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Allowance for Loan Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 17,132,907 | $ 14,251,675
Private Education Loans | Consumer Portfolio Segment | Student Loan | With Cosigner |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 15,395,579 | $ 12,816,512
Private Education Loans with cosigner (in percentage) | 90.00% | 90.00%
Private Education Loans | Consumer Portfolio Segment | Student Loan | Without Cosigner |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 1,737,328 | $ 1,435,163
Private Education Loans without cosigner (in percentage) | 10.00% | 10.00%
Private Education Loans | Consumer Portfolio Segment | Student Loan | Cosigners |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 17,132,907 | $ 14,251,675
Total in percent | 100.00% | 100.00%
Private Education Loans | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 17,132,907 | $ 14,251,675
Total in percent | 100.00% | 100.00%
Private Education Loans | Consumer Portfolio Segment | Student Loan | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 17,132,907 | $ 14,251,675
Total in percent | 100.00% | 100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments (in percentage) | 28.00% | 26.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments (in percentage) | 18.00% | 20.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments (in percentage) | 12.00% | 13.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments (in percentage) | 6.00% | 7.00%
Seasoning based on monthly scheduled payments due from more than 48 payments (in percentage) | 5.00% | 5.00%
Seasoning based on monthly scheduled payments due from not yet in repayment (in percentage) | 31.00% | 29.00%
Private Education Loans | FICO score less than 670 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 1,119,376 | $ 920,132
Private Education Loans At Origination | 6.00% | 6.00%
Private Education Loans | FICO score 670-699 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 2,532,389 | $ 2,092,722
Private Education Loans At Origination | 15.00% | 15.00%
Private Education Loans | FICO score 700-749 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 5,605,519 | $ 4,639,958
Private Education Loans At Origination | 33.00% | 33.00%
Private Education Loans | FICO score greater than or equal to 750 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 7,875,623 | $ 6,598,863
Private Education Loans At Origination | 46.00% | 46.00%
Private Education Loans | 1-12 payments | Consumer Portfolio Segment | Student Loan | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 4,758,249 | $ 3,737,110
Private Education Loans | 13-24 payments | Consumer Portfolio Segment | Student Loan | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | 3,017,316 | 2,841,107
Private Education Loans | 25-36 payments | Consumer Portfolio Segment | Student Loan | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | 2,033,608 | 1,839,764
Private Education Loans | 37-48 payments | Consumer Portfolio Segment | Student Loan | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | 1,105,979 | 917,633
Private Education Loans | More than 48 payments | Consumer Portfolio Segment | Student Loan | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | 865,084 | 726,106
Private Education Loans | Not yet in repayment | Consumer Portfolio Segment | Student Loan | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | 5,352,671 | 4,189,955
Personal Loans | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 132,100 | $ 12,894
Total in percent | 100.00% | 100.00%
Personal Loans | Consumer Portfolio Segment | Student Loan | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 132,100 | $ 12,894
Total in percent | 100.00% | 100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments (in percentage) | 100.00% | 100.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments (in percentage) | 0.00% | 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 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 10,910 | $ 1,189
Private Education Loans At Origination | 8.00% | 9.00%
Personal Loans | FICO score 670-699 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 38,330 | $ 3,139
Private Education Loans At Origination | 29.00% | 24.00%
Personal Loans | FICO score 700-749 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 60,601 | $ 5,678
Private Education Loans At Origination | 46.00% | 44.00%
Personal Loans | FICO score greater than or equal to 750 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 22,259 | $ 2,888
Private Education Loans At Origination | 17.00% | 23.00%
Personal Loans | 1-12 payments | Consumer Portfolio Segment | Student Loan | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 132,100 | $ 12,894
Personal Loans | 13-24 payments | Consumer Portfolio Segment | Student Loan | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | 0 | 0
Personal Loans | 25-36 payments | Consumer Portfolio Segment | Student Loan | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | 0 | 0
Personal Loans | 37-48 payments | Consumer Portfolio Segment | Student Loan | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | 0 | 0
Personal Loans | More than 48 payments | Consumer Portfolio Segment | Student Loan | Seasoning |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Private Education Loans | $ 0 | $ 0

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