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Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - USD ($) $ in Millions | Jun. 30, 2021 | Mar. 31, 2021 | Dec. 31, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2019
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Loan allowance for losses | $ (1,253) | $ (1,274) | $ (1,377) | $ (1,400) | $ (1,394) | $ (1,112)
Private Education Loans [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | 20,701 |  |  | 22,560 |  |
Loan allowance for losses | (976) | $ (992) | (1,089) | (1,098) | $ (1,083) | $ (1,048)
Loans, net | 19,725 |  | $ 21,079 | 21,462 |  |
FICO Scores [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | 2,728 |  |  | 2,036 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 2,526 |  |  | 3,871 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 2,322 |  |  | 1,395 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 859 |  |  | 456 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 267 |  |  | 97 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 11,999 |  |  | 14,705 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 20,701 |  |  | $ 22,560 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 100.00% |  |  | 100.00% |  |
FICO Scores [Member] | 640 and Above [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | $ 2,713 |  |  | $ 2,028 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 2,517 |  |  | 3,831 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 2,291 |  |  | 1,368 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 839 |  |  | 445 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 259 |  |  | 94 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 10,467 |  |  | 12,346 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 19,086 |  |  | $ 20,112 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 92.00% |  |  | 89.00% |  |
FICO Scores [Member] | Below 640 [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | $ 15 |  |  | $ 8 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 9 |  |  | 40 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 31 |  |  | 27 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 20 |  |  | 11 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 8 |  |  | 3 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 1,532 |  |  | 2,359 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 1,615 |  |  | $ 2,448 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 8.00% |  |  | 11.00% |  |
Loan Status [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | $ 2,728 |  |  | $ 2,036 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 2,526 |  |  | 3,871 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 2,322 |  |  | 1,395 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 859 |  |  | 456 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 267 |  |  | 97 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 11,999 |  |  | 14,705 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 20,701 |  |  | $ 22,560 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 100.00% |  |  | 100.00% |  |
Loan Status [Member] | School/Grace/Deferment [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | $ 12 |  |  | $ 89 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 29 |  |  | 180 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 37 |  |  | 80 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 19 |  |  | 28 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 7 |  |  | 8 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 905 |  |  | 1,974 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 1,009 |  |  | $ 2,359 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 5.00% |  |  | 10.00% |  |
Loan Status [Member] | Current/90 Days or Less Delinquent [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | $ 2,716 |  |  | $ 1,947 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 2,496 |  |  | 3,690 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 2,283 |  |  | 1,313 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 839 |  |  | 427 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 259 |  |  | 89 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 10,906 |  |  | 12,525 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 19,499 |  |  | $ 19,991 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 94.00% |  |  | 89.00% |  |
Loan Status [Member] | Greater than 90 days Delinquent [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | $ 1 |  |  | $ 1 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 2 |  |  | 2 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 1 |  |  | 1 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 1 |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 188 |  |  | 206 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 193 |  |  | $ 210 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 1.00% |  |  | 1.00% |  |
School Type [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | $ 2,728 |  |  | $ 2,036 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 2,526 |  |  | 3,871 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 2,322 |  |  | 1,395 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 859 |  |  | 456 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 267 |  |  | 97 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 11,999 |  |  | 14,705 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 20,701 |  |  | $ 22,560 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 100.00% |  |  | 100.00% |  |
School Type [Member] | Not For Profit [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | $ 2,583 |  |  | $ 1,918 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 2,412 |  |  | 3,618 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 2,166 |  |  | 1,292 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 792 |  |  | 436 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 256 |  |  | 96 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 9,960 |  |  | 12,155 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 18,169 |  |  | $ 19,515 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 88.00% |  |  | 86.00% |  |
School Type [Member] | For Profit [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | $ 145 |  |  | $ 118 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 114 |  |  | 253 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 156 |  |  | 103 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 67 |  |  | 20 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 11 |  |  | 1 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 2,039 |  |  | 2,550 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 2,532 |  |  | $ 3,045 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 12.00% |  |  | 14.00% |  |
Cosigners [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | $ 2,728 |  |  | $ 2,036 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 2,526 |  |  | 3,871 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 2,322 |  |  | 1,395 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 859 |  |  | 456 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 267 |  |  | 97 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 11,999 |  |  | 14,705 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 20,701 |  |  | $ 22,560 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 100.00% |  |  | 100.00% |  |
Cosigners [Member] | With Cosigner [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | $ 8 |  |  | $ 2 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 36 |  |  | 14 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 13 |  |  | 1 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 1 |  |  | 58 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 39 |  |  | 26 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 7,883 |  |  | 9,641 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 7,980 |  |  | $ 9,742 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 39.00% |  |  | 43.00% |  |
Cosigners [Member] | Without Cosigner [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | $ 2,720 |  |  | $ 2,034 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 2,490 |  |  | 3,857 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 2,309 |  |  | 1,394 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 858 |  |  | 398 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 228 |  |  | 71 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 4,116 |  |  | 5,064 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 12,721 |  |  | $ 12,818 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 61.00% |  |  | 57.00% |  |
Private Education Loans Troubled Debt Restructuring Status |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | $ 2,728 |  |  | $ 2,036 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 2,526 |  |  | 3,871 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 2,322 |  |  | 1,395 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 859 |  |  | 456 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 267 |  |  | 97 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 11,999 |  |  | 14,705 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 20,701 |  |  | $ 22,560 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 100.00% |  |  | 100.00% |  |
Private Education Loans Troubled Debt Restructuring Status | Private Education Loans - TDRs [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | $ 3 |  |  | $ 5 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 20 |  |  | 21 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 26 |  |  | 31 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 30 |  |  | 12 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 7,504 |  |  | 8,679 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 7,583 |  |  | $ 8,748 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 37.00% |  |  | 39.00% |  |
Private Education Loans Troubled Debt Restructuring Status | Private Education Loans - Non-TDRs [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | $ 2,728 |  |  | $ 2,036 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 2,523 |  |  | 3,866 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 2,302 |  |  | 1,374 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 833 |  |  | 425 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 237 |  |  | 85 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 4,495 |  |  | 6,026 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 13,118 |  |  | $ 13,812 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 63.00% |  |  | 61.00% |  |
Seasoning [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | $ 2,728 |  |  | $ 2,036 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 2,526 |  |  | 3,871 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 2,322 |  |  | 1,395 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 859 |  |  | 456 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 267 |  |  | 97 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 11,999 |  |  | 14,705 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 20,701 |  |  | $ 22,560 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 100.00% |  |  | 100.00% |  |
Seasoning [Member] | 1-12 Payments [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | $ 2,720 |  |  | $ 2,032 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 1,497 |  |  | 3,203 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 50 |  |  | 14 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 6 |  |  | 2 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 2 |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 152 |  |  | 214 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 4,427 |  |  | $ 5,465 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 22.00% |  |  | 24.00% |  |
Seasoning [Member] | 13-24 Payments [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | $ 1,010 |  |  | $ 651 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 1,869 |  |  | 1,017 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 27 |  |  | 14 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 4 |  |  | 2 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 181 |  |  | 283 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 3,091 |  |  | $ 1,967 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 15.00% |  |  | 9.00% |  |
Seasoning [Member] | 25-36 Payments [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | $ 381 |  |  | $ 352 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 621 |  |  | 350 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 16 |  |  | 6 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 297 |  |  | 465 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 1,315 |  |  | $ 1,173 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 6.00% |  |  | 5.00% |  |
Seasoning [Member] | 37-48 Payments [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | $ 195 |  |  | $ 86 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 197 |  |  | 86 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 463 |  |  | 698 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 855 |  |  | $ 870 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 4.00% |  |  | 4.00% |  |
Seasoning [Member] | More than 48 Payments [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | $ 46 |  |  | $ 2 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 10,564 |  |  | 12,571 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 10,610 |  |  | $ 12,573 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 51.00% |  |  | 56.00% |  |
Seasoning [Member] | School/Grace/Deferment [Member] |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2021 | $ 8 |  |  | $ 4 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2020 | 19 |  |  | 17 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2019 | 22 |  |  | 12 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2018 | 10 |  |  | 4 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year 2017 | 2 |  |  | 1 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year Prior | 342 |  |  | 474 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year | $ 403 |  |  | $ 512 |  |
Total Private Education Loan Credit Quality Indicators by Origination Year, Percent | 2.00% |  |  | 2.00% |  |

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