Borrowings - Summary of Exposure Related To Unconsolidated VIEs (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Debt Instrument [Line Items]
Estimated Fair Value	$ 2,315,978		$ 2,342,089
Total Exposure	113,847	$ 62,600	124,924	$ 107,794	$ 113,525	$ 72,713
Variable Interest Entity, Not Primary Beneficiary | Private Education Loan term securitizations
Debt Instrument [Line Items]
Estimated Fair Value	389,006		329,188
Equity Interests	52,561		50,786
Total Exposure	$ 441,567		$ 379,974