Borrowings - Summary of Exposure Related to Unconsolidated VIEs (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Estimated Fair Value	$ 2,411,622	$ 2,342,089
Liability for unfunded commitments	112,962	124,924	$ 72,713	$ 110,044
Variable Interest Entity, Not Primary Beneficiary | Private Education Loan term securitizations:
Debt Instrument [Line Items]
Estimated Fair Value	423,327	329,188
Equity Interests	54,481	50,786
Liability for unfunded commitments	$ 477,808	$ 379,974