Allowance for Credit Losses - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Allowance for Credit Losses
Beginning balance	$ 1,350,058	$ 1,479,306	$ 1,339,772	$ 1,357,075	$ 1,357,075
Transfer from unfunded commitment liability	29,715	28,188	161,329	176,701
Provision for current period	72,421	96,192	166,980	153,995
Loan sale reduction to provision	(102,751)	(136,531)	(235,955)	(136,531)
Total provisions	(30,330)	(40,339)	(68,975)	17,464
Net charge-offs:
Charge-offs	(91,168)	(114,134)	(185,165)	(210,216)
Recoveries	11,377	11,695	22,691	23,692
Net charge-offs	(79,791)	(102,439)	(162,474)	(186,524)
Ending Balance	1,269,652	1,364,716	1,269,652	1,364,716	1,339,772
Allowance
Ending balance: collectively evaluated for impairment	1,269,652	1,364,716	1,269,652	1,364,716
Loans
Ending balance: collectively evaluated for impairment	20,105,139	20,511,960	20,105,139	20,511,960
Accrued Interest to be Capitalized
Ending balance: collectively evaluated for impairment	1,231,754	1,136,973	1,231,754	1,136,973
Other impacts to the provisions for credit losses:	(441)	90	(358)	1,559
Provisions for credit losses reported in consolidated statements of income	16,830	17,729	28,871	131,841
FFELP Loans
Allowance for Credit Losses
Beginning balance	4,627	3,927	4,667	3,444	3,444
Transfer from unfunded commitment liability	0	0	0	0
Provision for current period	(441)	820	(358)	1,559
Loan sale reduction to provision	0	0	0	0
Total provisions	(441)	820	(358)	1,559
Net charge-offs:
Charge-offs	(126)	(325)	(249)	(581)
Recoveries	0	0	0	0
Net charge-offs	(126)	(325)	(249)	(581)
Ending Balance	4,060	4,422	4,060	4,422	4,667
Allowance
Ending balance: collectively evaluated for impairment	4,060	4,422	4,060	4,422
Loans
Ending balance: collectively evaluated for impairment	485,608	573,597	485,608	573,597
Accrued Interest to be Capitalized
Ending balance: collectively evaluated for impairment	$ 0	$ 0	$ 0	$ 0
Net charge-offs as a percentage of average loans in repayment (annualized)	0.13%	0.29%	0.13%	0.26%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized	0.84%	0.77%	0.84%	0.77%
Allowance as a percentage of the ending loans in repayment and accrued interest to be capitalized on loans in repayment	1.10%	1.02%	1.10%	1.02%
Allowance coverage of net charge-offs (annualized)	8.06	3.40	8.15	3.81
Ending total loans, gross	$ 485,608	$ 573,597	$ 485,608	$ 573,597
Average loans in repayment	378,667	441,749	388,510	446,655
Ending loans in repayment	369,681	431,543	369,681	431,543
Accrued interest to be capitalized on loans in repayment	0	0	0	0
Other impacts to the provisions for credit losses:	(441)	820	(358)	1,559
Private Education Loans
Allowance for Credit Losses
Beginning balance	1,345,431	1,475,379	1,335,105	1,353,631	1,353,631
Transfer from unfunded commitment liability	29,715	28,188	161,329	176,701
Provision for current period	72,862	96,102	167,338	152,436
Loan sale reduction to provision	(102,751)	(136,531)	(235,955)	(136,531)
Total provisions	(29,889)	(40,429)	(68,617)	15,905
Net charge-offs:
Charge-offs	(91,042)	(114,550)	(184,916)	(209,635)
Recoveries	11,377	11,706	22,691	23,692
Net charge-offs	(79,665)	(102,844)	(162,225)	(185,943)
Ending Balance	1,265,592	1,360,294	1,265,592	1,360,294	$ 1,335,105
Allowance
Ending balance: collectively evaluated for impairment	1,265,592	1,360,294	1,265,592	1,360,294
Loans
Ending balance: collectively evaluated for impairment	19,619,531	19,938,363	19,619,531	19,938,363
Accrued Interest to be Capitalized
Ending balance: collectively evaluated for impairment	$ 1,231,754	$ 1,136,973	$ 1,231,754	$ 1,136,973
Net charge-offs as a percentage of average loans in repayment (annualized)	2.19%	2.69%	2.17%	2.41%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized	6.07%	6.45%	6.07%	6.45%
Allowance as a percentage of the ending loans in repayment and accrued interest to be capitalized on loans in repayment	8.62%	9.03%	8.62%	9.03%
Allowance coverage of net charge-offs (annualized)	3.97	3.31	3.90	3.66
Ending total loans, gross	$ 19,619,531	$ 19,938,363	$ 19,619,531	$ 19,938,363
Average loans in repayment	14,543,669	15,269,101	14,977,567	15,448,931
Ending loans in repayment	14,231,581	14,652,527	14,231,581	14,652,527
Accrued interest to be capitalized on loans in repayment	453,150	408,923	453,150	408,923
Provisions for loan losses	(29,889)	(40,429)	(68,617)	15,905
Provisions for unfunded loan commitments	47,160	58,068	97,846	114,377
Total Private Education Loan provisions for credit losses	$ 17,271	17,639	$ 29,229	130,282
Credit Cards
Allowance for Credit Losses
Beginning balance		0
Transfer from unfunded commitment liability		0
Provision for current period		(730)
Loan sale reduction to provision		0
Total provisions		(730)
Net charge-offs:
Charge-offs		741
Recoveries		(11)
Net charge-offs		730
Ending Balance		0		0
Allowance
Ending balance: collectively evaluated for impairment		0		0
Loans
Ending balance: collectively evaluated for impairment		0		0
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)		0.00%
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		0
Average loans in repayment		0
Ending loans in repayment		0		0
Accrued interest to be capitalized on loans in repayment		0		$ 0
Other impacts to the provisions for credit losses:		$ (730)