Borrowings - Company Borrowings (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Debt Instrument [Line Items]
Short-Term	$ 0	$ 0
Long-Term	5,552,497	5,930,990
Total	5,552,497	5,930,990
Unsecured borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	986,591	986,138
Total	986,591	986,138
Secured borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	4,565,906	4,944,852
Total	4,565,906	4,944,852
Secured borrowings | Private Education Loan term securitizations
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	4,565,906	4,944,852
Total	4,565,906	4,944,852
Secured borrowings | Private Education Loan term securitizations | Fixed-rate
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	3,608,606	3,897,996
Total	3,608,606	3,897,996
Secured borrowings | Private Education Loan term securitizations | Variable-rate
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	957,300	1,046,856
Total	957,300	1,046,856
Secured borrowings | Secured Borrowing Facility
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	0	0
Total	$ 0	$ 0