Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Securities Financing Transaction [Line Items]
Document Period End Date	Sep. 30, 2018
Debt Outstanding
Short Term	$ 0	$ 0
Long Term	4,532,221	3,275,270
Total	4,532,221	3,275,270
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	115,658	101,836	$ 66,625
Other Assets	101,000	84,853
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short Term	0	0
Long Term	4,335,075	3,078,731
Total	4,335,075	3,078,731
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,174,592	3,691,024
Restricted Cash	111,555	96,983
Other Assets	382,190	240,369
Total	5,668,337	4,028,376
Variable Interest Entity, Primary Beneficiary | Private Education Loan securitization
Debt Outstanding
Short Term	0	0
Long Term	4,335,075	3,078,731
Total	4,335,075	3,078,731
Carrying Amount of Assets Securing Debt Outstanding
Loans	5,174,592	3,691,024
Restricted Cash	111,555	95,966
Other Assets	381,760	240,208
Total	5,667,907	4,027,198
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	0	1,017
Other Assets	430	161
Total	$ 430	$ 1,178