Loans Held for Investment - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022	Jul. 01, 2006	Jun. 30, 2006	Sep. 30, 1993
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percent of private loans indexed to LIBOR	40.00%		40.00%		45.00%
Tier 1 of government guarantee (at least)	97.00%		97.00%			97.00%
Tier 2 of government guarantee							98.00%
Tier 3 of government guarantee								100.00%
Gain on sales of loans, net	$ 124,754	$ 239,997	$ 124,745	$ 249,878
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gain on sales of loans, net			128,000	250,000
Proceeds from sale of loans receivable			2,100,000	2,240,000
Net proceeds from sales of loans held for investment, principal			1,960,000	2,100,000
Net proceeds from sales of loans held for investment, capitalized interest			$ 144,000	$ 142,000