Borrowings - Variable Interest Entity (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Debt Outstanding
Short-Term	$ 0	$ 0
Long-Term	3,275,270,000	2,167,979,000
Total	3,275,270,000	2,167,979,000
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	101,836,000	53,717,000
Other assets	84,853,000	52,153,000
ABCP Facility
Debt Outstanding
Short-Term	0	0
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	3,078,731,000	2,167,979,000
Total	3,078,731,000	2,167,979,000
Carrying Amount of Assets Securing Debt Outstanding
Loans	3,691,024,000	2,562,156,000
Restricted Cash	96,983,000	44,617,000
Other assets	240,369,000	160,783,000
Total	4,028,376,000	2,767,556,000
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations
Debt Outstanding
Short-Term	0	0
Long-Term	3,078,731,000	2,167,979,000
Total	3,078,731,000	2,167,979,000
Carrying Amount of Assets Securing Debt Outstanding
Loans	3,691,024,000	2,562,156,000
Restricted Cash	95,966,000	44,617,000
Other assets	240,208,000	160,783,000
Total	4,027,198,000	2,767,556,000
Variable Interest Entity, Primary Beneficiary | ABCP Facility
Debt Outstanding
Short-Term	0	0
Long-Term	0	0
Total	0	0
Carrying Amount of Assets Securing Debt Outstanding
Loans	0	0
Restricted Cash	1,017,000	0
Other assets	161,000	0
Total	$ 1,178,000	$ 0