Allowance for Credit Losses and Unfunded Loan Commitments - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended	12 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Sep. 30, 2023	Dec. 31, 2024	Dec. 31, 2023
Allowance For Credit Losses [Abstract]
Allowance for loan losses, beginning balance	$ 1,435,920	$ 1,339,772		$ 1,339,772
Transfer from allowance for unfunded loan commitments		131,614
Provision for current period		94,559
Loan sale reduction to provision		(133,204)	$ (133,000)
Total provisions		(38,645)
Net charge-offs:
Charge-offs		(93,997)
Recoveries		11,314
Net charge-offs		(82,683)
Allowance for loan losses, ending balance	1,443,715	1,350,058		1,435,920
Allowance
Beginning balance	84,568	112,962		112,962
Provision/New commitments - net	44,456	50,686
Transfer to allowance for loan losses	(105,134)	(131,614)
Ending balance	23,890	32,034		84,568
Total allowance for credit losses, ending balance		1,382,092
Accrued interest to be capitalized
Unfunded loan commitments	584,140	673,492		2,311,660	$ 2,221,077
Total accrued interest receivable	1,553,979			1,546,590
Provisions for credit losses reported in consolidated statements of income	23,286	12,041
Private Education Loans
Allowance For Credit Losses [Abstract]
Allowance for loan losses, beginning balance	1,435,920	1,335,105		1,335,105
Transfer from allowance for unfunded loan commitments	105,134	131,614
Provision for current period	95,289	94,476
Loan sale reduction to provision	(116,459)	(133,204)
Total provisions	(21,170)	(38,728)
Net charge-offs:
Charge-offs	(86,903)	(93,874)
Recoveries	10,734	11,314
Net charge-offs	(76,169)	(82,560)
Allowance for loan losses, ending balance	1,443,715	1,345,431		1,435,920
Allowance
Beginning balance	84,568	112,962		112,962
Provision/New commitments - net	44,456	50,686
Transfer to allowance for loan losses	(105,134)	(131,614)
Ending balance	23,890	32,034		84,568
Total allowance for credit losses, ending balance	$ 1,467,605	$ 1,377,465
Accrued interest to be capitalized
Net charge-offs as a percentage of average loans in repayment (annualized)	1.88%	2.14%
Allowance coverage of net charge-offs (annualized)	4.74	4.07
Total Allowance Percentage of Private Education Loan Exposure	5.97%	5.99%
Ending total loans, gross	$ 22,432,125	$ 20,952,346
Average loans in repayment	16,240,511	15,407,495
Ending loans in repayment	15,903,797	14,961,692
Unfunded loan commitments	584,140	673,492
Total accrued interest receivable	1,558,465	1,357,987
Provisions for loan losses	(21,170)	(38,728)
Provisions for unfunded loan commitments	44,456	50,686
Total Private Education Loan provisions for credit losses	23,286	11,958
FFELP Loans
Allowance For Credit Losses [Abstract]
Allowance for loan losses, beginning balance		4,667		4,667
Transfer from allowance for unfunded loan commitments		0
Provision for current period		83
Loan sale reduction to provision		0
Total provisions		83
Net charge-offs:
Charge-offs		(123)
Recoveries		0
Net charge-offs		(123)
Allowance for loan losses, ending balance		4,627
Allowance
Beginning balance		0		$ 0
Provision/New commitments - net		0
Transfer to allowance for loan losses		0
Ending balance		0
Total allowance for credit losses, ending balance		$ 4,627
Accrued interest to be capitalized
Net charge-offs as a percentage of average loans in repayment (annualized)		0.12%
Allowance coverage of net charge-offs (annualized)		9.40
Total Allowance Percentage of Private Education Loan Exposure		0.90%
Ending total loans, gross		$ 516,363
Average loans in repayment		399,680
Ending loans in repayment		393,820
Unfunded loan commitments		0
Total accrued interest receivable		0
Other impacts to the provisions for credit losses:	$ 0	$ 83