Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024
Debt Outstanding
Short-Term	$ 0	$ 0
Long-Term	6,147,473	6,440,345
Total	6,147,473	6,440,345
Carrying Amount of Net Assets Securing Debt Outstanding
Restricted Cash	173,485	173,894	$ 147,809
Other Assets, Net	55,160	36,846
Total assets	28,899,184	30,072,110
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short-Term	0	0
Long-Term	5,157,945	5,444,925
Total	5,157,945	5,444,925
Carrying Amount of Net Assets Securing Debt Outstanding
Loans	6,492,587	6,786,390
Restricted Cash	173,483	173,892
Other Assets, Net	412,716	418,803
Total assets	7,078,786	7,379,085
Variable Interest Entity, Primary Beneficiary | Private Education Loan term securitizations
Debt Outstanding
Short-Term	0	0
Long-Term	5,157,945	5,444,925
Total	5,157,945	5,444,925
Carrying Amount of Net Assets Securing Debt Outstanding
Loans	6,492,587	6,786,390
Restricted Cash	173,483	173,892
Other Assets, Net	413,202	418,705
Total assets	7,079,272	7,378,987
Variable Interest Entity, Primary Beneficiary | Secured Borrowing Facility
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	486	98
Total assets	$ 486	$ 98