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Allowance for Loan Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - Consumer Portfolio Segment - Student Loan - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Private Education Loans | With Cosigner |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 18,147,180 | $ 15,658,539
Private Education Loans with cosigner (in percentage) | 90.00% | 90.00%
Private Education Loans | Without Cosigner |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 2,092,811 | $ 1,773,628
Private Education Loans without cosigner (in percentage) | 10.00% | 10.00%
Private Education Loans | Cosigners |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 20,239,991 | $ 17,432,167
Total in percent | 100.00% | 100.00%
Private Education Loans | FICO at Original Approval |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 20,239,991 | $ 17,432,167
Total in percent | 100.00% | 100.00%
Private Education Loans | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 20,239,991 | $ 17,432,167
Total in percent | 100.00% | 100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments (in percentage) | 27.00% | 24.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments (in percentage) | 16.00% | 19.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments (in percentage) | 13.00% | 14.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments (in percentage) | 8.00% | 9.00%
Seasoning based on monthly scheduled payments due from more than 48 payments (in percentage) | 7.00% | 7.00%
Seasoning based on monthly scheduled payments due from not yet in repayment (in percentage) | 29.00% | 27.00%
Private Education Loans | FICO score less than 670 | FICO at Original Approval |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 1,373,155 | $ 1,153,591
Private Education Loans At Origination | 7.00% | 6.00%
Private Education Loans | FICO score 670-699 | FICO at Original Approval |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 3,042,228 | $ 2,596,959
Private Education Loans At Origination | 15.00% | 15.00%
Private Education Loans | FICO score 700-749 | FICO at Original Approval |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 6,655,405 | $ 5,714,554
Private Education Loans At Origination | 33.00% | 33.00%
Private Education Loans | FICO score greater than or equal to 750 | FICO at Original Approval |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 9,169,203 | $ 7,967,063
Private Education Loans At Origination | 45.00% | 46.00%
Private Education Loans | 1-12 payments | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 5,419,487 | $ 4,256,592
Private Education Loans | 13-24 payments | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | 3,238,910 | 3,229,465
Private Education Loans | 25-36 payments | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | 2,533,308 | 2,429,238
Private Education Loans | 37-48 payments | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | 1,638,309 | 1,502,327
Private Education Loans | More than 48 payments | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | 1,473,944 | 1,256,813
Private Education Loans | Not yet in repayment | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | 5,936,033 | 4,757,732
Personal Loans | FICO at Original Approval |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 1,133,005 | $ 400,280
Total in percent | 100.00% | 100.00%
Personal Loans | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 1,133,005 | $ 400,280
Total in percent | 100.00% | 100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments (in percentage) | 96.00% | 100.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments (in percentage) | 4.00% | 0.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments (in percentage) | 0.00% | 0.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments (in percentage) | 0.00% | 0.00%
Seasoning based on monthly scheduled payments due from more than 48 payments (in percentage) | 0.00% | 0.00%
Personal Loans | FICO score less than 670 | FICO at Original Approval |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 79,769 | $ 32,156
Private Education Loans At Origination | 7.00% | 8.00%
Personal Loans | FICO score 670-699 | FICO at Original Approval |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 327,364 | $ 114,731
Private Education Loans At Origination | 29.00% | 29.00%
Personal Loans | FICO score 700-749 | FICO at Original Approval |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 520,370 | $ 182,025
Private Education Loans At Origination | 46.00% | 45.00%
Personal Loans | FICO score greater than or equal to 750 | FICO at Original Approval |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 205,502 | $ 71,368
Private Education Loans At Origination | 18.00% | 18.00%
Personal Loans | 1-12 payments | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 1,084,706 | $ 400,280
Personal Loans | 13-24 payments | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | 48,299 | 0
Personal Loans | 25-36 payments | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | 0 | 0
Personal Loans | 37-48 payments | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | 0 | 0
Personal Loans | More than 48 payments | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 0 | $ 0

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