Borrowings - Company Borrowings (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Short-Term	$ 0	$ 0
Long-Term	4,918,670	5,189,217
Total	4,918,670	5,189,217
Unsecured borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	693,099	692,879
Total	693,099	692,879
Secured borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	4,225,571	4,496,338
Total	4,225,571	4,496,338
Secured borrowings | Private Education Loan securitization
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	4,225,571	4,496,338
Total	4,225,571	4,496,338
Secured borrowings | Private Education Loan securitization | Fixed-rate
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	3,109,329	3,261,233
Total	3,109,329	3,261,233
Secured borrowings | Private Education Loan securitization | Variable-rate
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	1,116,242	1,235,105
Total	1,116,242	1,235,105
Secured borrowings | Secured Borrowing Facility
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	0	0
Total	$ 0	$ 0