Borrowings - Narrative (Details) - USD ($)		1 Months Ended	3 Months Ended
	Mar. 16, 2022	Mar. 31, 2020	Mar. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Line of Credit Facility [Line Items]
Total loans and accrued interest amount securitized at inception			$ 0	$ 0	$ 1,656,263,000
Vertical risk retention interest		5.00%	5.00%	5.00%
Uncommitted federal funds			$ 125,000,000	$ 125,000,000
Lendable value of collateral			$ 2,900,000,000	$ 2,900,000,000	$ 3,300,000,000
2022-A
Line of Credit Facility [Line Items]
Vertical risk retention interest	5.00%
2022-A | Student Loan
Line of Credit Facility [Line Items]
Private education loans sold	$ 973,000,000
Additional net proceeds from sales of loans held for investment, capitalized interest	95,000,000
Gain on sale	$ 10,000,000