Borrowings - Secured Borrowings (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Secured borrowings:
Short-Term	$ 289,230	$ 0
Long-Term	4,354,037	4,284,304
Total	4,643,267	4,284,304
ABCP borrowings
Secured borrowings:
Short-Term	289,230	0
Unsecured borrowings
Secured borrowings:
Short-Term	0	0
Long-Term	198,159	197,348
Total	198,159	197,348
Secured borrowings
Secured borrowings:
Short-Term	289,230	0
Long-Term	4,155,878	4,086,956
Total	4,445,108	4,086,956
Secured borrowings | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	4,155,878	4,086,956
Total	4,155,878	4,086,956
Secured borrowings | ABCP borrowings
Secured borrowings:
Short-Term	289,230	0
Long-Term	0	0
Total	289,230	0
Secured borrowings | Fixed Income Interest Rate | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	2,629,902	2,284,347
Total	2,629,902	2,284,347
Secured borrowings | Variable Income Interest Rate | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	1,525,976	1,802,609
Total	$ 1,525,976	$ 1,802,609