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, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Allowance for Credit Losses
Beginning balance	$ 1,479,306	$ 1,227,362	$ 1,357,075	$ 1,165,335	$ 1,165,335
Transfer from unfunded commitment liability	28,188	40,528	176,701	135,214
Provision for current period	96,192	25,050	153,995	73,668
Loan sale reduction to provision	(136,531)	(115,852)	(136,531)	(121,099)
Total provisions	(40,339)	(90,802)	17,464	(47,431)
Net charge-offs:
Charge-offs	(114,134)	(106,999)	(210,216)	(191,065)
Recoveries	11,695	10,977	23,692	19,013
Net charge-offs	(102,439)	(96,022)	(186,524)	(172,052)
Ending Balance	1,364,716	1,081,066	1,364,716	1,081,066	1,357,075
Allowance
Ending balance: collectively evaluated for impairment	1,364,716	1,081,066	1,364,716	1,081,066
Loans
Ending balance: collectively evaluated for impairment	20,511,960	20,213,751	20,511,960	20,213,751
Accrued Interest to be Capitalized [Abstract]
Ending balance: collectively evaluated for impairment	1,136,973	1,029,381	1,136,973	1,029,381
Private Education Loan provision for credit losses, other impacts	90	519	1,559	677
Provisions for credit losses reported in consolidated statements of income	17,729	30,545	131,841	128,595
FFELP Loans
Allowance for Credit Losses
Beginning balance	3,927	3,999	3,444	4,077	4,077
Transfer from unfunded commitment liability	0	0	0	0
Provision for current period	820	60	1,559	81
Loan sale reduction to provision	0	0	0	0
Total provisions	820	60	1,559	81
Net charge-offs:
Charge-offs	(325)	(130)	(581)	(229)
Recoveries	0	0	0	0
Net charge-offs	(325)	(130)	(581)	(229)
Ending Balance	4,422	3,929	4,422	3,929	3,444
Allowance
Ending balance: collectively evaluated for impairment	4,422	3,929	4,422	3,929
Loans
Ending balance: collectively evaluated for impairment	573,597	665,666	573,597	665,666
Accrued Interest to be Capitalized [Abstract]
Ending balance: collectively evaluated for impairment	$ 0	$ 0	$ 0	$ 0
Net charge-offs as a percentage of average loans in repayment (annualized)	0.29%	0.10%	0.26%	0.08%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized	0.77%	0.59%	0.77%	0.59%
Allowance as a percentage of the ending loans in repayment and accrued interest to be capitalized on loans in repayment	1.02%	0.73%	1.02%	0.73%
Allowance coverage of net charge-offs (annualized)	3.40	7.56	3.81	8.58
Ending total loans, gross	$ 573,597	$ 665,666	$ 573,597	$ 665,666
Average loans in repayment	441,749	537,449	446,655	541,133
Ending loans in repayment	431,543	538,081	431,543	538,081
Accrued interest to be capitalized on loans in repayment	0	0	0	0
Private Education Loan provision for credit losses, other impacts	820	60	1,559	81
Private Education Loans
Allowance for Credit Losses
Beginning balance	1,475,379	1,221,053	1,353,631	1,158,977	1,158,977
Transfer from unfunded commitment liability	28,188	40,528	176,701	135,214
Provision for current period	96,102	24,531	152,436	72,991
Loan sale reduction to provision	(136,531)	(115,852)	(136,531)	(121,099)
Total provisions	(40,429)	(91,321)	15,905	(48,108)
Net charge-offs:
Charge-offs	(114,550)	(106,493)	(209,635)	(190,349)
Recoveries	11,706	10,977	23,692	19,010
Net charge-offs	(102,844)	(95,516)	(185,943)	(171,339)
Ending Balance	1,360,294	1,074,744	1,360,294	1,074,744	1,353,631
Allowance
Ending balance: collectively evaluated for impairment	1,360,294	1,074,744	1,360,294	1,074,744
Loans
Ending balance: collectively evaluated for impairment	19,938,363	19,519,185	19,938,363	19,519,185
Accrued Interest to be Capitalized [Abstract]
Ending balance: collectively evaluated for impairment	$ 1,136,973	$ 1,029,381	$ 1,136,973	$ 1,029,381
Net charge-offs as a percentage of average loans in repayment (annualized)	2.69%	2.56%	2.41%	2.23%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized	6.45%	5.23%	6.45%	5.23%
Allowance as a percentage of the ending loans in repayment and accrued interest to be capitalized on loans in repayment	9.03%	7.22%	9.03%	7.22%
Allowance coverage of net charge-offs (annualized)	3.31	2.81	3.66	3.14
Ending total loans, gross	$ 19,938,363	$ 19,519,185	$ 19,938,363	$ 19,519,185
Average loans in repayment	15,269,101	14,901,040	15,448,931	15,366,023
Ending loans in repayment	14,652,527	14,525,259	14,652,527	14,525,259
Accrued interest to be capitalized on loans in repayment	408,923	359,343	408,923	359,343
Provisions for loan losses	(40,429)	(91,321)	15,905	(48,108)
Provisions for unfunded loan commitments	58,068	121,347	114,377	176,026
Total Private Education Loan provisions for credit losses	17,639	30,026	130,282	127,918
Credit Cards
Allowance for Credit Losses
Beginning balance	0	2,310		2,281	$ 2,281
Transfer from unfunded commitment liability	0	0		0
Provision for current period	(730)	459		596
Loan sale reduction to provision	0	0		0
Total provisions	(730)	459		596
Net charge-offs:
Charge-offs		(376)		(487)
Charge-offs	741
Recoveries	(11)
Recoveries		0		3
Net charge-offs	730	(376)		(484)
Ending Balance	0	2,393	0	2,393
Allowance
Ending balance: collectively evaluated for impairment	0	2,393	0	2,393
Loans
Ending balance: collectively evaluated for impairment	0	28,900	0	28,900
Accrued Interest to be Capitalized [Abstract]
Ending balance: collectively evaluated for impairment	$ 0	$ 0	0	$ 0
Net charge-offs as a percentage of average loans in repayment (annualized)	0.00%	5.33%		3.54%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized	0.00%	8.28%		8.28%
Allowance as a percentage of the ending loans in repayment and accrued interest to be capitalized on loans in repayment	0.00%	8.28%		8.28%
Allowance coverage of net charge-offs (annualized)	0	1.59		2.47
Ending total loans, gross	$ 0	$ 28,900	0	$ 28,900
Average loans in repayment	0	28,243		27,375
Ending loans in repayment	0	28,900	0	28,900
Accrued interest to be capitalized on loans in repayment	0	0	$ 0	0
Private Education Loan provision for credit losses, other impacts	$ (730)	$ 459		$ 596