Borrowings - Secured Borrowings (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Secured borrowings:
Short-Term	$ 0	$ 0
Long-Term	4,284,304	3,275,270
Total	4,284,304	3,275,270
ABCP borrowings
Secured borrowings:
Short-Term	0	0
Unsecured borrowings
Secured borrowings:
Short-Term	0	0
Long-Term	197,348	196,539
Total	197,348	196,539
Secured borrowings
Secured borrowings:
Short-Term	0	0
Long-Term	4,086,956	3,078,731
Total	4,086,956	3,078,731
Secured borrowings | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	4,086,956	3,078,731
Total	4,086,956	3,078,731
Secured borrowings | ABCP borrowings
Secured borrowings:
Short-Term	0	0
Long-Term	0	0
Total	0	0
Fixed Income Interest Rate | Secured borrowings | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	2,284,347	1,565,760
Total	2,284,347	1,565,760
Variable Income Interest Rate | Secured borrowings | Private Education Loan term securitizations:
Secured borrowings:
Short-Term	0	0
Long-Term	1,802,609	1,512,971
Total	$ 1,802,609	$ 1,512,971