Borrowings - Company Borrowings (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Short Term	$ 0	$ 0
Long Term	4,217,119	3,275,270
Total	4,217,119	3,275,270
Unsecured borrowings
Debt Instrument [Line Items]
Short Term	0	0
Long Term	196,943	196,539
Total	196,943	196,539
Secured borrowings
Debt Instrument [Line Items]
Short Term	0	0
Long Term	4,020,176	3,078,731
Total	4,020,176	3,078,731
Secured borrowings | Private Education Loan securitization
Debt Instrument [Line Items]
Short Term	0	0
Long Term	4,020,176	3,078,731
Total	4,020,176	3,078,731
Secured borrowings | Private Education Loan securitization | Fixed rate
Debt Instrument [Line Items]
Short Term	0	0
Long Term	2,125,035	1,565,760
Total	2,125,035	1,565,760
Secured borrowings | Private Education Loan securitization | Variable rate
Debt Instrument [Line Items]
Short Term	0	0
Long Term	1,895,141	1,512,971
Total	1,895,141	1,512,971
Secured borrowings | ABCP Facility
Debt Instrument [Line Items]
Short Term	0	0
Long Term	0	0
Total	$ 0	$ 0