Borrowings - Secured Borrowings (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Secured borrowings:
Short-Term	$ 0	$ 0
Long-Term	3,275,270,000	2,167,979,000
Total	3,275,270,000	2,167,979,000
ABCP borrowings
Secured borrowings:
Short-Term	0	0
Unsecured borrowings
Secured borrowings:
Short-Term	0	0
Long-Term	196,539,000	0
Total	196,539,000	0
Secured borrowings
Secured borrowings:
Short-Term	0	0
Long-Term	3,078,731,000	2,167,979,000
Total	3,078,731,000	2,167,979,000
Secured borrowings | Private Education Loan term securitizations
Secured borrowings:
Short-Term	0	0
Long-Term	3,078,731,000	2,167,979,000
Total	3,078,731,000	2,167,979,000
Secured borrowings | ABCP borrowings
Secured borrowings:
Short-Term	0	0
Long-Term	0	0
Total	$ 0	$ 0