Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 21,389,370	$ 23,198,080	$ 21,559,053	$ 24,228,103
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 20,654,285	$ 21,590,905	$ 20,818,476	$ 22,546,874
Weighted Average Interest Rate	8.22%	8.33%	8.22%	8.60%
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 723,391	$ 761,469	$ 728,810	$ 768,897
Weighted Average Interest Rate	3.41%	3.82%	3.41%	4.06%
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 0	$ 836,342	$ 0	$ 905,007
Weighted Average Interest Rate	0.00%	12.54%	0.00%	12.31%
Credit Cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 11,694	$ 9,364	$ 11,767	$ 7,325
Weighted Average Interest Rate	6.64%	(9.34%)	3.71%	(7.67%)