Borrowings - Summary of Exposure Related to Unconsolidated VIEs (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Estimated Fair Value	$ 2,342,089	$ 2,517,956
Liability for unfunded commitments	124,924	72,713	$ 110,044	$ 115,758	$ 2,481
Variable Interest Entity, Not Primary Beneficiary | Private Education Loan term securitizations:
Debt Instrument [Line Items]
Estimated Fair Value	329,188	192,245
Equity Interests	50,786	37,465
Liability for unfunded commitments	$ 379,974	$ 229,710