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Borrowings - Company Borrowings (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Short Term $ 0 $ 0
Long Term 3,744,345 3,275,270
Total 3,744,345 3,275,270
Unsecured borrowings    
Debt Instrument [Line Items]    
Short Term 0 0
Long Term 196,741 196,539
Total 196,741 196,539
Secured borrowings    
Debt Instrument [Line Items]    
Short Term 0 0
Long Term 3,547,604 3,078,731
Total 3,547,604 3,078,731
Secured borrowings | Private Education Loan securitization    
Debt Instrument [Line Items]    
Short Term 0 0
Long Term 3,547,604 3,078,731
Total 3,547,604 3,078,731
Secured borrowings | ABCP Facility    
Debt Instrument [Line Items]    
Short Term 0 0
Long Term 0 0
Total $ 0 $ 0