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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% |  |

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