Borrowings - Secured Borrowings (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Secured borrowings:
Short-Term	$ 0	$ 500,175,000
Long-Term	2,167,979,000	579,101,000
Total	2,167,979,000	1,079,276,000
Private Education Loan term securitizations
Secured borrowings:
Short-Term	0	0
Long-Term	2,167,979,000	579,101,000
Total	2,167,979,000	579,101,000
ABCP borrowings
Secured borrowings:
Short-Term	0	500,175,000
Long-Term	0	0
Total	$ 0	$ 500,175,000