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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Allowance for Credit Losses
Beginning balance	$ 1,160,244	$ 1,929,323	$ 1,361,723	$ 441,912	$ 441,912
Transfer from unfunded commitment liability		100,470	262,049	279,555
Provision for current period	(6,530)	(90,014)	(260,335)	338,120
Loan sale reduction to provision	0	(42,916)	(10,335)	(204,709)
Loan transfer from held-for-sale			1,887
Total provisions	(6,530)	(132,930)	(268,783)	133,411
Net charge-offs:
Charge-offs	(56,225)	(60,666)	(161,569)	(178,120)
Recoveries	7,305	6,896	21,987	23,282
Net charge-offs	(48,920)	(53,770)	(139,582)	(154,838)
Loan sales			(108,534)	(108,534)
Ending Balance	1,215,407	1,734,559	1,215,407	1,734,559	1,361,723
Allowance
Ending balance: individually evaluated for impairment	69,626	138,663	69,626	138,663
Ending balance: collectively evaluated for impairment	1,145,781	1,595,896	1,145,781	1,595,896
Loans:
Ending balance: individually evaluated for impairment	1,148,282	1,495,161	1,148,282	1,495,161
Ending balance: collectively evaluated for impairment	21,278,012	21,876,262	21,278,012	21,876,262
Provisions for loan losses	(6,995)	(81,710)	(269,371)	134,374
Provisions for unfunded loan commitments	144,972	129,290	251,135	276,094
Total Private Education Loan provisions for credit losses	137,977	47,580	(18,236)	410,468
Private Education Loan provision for credit losses, other impacts	465	(51,220)	588	(963)
Less: provisions for credit losses	138,442	(3,640)	(17,648)	409,505
Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Beginning balance				1,584,965	1,584,965
Day 1 adjustment for the adoption of CECL						$ 1,143,053
Transfer from unfunded commitment liability	110,613
FFELP Loans
Allowance for Credit Losses
Beginning balance	4,262	4,385	4,378	1,633	1,633
Transfer from unfunded commitment liability		0	0	0
Provision for current period	50	67	77	277
Loan sale reduction to provision	0	0	0	0
Loan transfer from held-for-sale			0
Total provisions	50	67	77	277
Net charge-offs:
Charge-offs	(106)	(89)	(249)	(399)
Recoveries	0	0	0	0
Net charge-offs	(106)	(89)	(249)	(399)
Loan sales			0	0
Ending Balance	4,206	4,363	4,206	4,363	4,378
Allowance
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	4,206	4,363	4,206	4,363
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 705,691	$ 745,556	$ 705,691	$ 745,556
Net charge-offs as a percentage of average loans in repayment (annualized)	0.08%	0.07%	0.06%	0.10%
Allowance as a percentage of the ending total loan balance	0.60%	0.59%	0.60%	0.59%
Allowance as a percentage of the ending loans in repayment	0.79%	0.77%	0.79%	0.77%
Allowance coverage of net charge-offs (annualized)	9.92	12.26	12.67	8.20
Ending total loans, gross	$ 705,691	$ 745,556	$ 705,691	$ 745,556
Average loans in repayment	540,018	510,809	547,394	546,443
Ending loans in repayment	530,476	564,442	530,476	564,442
Private Education Loan provision for credit losses, other impacts	50	67	77	277
FFELP Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Beginning balance				4,485	4,485
Day 1 adjustment for the adoption of CECL						2,852
Transfer from unfunded commitment liability	0
Private Education Loans
Allowance for Credit Losses
Beginning balance	1,154,540	1,760,559	1,355,844	374,300	374,300
Transfer from unfunded commitment liability		100,470	262,049	279,555
Provision for current period	(6,995)	(81,710)	(260,923)	296,167
Loan sale reduction to provision	0	0	(10,335)	(161,793)
Loan transfer from held-for-sale			1,887
Total provisions	(6,995)	(81,710)	(269,371)	134,374
Net charge-offs:
Charge-offs	(56,000)	(55,298)	(161,039)	(138,546)
Recoveries	7,302	4,790	21,977	18,298
Net charge-offs	(48,698)	(50,508)	(139,062)	(120,248)
Loan sales			0	0
Ending Balance	1,209,460	1,728,811	1,209,460	1,728,811	1,355,844
Allowance
Ending balance: individually evaluated for impairment	69,626	138,663	69,626	138,663
Ending balance: collectively evaluated for impairment	1,139,834	1,590,148	1,139,834	1,590,148
Loans:
Ending balance: individually evaluated for impairment	1,148,282	1,495,161	1,148,282	1,495,161
Ending balance: collectively evaluated for impairment	$ 20,554,555	$ 21,119,166	$ 20,554,555	$ 21,119,166
Net charge-offs as a percentage of average loans in repayment (annualized)	1.29%	1.33%	1.25%	1.05%
Allowance as a percentage of the ending total loan balance	5.57%	7.64%	5.57%	7.64%
Allowance as a percentage of the ending loans in repayment	7.81%	10.91%	7.81%	10.91%
Allowance coverage of net charge-offs (annualized)	6.21	8.56	6.52	10.78
Ending total loans, gross	$ 21,702,837	$ 22,614,327	$ 21,702,837	$ 22,614,327
Average loans in repayment	15,108,802	15,182,703	14,877,937	15,336,253
Ending loans in repayment	15,490,132	15,853,309	15,490,132	15,853,309
Private Education Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Beginning balance				1,435,130	1,435,130
Day 1 adjustment for the adoption of CECL						1,060,830
Transfer from unfunded commitment liability	110,613
Personal Loans
Allowance for Credit Losses
Beginning balance		163,337		65,877	65,877
Transfer from unfunded commitment liability		0		0
Provision for current period		(8,762)		40,485
Loan sale reduction to provision		(42,916)		(42,916)
Total provisions		(51,678)		(2,431)
Net charge-offs:
Charge-offs		(5,231)		(39,079)
Recoveries		2,106		4,984
Net charge-offs		(3,125)		(34,095)
Loan sales			(108,534)	(108,534)
Ending Balance		0		0
Allowance
Ending balance: individually evaluated for impairment		0		0
Ending balance: collectively evaluated for impairment		0		0
Loans:
Ending balance: individually evaluated for impairment		0		0
Ending balance: collectively evaluated for impairment		$ 0		$ 0
Net charge-offs as a percentage of average loans in repayment (annualized)		0.00%		0.00%
Allowance as a percentage of the ending total loan balance		0.00%		0.00%
Allowance as a percentage of the ending loans in repayment		0.00%		0.00%
Allowance coverage of net charge-offs (annualized)		0		0
Ending total loans, gross		$ 0		$ 0
Average loans in repayment		0		0
Ending loans in repayment		0		0
Private Education Loan provision for credit losses, other impacts		(51,678)		(2,431)
Personal Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Beginning balance				145,060	145,060
Day 1 adjustment for the adoption of CECL						79,183
Credit Cards
Allowance for Credit Losses
Beginning balance	1,442	1,042	1,501	102	102
Transfer from unfunded commitment liability		0	0	0
Provision for current period	415	391	511	1,191
Loan sale reduction to provision	0	0	0	0
Loan transfer from held-for-sale			0
Total provisions	415	391	511	1,191
Net charge-offs:
Charge-offs	(119)	(48)	(281)	(96)
Recoveries	3	0	10	0
Net charge-offs	(116)	(48)	(271)	(96)
Loan sales			0	0
Ending Balance	1,741	1,385	1,741	1,385	1,501
Allowance
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	1,741	1,385	1,741	1,385
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 17,766	$ 11,540	$ 17,766	$ 11,540
Net charge-offs as a percentage of average loans in repayment (annualized)	3.07%	1.73%	2.75%	1.51%
Allowance as a percentage of the ending total loan balance	9.80%	12.00%	9.80%	12.00%
Allowance as a percentage of the ending loans in repayment	9.80%	12.00%	9.80%	12.00%
Allowance coverage of net charge-offs (annualized)	3.75	7.21	4.82	10.82
Ending total loans, gross	$ 17,766	$ 11,540	$ 17,766	$ 11,540
Average loans in repayment	15,098	11,103	13,136	8,499
Ending loans in repayment	17,766	11,540	17,766	11,540
Private Education Loan provision for credit losses, other impacts	415	$ 391	$ 511	1,191
Credit Cards | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Beginning balance				$ 290	$ 290
Day 1 adjustment for the adoption of CECL						$ 188
Transfer from unfunded commitment liability	$ 0