Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 23,188,716	$ 23,718,632	$ 24,220,778	$ 23,732,858
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 21,590,905	$ 21,748,247	$ 22,546,874	$ 21,740,579
Weighted Average Interest Rate	8.33%	9.39%	8.60%	9.44%
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 761,469	$ 821,138	$ 768,897	$ 829,497
Weighted Average Interest Rate	3.82%	5.01%	4.06%	4.98%
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 836,342	$ 1,149,247	$ 905,007	$ 1,162,782
Weighted Average Interest Rate	12.54%	12.00%	12.31%	11.90%