Borrowings - Secured Borrowings (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Secured borrowings:
Short-Term	$ 0	$ 0
Long-Term	5,930,990	5,189,217
Total	5,930,990	5,189,217
ABCP borrowings
Secured borrowings:
Short-Term	0	0
Unsecured borrowings
Secured borrowings:
Short-Term	0	0
Long-Term	986,138	692,879
Total	986,138	692,879
Secured borrowings
Secured borrowings:
Short-Term	0	0
Long-Term	4,944,852	4,496,338
Total	4,944,852	4,496,338
Secured borrowings | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	4,944,852	4,496,338
Total	4,944,852	4,496,338
Secured borrowings | ABCP borrowings
Secured borrowings:
Short-Term	0	0
Long-Term	0	0
Total	0	0
Secured borrowings | Fixed Income Interest Rate | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	3,897,996	3,261,233
Total	3,897,996	3,261,233
Secured borrowings | Variable Income Interest Rate | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	1,046,856	1,235,105
Total	$ 1,046,856	$ 1,235,105