Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 23,747,242	$ 20,108,078
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 21,732,826	$ 18,659,717
Weighted Average Interest Rate	9.50%	8.84%
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 837,950	$ 919,717
Weighted Average Interest Rate	4.94%	4.25%
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 1,176,466	$ 528,644
Weighted Average Interest Rate	11.81%	10.64%