Borrowings - Variable Interest Entity (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
Restricted Cash	53,717,000	27,980,000
Other assets	52,153,000	57,227,000
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 Facility
Secured borrowings:
Short-Term	0	500,175,000
Long-Term	0	0
Total	0	500,175,000
Variable Interest Entity, Primary Beneficiary
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
Loans	2,562,156,000	1,610,985,000
Restricted Cash	44,617,000	22,439,000
Other assets	160,783,000	103,661,000
Total	2,767,556,000	1,737,085,000
Variable Interest Entity, Primary Beneficiary | 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
Loans	2,562,156,000	687,298,000
Restricted Cash	44,617,000	9,996,000
Other assets	160,783,000	45,566,000
Total	2,767,556,000	742,860,000
Variable Interest Entity, Primary Beneficiary | ABCP Facility
Secured borrowings:
Short-Term	0	500,175,000
Long-Term	0	0
Total	0	500,175,000
Loans	0	923,687,000
Restricted Cash	0	12,443,000
Other assets	0	58,095,000
Total	$ 0	$ 994,225,000