Borrowings - Company Borrowings (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Short Term	$ 297,800	$ 0
Long Term	4,601,888	4,284,304
Total	4,899,688	4,284,304
Unsecured borrowings
Debt Instrument [Line Items]
Short Term	0	0
Long Term	197,956	197,348
Total	197,956	197,348
Secured borrowings
Debt Instrument [Line Items]
Short Term	297,800	0
Long Term	4,403,932	4,086,956
Total	4,701,732	4,086,956
Secured borrowings | Private Education Loan securitization
Debt Instrument [Line Items]
Short Term	0	0
Long Term	4,403,932	4,086,956
Total	4,403,932	4,086,956
Secured borrowings | Private Education Loan securitization | Fixed-rate
Debt Instrument [Line Items]
Short Term	0	0
Long Term	2,699,378	2,284,347
Total	2,699,378	2,284,347
Secured borrowings | Private Education Loan securitization | Variable-rate
Debt Instrument [Line Items]
Short Term	0	0
Long Term	1,704,554	1,802,609
Total	1,704,554	1,802,609
Secured borrowings | Secured Borrowing Facility
Debt Instrument [Line Items]
Short Term	297,800	0
Long Term	0	0
Total	$ 297,800	$ 0