Allowance for Loan Losses - Loan Status and Aging of Past Due TDR Loans (Details) - Student Loan - Consumer Portfolio Segment - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
TDR loans in repayment and percentage of each status:
Total Private Education Loans in repayment	$ 455,960	$ 2,355,395
Total	14,516,211	16,315,651
Troubled Debt Restructured Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR loans in in-school/grace/deferment	83,964	87,749
TDR loans in forbearance	162,048	99,054
TDR loans in repayment and percentage of each status:
Loans current	1,113,104	1,230,954
Total Private Education Loans in repayment	$ 1,272,751	$ 1,395,163
Loans current, percentage	87.50%	88.20%
Total TDR loans in repayment	100.00%	100.00%
Total	$ 1,518,763	$ 1,581,966
Loan delinquency 31-60 days
TDR loans in repayment and percentage of each status:
Total	254,625	288,051
Loan delinquency 31-60 days | Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 74,856	$ 85,555
Loans delinquent	5.90%	6.10%
Loan delinquency 61-90
TDR loans in repayment and percentage of each status:
Total	$ 135,896	$ 121,302
Loan delinquency 61-90 | Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 50,237	$ 49,626
Loans delinquent	3.90%	3.60%
Loan delinquency greater than 90 days
TDR loans in repayment and percentage of each status:
Total	$ 81,613	$ 62,666
Loan delinquency greater than 90 days | Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 34,554	$ 29,028
Loans delinquent	2.70%	2.10%