Borrowings - Company Borrowings (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Short-Term	$ 0	$ 289,230
Long-Term	4,708,036	4,354,037
Total	4,708,036	4,643,267
Unsecured borrowings
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	198,362	198,159
Total	198,362	198,159
Secured borrowings
Debt Instrument [Line Items]
Short-Term	0	289,230
Long-Term	4,509,674	4,155,878
Total	4,509,674	4,445,108
Secured borrowings | Private Education Loan securitization
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	4,509,674	4,155,878
Total	4,509,674	4,155,878
Secured borrowings | Private Education Loan securitization | Fixed-rate
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	2,912,346	2,629,902
Total	2,912,346	2,629,902
Secured borrowings | Private Education Loan securitization | Variable-rate
Debt Instrument [Line Items]
Short-Term	0	0
Long-Term	1,597,328	1,525,976
Total	1,597,328	1,525,976
Secured borrowings | Secured Borrowing Facility
Debt Instrument [Line Items]
Short-Term	0	289,230
Long-Term	0	0
Total	$ 0	$ 289,230