Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 25,252,841	$ 23,747,242
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 23,502,844	$ 21,732,826
Weighted Average Interest Rate	8.86%	9.50%
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 776,326	$ 837,950
Weighted Average Interest Rate	4.29%	4.94%
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 973,671	$ 1,176,466
Weighted Average Interest Rate	12.11%	11.81%