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, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Allowance for Credit Losses
Beginning balance	$ 1,227,362	$ 1,179,021	$ 1,165,335	$ 1,361,723	$ 1,361,723
Transfer from unfunded commitment liability	40,528	24,556	135,214	151,436
Provision for current period	25,050	1,194	73,668	(253,805)
Loan sale reduction to provision	(115,852)	(1,477)	(121,099)	(10,335)
Loan transfer from held-for-sale				1,887
Total provisions	(90,802)	(283)	(47,431)	(262,253)
Net charge-offs:
Charge-offs	(106,999)	(50,028)	(191,065)	(105,344)
Recoveries	10,977	6,978	19,013	14,682
Net charge-offs	(96,022)	(43,050)	(172,052)	(90,662)
Ending Balance	1,081,066	1,160,244	1,081,066	1,160,244	1,165,335
Allowance
Ending balance: individually evaluated for impairment	0	80,495	0	80,495
Ending balance: collectively evaluated for impairment	1,081,066	1,079,749	1,081,066	1,079,749
Loans:
Ending balance: individually evaluated for impairment	0	1,192,743	0	1,192,743
Ending balance: collectively evaluated for impairment	20,213,751	20,014,756	20,213,751	20,014,756
Private Education Loan provision for credit losses, other impacts	519	180	677	123
Less: provisions for credit losses	30,545	69,677	128,595	(156,090)
FFELP Loans
Allowance for Credit Losses
Beginning balance	3,999	4,318	4,077	4,378	4,378
Transfer from unfunded commitment liability	0	0	0	0
Provision for current period	60	(2)	81	27
Loan sale reduction to provision	0	0	0	0
Loan transfer from held-for-sale				0
Total provisions	60	(2)	81	27
Net charge-offs:
Charge-offs	(130)	(54)	(229)	(143)
Recoveries	0	0	0	0
Net charge-offs	(130)	(54)	(229)	(143)
Ending Balance	3,929	4,262	3,929	4,262	4,077
Allowance
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	3,929	4,262	3,929	4,262
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 665,666	$ 716,958	$ 665,666	$ 716,958
Net charge-offs as a percentage of average loans in repayment (annualized)	0.10%	0.04%	0.08%	0.05%
Allowance as a percentage of the ending total loan balance	0.59%	0.59%	0.59%	0.59%
Allowance as a percentage of the ending loans in repayment	0.73%	0.78%	0.73%	0.78%
Allowance coverage of net charge-offs (annualized)	7.56	19.73	8.58	14.90
Ending total loans, gross	$ 665,666	$ 716,958	$ 665,666	$ 716,958
Average loans in repayment	537,449	546,306	541,133	551,765
Ending loans in repayment	538,081	548,488	538,081	548,488
Private Education Loan provision for credit losses, other impacts	60	(2)	81	27
Private Education Loans
Allowance for Credit Losses
Beginning balance	1,221,053	1,173,375	1,158,977	1,355,844	1,355,844
Transfer from unfunded commitment liability	40,528	24,556	135,214	151,436
Provision for current period	24,531	1,014	72,991	(253,928)
Loan sale reduction to provision	(115,852)	(1,477)	(121,099)	(10,335)
Loan transfer from held-for-sale				1,887
Total provisions	(91,321)	(463)	(48,108)	(262,376)
Net charge-offs:
Charge-offs	(106,493)	(49,900)	(190,349)	(105,039)
Recoveries	10,977	6,972	19,010	14,675
Net charge-offs	(95,516)	(42,928)	(171,339)	(90,364)
Ending Balance	1,074,744	1,154,540	1,074,744	1,154,540	1,158,977
Allowance
Ending balance: individually evaluated for impairment	0	80,495	0	80,495
Ending balance: collectively evaluated for impairment	1,074,744	1,074,045	1,074,744	1,074,045
Loans:
Ending balance: individually evaluated for impairment	0	1,192,743	0	1,192,743
Ending balance: collectively evaluated for impairment	$ 19,519,185	$ 19,285,014	$ 19,519,185	$ 19,285,014
Net charge-offs as a percentage of average loans in repayment (annualized)	2.56%	1.16%	2.23%	1.23%
Allowance as a percentage of the ending total loan balance	5.51%	5.64%	5.51%	5.64%
Allowance as a percentage of the ending loans in repayment	7.40%	7.79%	7.40%	7.79%
Allowance coverage of net charge-offs (annualized)	2.81	6.72	3.14	6.39
Ending total loans, gross	$ 19,519,185	$ 20,477,757	$ 19,519,185	$ 20,477,757
Average loans in repayment	14,901,040	14,743,360	15,366,023	14,738,505
Ending loans in repayment	14,525,259	14,825,375	14,525,259	14,825,375
Provisions for loan losses	(91,321)	(463)	(48,108)	(262,376)
Provisions for unfunded loan commitments	121,347	69,960	176,026	106,163
Total Private Education Loan provisions for credit losses	30,026	69,497	127,918	(156,213)
Credit Cards
Allowance for Credit Losses
Beginning balance	2,310	1,328	2,281	1,501	1,501
Transfer from unfunded commitment liability	0	0	0	0
Provision for current period	459	182	596	96
Loan sale reduction to provision	0	0	0	0
Loan transfer from held-for-sale				0
Total provisions	459	182	596	96
Net charge-offs:
Charge-offs	(376)	(74)	(487)	(162)
Recoveries	0	6	3	7
Net charge-offs	(376)	(68)	(484)	(155)
Ending Balance	2,393	1,442	2,393	1,442	$ 2,281
Allowance
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	2,393	1,442	2,393	1,442
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 28,900	$ 12,784	$ 28,900	$ 12,784
Net charge-offs as a percentage of average loans in repayment (annualized)	5.33%	2.29%	3.54%	2.59%
Allowance as a percentage of the ending total loan balance	8.28%	11.28%	8.28%	11.28%
Allowance as a percentage of the ending loans in repayment	8.28%	11.28%	8.28%	11.28%
Allowance coverage of net charge-offs (annualized)	1.59	5.30	2.47	4.65
Ending total loans, gross	$ 28,900	$ 12,784	$ 28,900	$ 12,784
Average loans in repayment	28,243	11,855	27,375	11,964
Ending loans in repayment	28,900	12,784	28,900	12,784
Private Education Loan provision for credit losses, other impacts	$ 459	$ 182	$ 596	$ 96