Allowance for Credit Losses - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Allowance for Credit Losses
Beginning balance	$ 1,269,652	$ 1,364,716	$ 1,339,772	$ 1,357,075	$ 1,357,075
Transfer from unfunded commitment liability	115,421	101,687	276,750	278,388
Provision for current period	113,564	45,089	280,544	199,084
Loan sale reduction to provision			(235,955)	(136,531)
Total provisions	113,564	45,089	44,589	62,553
Net charge-offs:
Charge-offs	(87,868)	(105,137)	(273,033)	(315,353)
Recoveries	11,149	9,693	33,840	33,385
Net charge-offs	(76,719)	(95,444)	(239,193)	(281,968)
Write-downs arising from transfer of loans to held for sale	(8,297)		(8,297)
Ending Balance	1,413,621	1,416,048	1,413,621	1,416,048	1,339,772
Allowance
Ending balance: collectively evaluated for impairment	1,413,621	1,416,048	1,413,621	1,416,048
Loans
Ending balance: collectively evaluated for impairment	21,777,466	22,235,176	21,777,466	22,235,176
Accrued interest to be capitalized
Ending balance: collectively evaluated for impairment	1,390,774	1,283,388	1,390,774	1,283,388
Other impacts to the provisions for credit losses:	4,368	666	4,010	2,225
Provisions for credit losses reported in consolidated statements of operations	271,465	198,023	300,336	329,864
FFELP Loans
Allowance for Credit Losses
Beginning balance	4,060	4,422	4,667	3,444	3,444
Transfer from unfunded commitment liability	0	0	0	0
Provision for current period	4,368	666	4,010	2,225
Loan sale reduction to provision			0	0
Total provisions	4,368	666	4,010	2,225
Net charge-offs:
Charge-offs	(131)	(272)	(380)	(853)
Recoveries	0	0	0	0
Net charge-offs	(131)	(272)	(380)	(853)
Write-downs arising from transfer of loans to held for sale	(8,297)		(8,297)
Ending Balance	0	4,816	0	4,816	4,667
Allowance
Ending balance: collectively evaluated for impairment	0	4,816	0	4,816
Loans
Ending balance: collectively evaluated for impairment	0	554,309	0	554,309
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)	0.00%	0.25%	0.00%	0.26%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized	0.00%	0.87%	0.00%	0.87%
Allowance as a percentage of the ending loans in repayment and accrued interest to be capitalized on loans in repayment	0.00%	1.15%	0.00%	1.15%
Allowance coverage of net charge-offs (annualized)	0	4.43	0	4.23
Ending total loans, gross	$ 0	$ 554,309	$ 0	$ 554,309
Average loans in repayment	0	428,028	0	440,716
Ending loans in repayment	0	418,022	0	418,022
Accrued interest to be capitalized on loans in repayment	0	0	0	0
Other impacts to the provisions for credit losses:	4,368	666	4,010	2,225
Private Education Loans
Allowance for Credit Losses
Beginning balance	1,265,592	1,360,294	1,335,105	1,353,631	1,353,631
Transfer from unfunded commitment liability	115,421	101,687	276,750	278,388
Provision for current period	109,196	44,423	276,534	196,859
Loan sale reduction to provision			(235,955)	(136,531)
Total provisions	109,196	44,423	40,579	60,328
Net charge-offs:
Charge-offs	(87,737)	(104,865)	(272,653)	(314,500)
Recoveries	11,149	9,693	33,840	33,385
Net charge-offs	(76,588)	(95,172)	(238,813)	(281,115)
Write-downs arising from transfer of loans to held for sale	0		0
Ending Balance	1,413,621	1,411,232	1,413,621	1,411,232	$ 1,335,105
Allowance
Ending balance: collectively evaluated for impairment	1,413,621	1,411,232	1,413,621	1,411,232
Loans
Ending balance: collectively evaluated for impairment	21,777,466	21,680,867	21,777,466	21,680,867
Accrued interest to be capitalized
Ending balance: collectively evaluated for impairment	$ 1,390,774	$ 1,283,388	$ 1,390,774	$ 1,283,388
Net charge-offs as a percentage of average loans in repayment (annualized)	2.08%	2.53%	2.13%	2.44%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized	6.10%	6.15%	6.10%	6.15%
Allowance as a percentage of the ending loans in repayment and accrued interest to be capitalized on loans in repayment	8.91%	8.84%	8.91%	8.84%
Allowance coverage of net charge-offs (annualized)	4.61	3.71	4.44	3.77
Ending total loans, gross	$ 21,777,466	$ 21,680,867	$ 21,777,466	$ 21,680,867
Average loans in repayment	14,708,205	15,023,993	14,944,421	15,358,596
Ending loans in repayment	15,360,255	15,505,145	15,360,255	15,505,145
Accrued interest to be capitalized on loans in repayment	513,121	464,807	513,121	464,807
Provisions for loan losses	109,196	44,423	40,579	60,328
Provisions for unfunded loan commitments	157,901	152,934	255,747	267,311
Total Private Education Loan provisions for credit losses	$ 267,097	$ 197,357	$ 296,326	$ 327,639