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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, 2019 | Dec. 31, 2018 | Mar. 31, 2019 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017
Loans in repayment and percentage of each status: |  |  |  |  |  |
Loans allowance for losses | $ (1,218) | $ (1,286) | $ (1,252) | $ (1,389) | $ (1,367) | $ (1,367)
FFELP Loans [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Total loans in repayment | 55,684 |  |  | 62,952 |  |
Loans allowance for losses | (67) | (76) | $ (67) | $ (82) | $ (59) | $ (60)
Loans, net | 67,956 | 72,253 |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Private Education Loans - TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Loans current | 7,816 | 7,890 |  |  |  |
Total loans in repayment | 8,760 | 9,025 |  |  |  |
Total loans, gross | 9,636 | 9,969 |  |  |  |
Loans unamortized premium (discount) | (209) | (212) |  |  |  |
Total loans | 9,427 | 9,757 |  |  |  |
Loans receivable for partially charged-off loans | 364 | 367 |  |  |  |
Loans allowance for losses | (1,040) | (1,100) |  |  |  |
Loans, net | $ 8,751 | $ 9,024 |  |  |  |
Dimensions of concentration risk | 100.00% | 100.00% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Private Education Loans - Non-TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Loans current | $ 12,553 | $ 12,851 |  |  |  |
Total loans in repayment | 12,679 | 13,012 |  |  |  |
Total loans, gross | 13,130 | 13,562 |  |  |  |
Loans unamortized premium (discount) | (482) | (547) |  |  |  |
Total loans | 12,648 | 13,015 |  |  |  |
Loans receivable for partially charged-off loans | 276 | 307 |  |  |  |
Loans allowance for losses | (111) | (101) |  |  |  |
Loans, net | $ 12,813 | $ 13,221 |  |  |  |
Dimensions of concentration risk | 100.00% | 100.00% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | School/Grace/Deferment [Member] | Private Education Loans - TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Total loans, gross | $ 371 | $ 426 |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | School/Grace/Deferment [Member] | Private Education Loans - Non-TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Total loans, gross | 315 | 392 |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Forbearance [Member] | Private Education Loans - TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Total loans, gross | 505 | 518 |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Forbearance [Member] | Private Education Loans - Non-TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Total loans, gross | 136 | 158 |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | FFELP Loans [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Loans current | 49,855 | 53,500 |  |  |  |
Total loans in repayment | 55,684 | 59,551 |  |  |  |
Total loans, gross | 67,454 | 71,730 |  |  |  |
Loans unamortized premium (discount) | 569 | 599 |  |  |  |
Total loans | 68,023 | 72,329 |  |  |  |
Loans allowance for losses | (67) | (76) |  |  |  |
Loans, net | $ 67,956 | $ 72,253 |  |  |  |
Dimensions of concentration risk | 100.00% | 100.00% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | FFELP Loans [Member] | School/Grace/Deferment [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Total loans, gross | $ 3,530 | $ 3,793 |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | FFELP Loans [Member] | Forbearance [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Total loans, gross | 8,240 | 8,386 |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, 31 to 60 Days Past Due [Member] | Private Education Loans - TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Loans delinquent | $ 276 | $ 344 |  |  |  |
Loans delinquent, percentage | 3.20% | 3.80% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, 31 to 60 Days Past Due [Member] | Private Education Loans - Non-TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Loans delinquent | $ 51 | $ 71 |  |  |  |
Loans delinquent, percentage | 0.40% | 0.50% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, 31 to 60 Days Past Due [Member] | FFELP Loans [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Loans delinquent | $ 1,540 | $ 1,964 |  |  |  |
Loans delinquent, percentage | 2.80% | 3.40% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, 61 to 90 Days Past Due [Member] | Private Education Loans - TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Loans delinquent | $ 191 | $ 235 |  |  |  |
Loans delinquent, percentage | 2.20% | 2.60% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, 61 to 90 Days Past Due [Member] | Private Education Loans - Non-TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Loans delinquent | $ 28 | $ 32 |  |  |  |
Loans delinquent, percentage | 0.20% | 0.30% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, 61 to 90 Days Past Due [Member] | FFELP Loans [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Loans delinquent | $ 870 | $ 910 |  |  |  |
Loans delinquent, percentage | 1.60% | 1.50% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, 90 Days Past Due [Member] | Private Education Loans - TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Loans delinquent | $ 477 | $ 556 |  |  |  |
Loans delinquent, percentage | 5.40% | 6.20% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, 90 Days Past Due [Member] | Private Education Loans - Non-TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Loans delinquent | $ 47 | $ 58 |  |  |  |
Loans delinquent, percentage | 0.40% | 0.40% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, 90 Days Past Due [Member] | FFELP Loans [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Loans delinquent | $ 3,419 | $ 3,177 |  |  |  |
Loans delinquent, percentage | 6.10% | 5.30% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, Current [Member] | Private Education Loans - TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Dimensions of concentration risk | 89.20% | 87.40% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, Current [Member] | Private Education Loans - Non-TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Dimensions of concentration risk | 99.00% | 98.80% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, Current [Member] | FFELP Loans [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Dimensions of concentration risk | 89.50% | 89.80% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, Loans In Repayment [Member] | Private Education Loans - TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Dimensions of concentration risk | 90.90% | 90.50% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, Loans In Repayment [Member] | Private Education Loans - Non-TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Dimensions of concentration risk | 96.60% | 95.90% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, Loans In Repayment [Member] | FFELP Loans [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Dimensions of concentration risk | 82.60% | 83.00% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, Delinquent Loans in Repayment [Member] | Private Education Loans - TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Dimensions of concentration risk | 10.80% | 12.60% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, Delinquent Loans in Repayment [Member] | Private Education Loans - Non-TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Dimensions of concentration risk | 1.00% | 1.20% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, Delinquent Loans in Repayment [Member] | FFELP Loans [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Dimensions of concentration risk | 10.50% | 10.20% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, Forbearance, Loans In Repayment [Member] | Private Education Loans - TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Dimensions of concentration risk | 5.40% | 5.40% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, Forbearance, Loans In Repayment [Member] | Private Education Loans - Non-TDRs [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Dimensions of concentration risk | 1.10% | 1.20% |  |  |  |
Credit Concentration Risk [Member] | Accounts Receivable [Member] | Financing Receivables, Forbearance, Loans In Repayment [Member] | FFELP Loans [Member] |  |  |  |  |  |
Loans in repayment and percentage of each status: |  |  |  |  |  |
Dimensions of concentration risk | 12.90% | 12.30% |  |  |  |

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