Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 23,215,887	$ 24,141,470	$ 23,883,369	$ 23,870,559
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 21,937,758	$ 22,202,420	$ 22,342,353	$ 21,896,218
Weighted Average Interest Rate	8.24%	9.30%	8.48%	9.40%
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 750,925	$ 806,865	$ 762,863	$ 821,870
Weighted Average Interest Rate	3.46%	4.54%	3.86%	4.83%
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 527,204	$ 1,132,185	$ 778,153	$ 1,152,471
Weighted Average Interest Rate	12.86%	12.16%	12.43%	11.99%