  |  |  |
Loans in forbearance | 792 | 863 |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Loans current | 20,225 | 21,085 |  |  |  |
Total loans in repayment | 21,416 | 22,558 |  |  |  |
Total loans, gross | 23,697 | 25,280 |  |  |  |
Loans unamortized premium (discount) | (431) | (470) |  |  |  |
Total loans | 23,266 | 24,810 |  |  |  |
Loans receivable for partially charged-off loans | 541 | 560 |  |  |  |
Loans allowance for losses | (1,186) | (1,236) |  |  |  |
Loans, net | 22,621 | 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 | 147 | 181 |  |  |  |
Loans in forbearance | 100 | 110 |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Total loans in repayment | 1,849 | 1,944 |  |  |  |
Total loans, gross | 2,096 | 2,235 |  |  |  |
Loans unamortized premium (discount) | (58) | (61) |  |  |  |
Total loans | 2,038 | 2,174 |  |  |  |
Loans receivable for partially charged-off loans | 306 | 321 |  |  |  |
Loans allowance for losses | (224) | (235) |  |  |  |
Loans, net | 2,120 | 2,260 |  |  |  |
Credit Concentration Risk [Member] | Troubled Debt Restructuring Loans [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Loans in-school/grace/deferment | 609 | 706 |  |  |  |
Loans in forbearance | 648 | 695 |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Total loans in repayment | 9,562 | 9,241 |  |  |  |
Total loans, gross | 10,819 | 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 | 1,618 | 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 | 8,424 | 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 | 401 | 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 | 66 | 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 | 356 | 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 | 242 | 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 | 45 | 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 | 228 | 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 | 548 | 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 | 120 | 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 | 554 | 679 |  |  |  |
Credit Concentration Risk [Member] | FFELP Loans [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Loans in-school/grace/deferment | 7,149 | 8,257 |  |  |  |
Loans in forbearance | 12,512 | 13,298 |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Loans current | 62,581 | 62,651 |  |  |  |
Total loans in repayment | 72,058 | 73,934 |  |  |  |
Total loans, gross | 91,719 | 95,489 |  |  |  |
Loans unamortized premium (discount) | 961 | 1,087 |  |  |  |
Total loans | 92,680 | 96,576 |  |  |  |
Loans allowance for losses | (62) | (78) |  |  |  |
Loans, net | 92,618 | 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 | 2,668 | 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 | 1,605 | 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 | $ 5,204 | $ 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 | 94.40% | 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 | 87.50% | 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 | 88.10% | 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 | 1.90% | 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 | 3.60% | 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 | 3.70% | 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 | 1.10% | 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 | 2.40% | 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 | 2.40% | 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 | 2.60% | 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 | 6.50% | 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 | 5.80% | 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 | 90.40% | 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 | 88.20% | 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 | 5.60% | 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 | 12.50% | 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 | 3.60% | 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 | 5.10% | 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 | 86.80% | 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 | 3.70% | 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 | 2.30% | 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 | 7.20% | 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 | 78.60% | 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 | 13.20% | 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 | 14.80% | 15.20% |  |  |  |

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- Definition Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953423-111524 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953423-111524 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(d)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SAB Topic 6.L.1) -URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (b)(1)(ii) -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544 + Details
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