Allowance for Credit Losses and Unfunded Loan Commitments - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Allowance For Credit Losses [Abstract]
Allowance for loan losses, beginning balance	$ 1,339,772	$ 1,357,075	$ 1,165,335
Transfer from allowance for unfunded loan commitments	311,787	320,237	344,310
Provision for current period	361,077	242,571	413,535
Loan sale reduction to provision	(235,955)	(205,383)	(174,231)
Loans transferred to held-for-sale			(2,372)
Total provisions	125,122	37,188	236,932
Net charge-offs:
Charge-offs	(377,220)	(421,096)	(431,244)
Recoveries	44,756	46,368	41,742
Net charge-offs	(332,464)	(374,728)	(389,502)
Write-downs arising from transfer of loans to held for sale	(8,297)
Allowance for loan losses, ending balance	1,435,920	1,339,772	1,357,075
Allowance
Beginning balance	112,962	124,924	72,713
Provision	283,393	308,275	396,521
Transfer to allowance for loan losses	(311,787)	(320,237)	(344,310)
Ending balance	84,568	112,962	124,924
Total allowance for credit losses, ending balance	1,520,488	1,452,734	1,481,999
Accrued interest to be capitalized
Unfunded loan commitments	2,311,660	2,221,077	1,995,808	$ 1,776,976
Accrued interest receivable	1,546,590	1,379,904
Other impacts to the provisions for credit losses:	4,010	2,224	909
Provisions for credit losses	408,515	345,463	633,453
FFELP Loans
Allowance For Credit Losses [Abstract]
Allowance for loan losses, beginning balance	4,667	3,444	4,077
Transfer from allowance for unfunded loan commitments	0	0	0
Provision for current period	4,010	2,224	(20)
Loan sale reduction to provision	0	0	0
Loans transferred to held-for-sale			0
Total provisions	4,010	2,224	(20)
Net charge-offs:
Charge-offs	(380)	(1,001)	(613)
Recoveries	0	0	0
Net charge-offs	(380)	(1,001)	(613)
Write-downs arising from transfer of loans to held for sale	(8,297)
Allowance for loan losses, ending balance	0	4,667	3,444
Allowance
Beginning balance	0	0	0
Provision	0	0	0
Transfer to allowance for loan losses	0	0	0
Ending balance	0	0	0
Total allowance for credit losses, ending balance	$ 0	$ 4,667	$ 3,444
Accrued interest to be capitalized
Net charge-offs as a percentage of average loans in repayment	0.00%	0.23%	0.12%
Allowance for loan losses coverage of net charge-offs	0	4.66	5.62
Total allowance for credit losses as a percentage of the ending total loan balance, plus unfunded loan commitments and total accrued interest receivable	0.00%	0.87%	0.57%
Ending total loans, gross	$ 0	$ 537,401	$ 609,050
Average loans in repayment	0	433,225	517,139
Ending loans in repayment	0	406,568	453,915
Unfunded loan commitments	0	0	0
Accrued interest receivable	0	0	0
Other impacts to the provisions for credit losses:	4,010	2,224	(20)
Private Education Loans
Allowance For Credit Losses [Abstract]
Allowance for loan losses, beginning balance	1,335,105	1,353,631	1,158,977
Transfer from allowance for unfunded loan commitments	311,787	320,237	344,310
Provision for current period	357,067	240,347	410,254
Loan sale reduction to provision	(235,955)	(205,383)	(174,231)
Loans transferred to held-for-sale			0
Total provisions	121,112	34,964	236,023
Net charge-offs:
Charge-offs	(376,840)	(420,095)	(427,416)
Recoveries	44,756	46,368	41,737
Net charge-offs	(332,084)	(373,727)	(385,679)
Write-downs arising from transfer of loans to held for sale	0
Allowance for loan losses, ending balance	1,435,920	1,335,105	1,353,631
Allowance
Beginning balance	112,962	124,924	72,713
Provision	283,393	308,275	396,521
Transfer to allowance for loan losses	(311,787)	(320,237)	(344,310)
Ending balance	84,568	112,962	124,924
Total allowance for credit losses, ending balance	$ 1,520,488	$ 1,448,067	$ 1,478,555
Accrued interest to be capitalized
Net charge-offs as a percentage of average loans in repayment	2.19%	2.44%	2.55%
Allowance for loan losses coverage of net charge-offs	4.32	3.57	3.51
Total allowance for credit losses as a percentage of the ending total loan balance, plus unfunded loan commitments and total accrued interest receivable	5.83%	5.89%	6.30%
Ending total loans, gross	$ 22,235,008	$ 21,025,844	$ 20,303,688
Average loans in repayment	15,139,184	15,310,934	15,103,123
Ending loans in repayment	16,106,751	15,409,814	15,129,550
Unfunded loan commitments	2,311,660	2,221,077	1,995,808
Accrued interest receivable	1,549,415	1,354,565	1,177,562
Provisions for loan losses	121,112	34,964	236,023
Provisions for unfunded loan commitments	283,393	308,275	396,521
Total Private Education Loan provisions for credit losses	404,505	343,239	632,544
Credit Cards
Allowance For Credit Losses [Abstract]
Allowance for loan losses, beginning balance		0	2,281
Transfer from allowance for unfunded loan commitments			0
Provision for current period			3,301
Loan sale reduction to provision			0
Loans transferred to held-for-sale			(2,372)
Total provisions			929
Net charge-offs:
Charge-offs			(3,215)
Recoveries			5
Net charge-offs			(3,210)
Allowance for loan losses, ending balance			0
Allowance
Beginning balance		0	0
Provision			0
Transfer to allowance for loan losses			0
Ending balance			0
Total allowance for credit losses, ending balance			$ 0
Accrued interest to be capitalized
Net charge-offs as a percentage of average loans in repayment			0.00%
Allowance for loan losses coverage of net charge-offs			0
Total allowance for credit losses as a percentage of the ending total loan balance, plus unfunded loan commitments and total accrued interest receivable			0.00%
Ending total loans, gross			$ 0
Average loans in repayment			0
Ending loans in repayment			0
Unfunded loan commitments			0
Accrued interest receivable			0
Other impacts to the provisions for credit losses:	$ 0	$ 0	$ 929