Allowance for Loan Losses - Loan Status and Aging of Past Due TDR Loans (Details) - Student Loan - Consumer Portfolio Segment - Troubled Debt Restructured Loans - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR loans in in-school/grace/deferment	$ 89,046	$ 69,212
TDR loans in forbearance	97,175	69,796
TDR loans in repayment and percentage of each status:
Loans current	1,137,771	994,411
Total TDR loans in repayment	$ 1,288,598	$ 1,118,848
Loans current	88.30%	88.90%
Total TDR loans in repayment	100.00%	100.00%
Total Private Education Loans	$ 1,474,819	$ 1,257,856
Loans delinquent 31-60 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 74,243	$ 63,074
Loans delinquent	5.70%	5.60%
Loans delinquent 61-90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 47,500	$ 36,804
Loans delinquent	3.70%	3.30%
Loans delinquent, greater than 90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 29,084	$ 24,559
Loans delinquent	2.30%	2.20%

Loan status forbearance recorded investment.

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Loan status in school grace and deferment recorded investment.

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Total loans in repayment.

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Loans in repayment and percentage of each status.

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Loans in repayment current in percentage.

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Total loans in repayment, in percentage.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Percentage of financing receivable balance that is past due.

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Amount of financing receivable past due.

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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