Loans Held for Investment - Student Loan Portfolio Average Balances (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 20,108,078		$ 16,488,513
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 18,659,717		15,449,555
Weighted Average Interest Rate	8.84%	8.26%
FFELP Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 919,717		1,003,128
Weighted Average Interest Rate	4.25%	3.69%
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Balance	$ 528,644		$ 35,830
Weighted Average Interest Rate	10.64%	9.16%