Borrowings - Company Borrowings (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Short Term	$ 300,000	$ 0
Long Term	2,738,662	2,167,979
Total	3,038,662	2,167,979
Unsecured borrowings
Debt Instrument [Line Items]
Short Term	0	0
Long Term	196,337	0
Total	196,337	0
Secured borrowings
Debt Instrument [Line Items]
Short Term	300,000	0
Long Term	2,542,325	2,167,979
Total	2,842,325	2,167,979
Secured borrowings | Private Education Loan securitization
Debt Instrument [Line Items]
Short Term	0	0
Long Term	2,542,325	2,167,979
Total	2,542,325	2,167,979
Secured borrowings | ABCP borrowings
Debt Instrument [Line Items]
Short Term	300,000	0
Long Term	0	0
Total	$ 300,000	$ 0