Borrowings - Secured Borrowings (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Secured borrowings:
Short-Term	$ 0	$ 289,230
Long-Term	5,189,217	4,354,037
Total	5,189,217	4,643,267
ABCP borrowings
Secured borrowings:
Short-Term	0	289,230
Unsecured borrowings
Secured borrowings:
Short-Term	0	0
Long-Term	692,879	198,159
Total	692,879	198,159
Secured borrowings
Secured borrowings:
Short-Term	0	289,230
Long-Term	4,496,338	4,155,878
Total	4,496,338	4,445,108
Secured borrowings | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	4,496,338	4,155,878
Total	4,496,338	4,155,878
Secured borrowings | ABCP borrowings
Secured borrowings:
Short-Term	0	289,230
Long-Term	0	0
Total	0	289,230
Secured borrowings | Fixed Income Interest Rate | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	3,261,233	2,629,902
Total	3,261,233	2,629,902
Secured borrowings | Variable Income Interest Rate | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	1,235,105	1,525,976
Total	$ 1,235,105	$ 1,525,976