Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 21,730,621	$ 25,258,127
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 20,984,491	$ 23,502,844
Weighted Average Interest Rate	8.22%	8.86%
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 734,289	$ 776,326
Weighted Average Interest Rate	3.41%	4.29%
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 0	$ 973,671
Weighted Average Interest Rate	0.00%	12.11%
Credit Cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 11,841	$ 5,286
Weighted Average Interest Rate	0.78%	(4.72%)