Allowance for Credit Losses - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Allowance for Credit Losses
Beginning balance	$ 1,339,772	$ 1,357,075	$ 1,357,075
Transfer from unfunded commitment liability	131,614	148,513
Provision for current period	94,559	57,803
Loan sale reduction to provision	(133,204)
Total provisions	(38,645)	57,803
Net charge-offs:
Charge-offs	(93,997)	(96,082)
Recoveries	11,314	11,997
Net charge-offs	(82,683)	(84,085)
Ending Balance	1,350,058	1,479,306	1,339,772
Allowance
Ending balance: collectively evaluated for impairment	1,350,058	1,479,306
Loans
Ending balance: collectively evaluated for impairment	21,468,709	22,490,321
Accrued Interest to be Capitalized
Ending balance: collectively evaluated for impairment	1,217,295	1,150,802
Other impacts to the provisions for credit losses	83	1,469
Provisions for credit losses reported in consolidated statements of income	12,041	114,112
FFELP Loans
Allowance for Credit Losses
Beginning balance	4,667	3,444	3,444
Transfer from unfunded commitment liability	0	0
Provision for current period	83	739
Loan sale reduction to provision	0
Total provisions	83	739
Net charge-offs:
Charge-offs	(123)	(256)
Recoveries	0	0
Net charge-offs	(123)	(256)
Ending Balance	4,627	3,927	4,667
Allowance
Ending balance: collectively evaluated for impairment	4,627	3,927
Loans
Ending balance: collectively evaluated for impairment	516,363	592,318
Accrued Interest to be Capitalized
Ending balance: collectively evaluated for impairment	$ 0	$ 0
Net charge-offs as a percentage of average loans in repayment (annualized)	0.12%	0.23%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized	0.90%	0.66%
Allowance as a percentage of the ending loans in repayment and accrued interest to be capitalized on loans in repayment	1.17%	0.88%
Allowance coverage of net charge-offs (annualized)	9.40	3.83
Ending total loans, gross	$ 516,363	$ 592,318
Average loans in repayment	399,680	451,451
Ending loans in repayment	393,820	446,214
Accrued interest to be capitalized on loans in repayment	0	0
Other impacts to the provisions for credit losses	83	739
Private Education Loans
Allowance for Credit Losses
Beginning balance	1,335,105	1,353,631	1,353,631
Transfer from unfunded commitment liability	131,614	148,513
Provision for current period	94,476	56,334
Loan sale reduction to provision	(133,204)
Total provisions	(38,728)	56,334
Net charge-offs:
Charge-offs	(93,874)	(95,085)
Recoveries	11,314	11,986
Net charge-offs	(82,560)	(83,099)
Ending Balance	1,345,431	1,475,379	1,335,105
Allowance
Ending balance: collectively evaluated for impairment	1,345,431	1,475,379
Loans
Ending balance: collectively evaluated for impairment	20,952,346	21,898,003
Accrued Interest to be Capitalized
Ending balance: collectively evaluated for impairment	$ 1,217,295	$ 1,150,802
Net charge-offs as a percentage of average loans in repayment (annualized)	2.14%	2.11%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized	6.07%	6.40%
Allowance as a percentage of the ending loans in repayment and accrued interest to be capitalized on loans in repayment	8.74%	9.00%
Allowance coverage of net charge-offs (annualized)	4.07	4.44
Ending total loans, gross	$ 20,952,346	$ 21,898,003
Average loans in repayment	15,407,495	15,764,143
Ending loans in repayment	14,961,692	15,990,459
Accrued interest to be capitalized on loans in repayment	440,259	408,263
Provisions for loan losses	(38,728)	56,334
Provisions for unfunded loan commitments	50,686	56,309
Total Private Education Loan provisions for credit losses	$ 11,958	112,643
Credit Cards
Allowance for Credit Losses
Beginning balance		0	$ 0
Transfer from unfunded commitment liability		0
Provision for current period		730
Total provisions		730
Net charge-offs:
Charge-offs		(741)
Recoveries		11
Net charge-offs		(730)
Ending Balance		0
Allowance
Ending balance: collectively evaluated for impairment		0
Loans
Ending balance: collectively evaluated for impairment		0
Accrued Interest to be Capitalized
Ending balance: collectively evaluated for impairment		$ 0
Net charge-offs as a percentage of average loans in repayment (annualized)		0.00%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized		0.00%
Allowance as a percentage of the ending loans in repayment and accrued interest to be capitalized on loans in repayment		0.00%
Allowance coverage of net charge-offs (annualized)		0
Ending total loans, gross		$ 0
Average loans in repayment		0
Ending loans in repayment		0
Accrued interest to be capitalized on loans in repayment		0
Other impacts to the provisions for credit losses		$ 730