Borrowings - Company Borrowings (Details) - USD ($)	Sep. 30, 2022	Dec. 31, 2021
Debt Instrument [Line Items]
Short-Term	$ 0	$ 0
Long-Term	5,522,311,000	5,930,990,000
Total	5,522,311,000	5,930,990,000
Unsecured borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	988,182,000	986,138,000
Total	988,182,000	986,138,000
Secured borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	4,534,129,000	4,944,852,000
Total	4,534,129,000	4,944,852,000
Secured borrowings | Private Education Loan term securitizations
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	4,534,129,000	4,944,852,000
Total	4,534,129,000	4,944,852,000
Secured borrowings | Private Education Loan term securitizations | Fixed-rate
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	3,673,627,000	3,897,996,000
Total	3,673,627,000	3,897,996,000
Secured borrowings | Private Education Loan term securitizations | Variable-rate
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	860,502,000	1,046,856,000
Total	860,502,000	1,046,856,000
Secured borrowings | Secured Borrowing Facility
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	0	0
Total	$ 0	$ 0