Allowance for Credit Losses - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Allowance for Credit Losses
Beginning balance	$ 1,165,335	$ 1,361,723	$ 1,361,723
Transfer from unfunded commitment liability	94,686	126,880
Provision for current period	48,618	(254,999)
Loan sale reduction to provision	(5,247)	(8,858)
Loan transfer from held-for-sale		1,887
Total provisions	43,371	(261,970)
Net charge-offs:
Charge-offs	(84,066)	(55,316)
Recoveries	8,036	7,704
Net charge-offs	(76,030)	(47,612)
Ending Balance	1,227,362	1,179,021	1,165,335
Allowance
Ending balance: individually evaluated for impairment	0	95,536
Ending balance: collectively evaluated for impairment	1,227,362	1,083,485
Loans:
Ending balance: individually evaluated for impairment	0	1,225,604
Ending balance: collectively evaluated for impairment	22,445,189	20,255,209
Provisions for loan losses	43,213	(261,913)
Provisions for unfunded loan commitments	54,679	36,203
Total Private Education Loan provisions for credit losses	97,892	(225,710)
Private Education Loan provision for credit losses, other impacts	158	(57)
Less: provisions for credit losses	98,050	(225,767)
FFELP Loans
Allowance for Credit Losses
Beginning balance	4,077	4,378	4,378
Transfer from unfunded commitment liability	0	0
Provision for current period	21	29
Loan sale reduction to provision	0	0
Loan transfer from held-for-sale		0
Total provisions	21	29
Net charge-offs:
Charge-offs	(99)	(89)
Recoveries	0	0
Net charge-offs	(99)	(89)
Ending Balance	3,999	4,318	4,077
Allowance
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	3,999	4,318
Loans:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	$ 682,273	$ 727,664
Net charge-offs as a percentage of average loans in repayment (annualized)	0.07%	0.06%
Allowance as a percentage of the ending total loan balance	0.59%	0.59%
Allowance as a percentage of the ending loans in repayment	0.75%	0.80%
Allowance coverage of net charge-offs (annualized)	10.10	12.13
Ending total loans, gross	$ 682,273	$ 727,664
Average loans in repayment	543,303	554,510
Ending loans in repayment	535,080	540,903
Private Education Loan provision for credit losses, other impacts	21	29
Private Education Loans
Allowance for Credit Losses
Beginning balance	1,158,977	1,355,844	1,355,844
Transfer from unfunded commitment liability	94,686	126,880
Provision for current period	48,460	(254,942)
Loan sale reduction to provision	(5,247)	(8,858)
Loan transfer from held-for-sale		1,887
Total provisions	43,213	(261,913)
Net charge-offs:
Charge-offs	(83,856)	(55,139)
Recoveries	8,033	7,703
Net charge-offs	(75,823)	(47,436)
Ending Balance	1,221,053	1,173,375	1,158,977
Allowance
Ending balance: individually evaluated for impairment	0	95,536
Ending balance: collectively evaluated for impairment	1,221,053	1,077,839
Loans:
Ending balance: individually evaluated for impairment	0	1,225,604
Ending balance: collectively evaluated for impairment	$ 21,735,369	$ 19,516,236
Net charge-offs as a percentage of average loans in repayment (annualized)	1.89%	1.29%
Allowance as a percentage of the ending total loan balance	5.62%	5.66%
Allowance as a percentage of the ending loans in repayment	7.59%	7.94%
Allowance coverage of net charge-offs (annualized)	4.03	6.18
Ending total loans, gross	$ 21,735,369	$ 20,741,840
Average loans in repayment	16,013,289	14,743,508
Ending loans in repayment	16,095,157	14,777,939
Credit Cards
Allowance for Credit Losses
Beginning balance	2,281	1,501	1,501
Transfer from unfunded commitment liability	0	0
Provision for current period	137	(86)
Loan sale reduction to provision	0	0
Loan transfer from held-for-sale		0
Total provisions	137	(86)
Net charge-offs:
Charge-offs	(111)	(88)
Recoveries	3	1
Net charge-offs	(108)	(87)
Ending Balance	2,310	1,328	$ 2,281
Allowance
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	2,310	1,328
Loans:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	$ 27,547	$ 11,309
Net charge-offs as a percentage of average loans in repayment (annualized)	1.63%	2.92%
Allowance as a percentage of the ending total loan balance	8.39%	11.74%
Allowance as a percentage of the ending loans in repayment	8.39%	11.74%
Allowance coverage of net charge-offs (annualized)	5.35	3.82
Ending total loans, gross	$ 27,547	$ 11,309
Average loans in repayment	26,551	11,909
Ending loans in repayment	27,547	11,309
Private Education Loan provision for credit losses, other impacts	$ 137	$ (86)