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Borrowings - Company Borrowings (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Document Period End Date Sep. 30, 2018  
Short Term $ 0 $ 0
Long Term 4,532,221 3,275,270
Total 4,532,221 3,275,270
Unsecured borrowings    
Debt Instrument [Line Items]    
Short Term 0 0
Long Term 197,146 196,539
Total 197,146 196,539
Secured borrowings    
Debt Instrument [Line Items]    
Short Term 0 0
Long Term 4,335,075 3,078,731
Total 4,335,075 3,078,731
Secured borrowings | Private Education Loan securitization    
Debt Instrument [Line Items]    
Short Term 0 0
Long Term 4,335,075 3,078,731
Total 4,335,075 3,078,731
Secured borrowings | Private Education Loan securitization | Fixed rate    
Debt Instrument [Line Items]    
Short Term 0 0
Long Term 2,337,004 1,565,760
Total 2,337,004 1,565,760
Secured borrowings | Private Education Loan securitization | Variable rate    
Debt Instrument [Line Items]    
Short Term 0 0
Long Term 1,998,071 1,512,971
Total 1,998,071 1,512,971
Secured borrowings | ABCP Facility    
Debt Instrument [Line Items]    
Short Term 0 0
Long Term 0 0
Total $ 0 $ 0