Allowance for Credit Losses - Loan Status and Aging of Past Due TDR Loans (Details) - Student Loan - Consumer Portfolio Segment - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
TDR loans in repayment and percentage of each status:
Total Private Education Loans in repayment, 2020	$ 1,763,684
Total	15,375,006	$ 16,315,651
Troubled Debt Restructured Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR loans in in-school/grace/deferment	96,489	87,749
TDR loans in repayment and percentage of each status:
TDR loans in forbearance - current	102,973	99,054
Total TDR loans in forbearance	$ 103,112	$ 99,054
Loans in forbearance, current	99.90%	100.00%
Total TDR loans in forbearance	100.00%	100.00%
Loans current	$ 1,139,598	$ 1,230,954
Total Private Education Loans in repayment, 2020	$ 1,295,560	$ 1,395,163
Loans current	87.90%	88.20%
Total TDR loans in repayment(3)	100.00%	100.00%
Total	$ 1,495,161	$ 1,581,966
Loan delinquency 31-60 days
TDR loans in repayment and percentage of each status:
Total	265,251	288,051
Loan delinquency 31-60 days \| Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
TDR loans in forbearance - delinquent	$ 139	$ 0
Loans in forbearance, delinquent	0.10%	0.00%
Loans delinquent	$ 70,790	$ 85,555
Loans delinquent	5.50%	6.10%
Loan delinquency 61-90
TDR loans in repayment and percentage of each status:
Total	$ 139,823	$ 121,302
Loan delinquency 61-90 \| Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
TDR loans in forbearance - delinquent	$ 0	$ 0
Loans in forbearance, delinquent	0.00%	0.00%
Loans delinquent	$ 51,537	$ 49,626
Loans delinquent	4.00%	3.60%
Loan delinquency greater than 90 days
TDR loans in repayment and percentage of each status:
Total	$ 73,229	$ 62,666
Loan delinquency greater than 90 days \| Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
TDR loans in forbearance - delinquent	$ 0	$ 0
Loans in forbearance, delinquent	0.00%	0.00%
Loans delinquent	$ 33,635	$ 29,028
Loans delinquent	2.60%	2.10%

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