Allowance for Credit Losses - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022
Allowance for Credit Losses
Beginning Balance	$ 1,357,075	$ 1,165,335	$ 1,165,335
Transfer from unfunded commitment liability	148,513	94,686
Provision for current period	57,803	48,618
Loan sale reduction to provision		(5,247)
Total provisions	57,803	43,371
Net charge-offs:
Charge-offs	(96,082)	(84,066)
Recoveries	11,997	8,036
Net charge-offs	(84,085)	(76,030)
Ending Balance	1,479,306	1,227,362	1,357,075
Allowance
Ending balance: collectively evaluated for impairment	1,479,306	1,227,362
Loans
Ending balance: collectively evaluated for impairment	22,490,321	22,445,189
Accrued Interest to be Capitalized [Abstract]
Ending balance: collectively evaluated for impairment	1,150,802	993,698
Private Education Loan provision for credit losses, other impacts	1,469	158
Provisions for credit losses reported in consolidated statements of income	114,112	98,050
FFELP Loans
Allowance for Credit Losses
Beginning Balance	3,444	4,077	4,077
Transfer from unfunded commitment liability	0	0
Provision for current period	739	21
Loan sale reduction to provision		0
Total provisions	739	21
Net charge-offs:
Charge-offs	(256)	(99)
Recoveries	0	0
Net charge-offs	(256)	(99)
Ending Balance	3,927	3,999	3,444
Allowance
Ending balance: collectively evaluated for impairment	3,927	3,999
Loans
Ending balance: collectively evaluated for impairment	592,318	682,273
Accrued Interest to be Capitalized [Abstract]
Ending balance: collectively evaluated for impairment	$ 0	$ 0
Net charge-offs as a percentage of average loans in repayment (annualized)	0.23%	0.07%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized(5)	0.66%	0.59%
Allowance as a percentage of the ending loans in repayment and accrued interest to be capitalized on loans in repayment	0.88%	0.75%
Allowance coverage of net charge-offs (annualized)	3.83	10.10
Ending total loans, gross	$ 592,318	$ 682,273
Average loans in repayment	451,451	543,303
Ending loans in repayment	446,214	535,080
Accrued interest to be capitalized on loans in repayment	0	0
Private Education Loan provision for credit losses, other impacts	739	21
Private Education Loans
Allowance for Credit Losses
Beginning Balance	1,353,631	1,158,977	1,158,977
Transfer from unfunded commitment liability	148,513	94,686
Provision for current period	56,334	48,460
Loan sale reduction to provision		(5,247)
Total provisions	56,334	43,213
Net charge-offs:
Charge-offs	(95,085)	(83,856)
Recoveries	11,986	8,033
Net charge-offs	(83,099)	(75,823)
Ending Balance	1,475,379	1,221,053	1,353,631
Allowance
Ending balance: collectively evaluated for impairment	1,475,379	1,221,053
Loans
Ending balance: collectively evaluated for impairment	21,898,003	21,735,369
Accrued Interest to be Capitalized [Abstract]
Ending balance: collectively evaluated for impairment	$ 1,150,802	$ 993,698
Net charge-offs as a percentage of average loans in repayment (annualized)	2.11%	1.89%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized(5)	6.40%	5.37%
Allowance as a percentage of the ending loans in repayment and accrued interest to be capitalized on loans in repayment	9.00%	7.43%
Allowance coverage of net charge-offs (annualized)	4.44	4.03
Ending total loans, gross	$ 21,898,003	$ 21,735,369
Average loans in repayment	15,764,143	16,013,289
Ending loans in repayment	15,990,459	16,095,157
Accrued interest to be capitalized on loans in repayment	408,263	331,405
Provisions for loan losses	56,334	43,213
Provisions for unfunded loan commitments	56,309	54,679
Total Private Education Loan provisions for credit losses	112,643	97,892
Credit Cards
Allowance for Credit Losses
Beginning Balance	0	2,281	2,281
Transfer from unfunded commitment liability	0	0
Provision for current period	730	137
Loan sale reduction to provision		0
Total provisions	730	137
Net charge-offs:
Charge-offs	(741)	(111)
Recoveries	11	3
Net charge-offs	(730)	(108)
Ending Balance	0	2,310	$ 0
Allowance
Ending balance: collectively evaluated for impairment	0	2,310
Loans
Ending balance: collectively evaluated for impairment	0	27,547
Accrued Interest to be Capitalized [Abstract]
Ending balance: collectively evaluated for impairment	$ 0	$ 0
Net charge-offs as a percentage of average loans in repayment (annualized)	0.00%	1.63%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized(5)	0.00%	8.39%
Allowance as a percentage of the ending loans in repayment and accrued interest to be capitalized on loans in repayment	0.00%	8.39%
Allowance coverage of net charge-offs (annualized)	0	5.35
Ending total loans, gross	$ 0	$ 27,547
Average loans in repayment	0	26,551
Ending loans in repayment	0	27,547
Accrued interest to be capitalized on loans in repayment	0	0
Private Education Loan provision for credit losses, other impacts	$ 730	$ 137