Borrowings - Company Borrowings (Details) - USD ($)	Jun. 30, 2022	Dec. 31, 2021
Debt Instrument [Line Items]
Short-Term	$ 0	$ 0
Long-Term	5,219,310,000	5,930,990,000
Total	5,219,310,000	5,930,990,000
Unsecured borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	987,384,000	986,138,000
Total	987,384,000	986,138,000
Secured borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	4,231,926,000	4,944,852,000
Total	4,231,926,000	4,944,852,000
Secured borrowings | Private Education Loan term securitizations
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	4,231,926,000	4,944,852,000
Total	4,231,926,000	4,944,852,000
Secured borrowings | Private Education Loan term securitizations | Fixed-rate
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	3,368,758,000	3,897,996,000
Total	3,368,758,000	3,897,996,000
Secured borrowings | Private Education Loan term securitizations | Variable-rate
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	863,168,000	1,046,856,000
Total	863,168,000	1,046,856,000
Secured borrowings | Secured Borrowing Facility
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	0	0
Total	$ 0	$ 0