Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Debt Outstanding
Short-Term	$ 0	$ 0
Long-Term	6,440,345	5,227,512
Total	6,440,345	5,227,512
Carrying Amount of Net Assets Securing Debt Outstanding
Restricted Cash	173,894	149,669	$ 156,719
Other Assets, Net	36,846	52,342
Total	30,072,110	29,169,468
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	5,444,925	4,235,312
Total	5,444,925	4,235,312
Carrying Amount of Net Assets Securing Debt Outstanding
Loans	6,786,390	5,539,964
Restricted Cash	173,892	149,412
Other Assets, Net	418,803	303,753
Total	7,379,085	5,993,129
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations:
Debt Outstanding
Short-Term	0	0
Long-Term	5,444,925	4,235,312
Total	5,444,925	4,235,312
Carrying Amount of Net Assets Securing Debt Outstanding
Loans	6,786,390	5,539,964
Restricted Cash	173,892	149,412
Other Assets, Net	418,705	303,653
Total	7,378,987	5,993,029
Variable Interest Entity, Primary Beneficiary | Fixed-rate borrowings
Debt Outstanding
Short-Term	0	0
Long-Term	0	0
Total	0	0
Carrying Amount of Net Assets Securing Debt Outstanding
Loans	0	0
Restricted Cash	0	0
Other Assets, Net	98	100
Total	$ 98	$ 100