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Allowance for Loan Losses - Loan Status and Aging of Past Due TDR Loans (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016
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TDR loans in repayment and percentage of each status: |  |  |  |  |  |
Allowance for loan losses | $ (295,277) | $ (272,123) | $ (251,475) | $ (207,448) | $ (187,086) | $ (184,701)
Loans held for investment, net | 20,308,581 |  | 18,567,641 |  |  |
Student Loan | Consumer Portfolio Segment | Troubled Debt Restructured Loans |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
TDR loans in in-school/grace/deferment | 57,595 |  | 51,745 |  |  |
TDR loans in forbearance | 59,682 |  | 69,652 |  |  |
TDR loans in repayment and percentage of each status: |  |  |  |  |  |
Loans current | 917,138 |  | 774,222 |  |  |
Total TDR loans in repayment | $ 1,004,539 |  | $ 868,954 |  |  |
Loans current (as a percentage) | 91.30% |  | 89.10% |  |  |
Total TDR loans in repayment | 100.00% |  | 100.00% |  |  |
Total Private Education Loans, gross | $ 1,121,816 |  | $ 990,351 |  |  |
Student Loan | Consumer Portfolio Segment | Loans delinquent 31-60 days | Troubled Debt Restructured Loans |  |  |  |  |  |
TDR loans in repayment and percentage of each status: |  |  |  |  |  |
Loans delinquent | $ 45,080 |  | $ 48,377 |  |  |
Loans delinquent (as a percentage) | 4.50% |  | 5.60% |  |  |
Student Loan | Consumer Portfolio Segment | Loans delinquent 61-90 days | Troubled Debt Restructured Loans |  |  |  |  |  |
TDR loans in repayment and percentage of each status: |  |  |  |  |  |
Loans delinquent | $ 25,580 |  | $ 28,778 |  |  |
Loans delinquent (as a percentage) | 2.50% |  | 3.30% |  |  |
Student Loan | Consumer Portfolio Segment | Loans delinquent, greater than 90 days | Troubled Debt Restructured Loans |  |  |  |  |  |
TDR loans in repayment and percentage of each status: |  |  |  |  |  |
Loans delinquent | $ 16,741 |  | $ 17,577 |  |  |
Loans delinquent (as a percentage) | 1.70% |  | 2.00% |  |  |

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