Allowance for Credit Losses - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Allowance for Credit Losses
Beginning balance	$ 1,361,723	$ 441,912	$ 441,912
Transfer from unfunded commitment liability		142,075
Provision for current period	(254,999)	169,508
Loan sale reduction to provision	(8,858)	(161,793)
Loan transfer from held-for-sale	1,887
Total provision	261,970	(7,715)
Net charge-offs:
Charge-offs	(55,316)	(70,949)
Recoveries	7,704	9,518
Net charge-offs	(47,612)	(61,431)
Loan sales	0
Ending Balance	1,179,021	1,673,324	1,361,723
Allowance
Ending balance: individually evaluated for impairment	95,536	150,822
Ending balance: collectively evaluated for impairment	1,083,485	1,522,502
Loans:
Ending balance: individually evaluated for impairment	1,225,604	1,518,763
Ending balance: collectively evaluated for impairment	20,255,209	21,781,876
Provisions for loan losses	(261,913)	(17,931)
Provisions for unfunded loan commitments	36,203	53,543
Total Private Education Loan provisions for credit losses	(225,710)	35,612
Private Education Loan provision for credit losses, other impacts	(57)	25,646
Less: provisions for credit losses	(225,767)	61,258
Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Beginning balance		1,584,965	1,584,965
Transfer from unfunded commitment liability	126,880
Net charge-offs:
Day 1 adjustment for the adoption of CECL				$ 1,143,053
FFELP Loans
Allowance for Credit Losses
Beginning balance	4,378	1,633	1,633
Transfer from unfunded commitment liability		0
Provision for current period	29	37
Loan sale reduction to provision	0	0
Loan transfer from held-for-sale	0
Total provision	(29)	(37)
Net charge-offs:
Charge-offs	(89)	(226)
Recoveries	0	0
Net charge-offs	(89)	(226)
Loan sales	0
Ending Balance	4,318	4,296	4,378
Allowance
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	4,318	4,296
Loans:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	$ 727,664	$ 766,954
Net charge-offs as a percentage of average loans in repayment (annualized)	0.06%	0.15%
Allowance as a percentage of the ending total loan balance	0.59%	0.56%
Allowance as a percentage of the ending loans in repayment	0.80%	0.74%
Allowance coverage of net charge-offs (annualized)	12.13	4.75
Ending total loans, gross	$ 727,664	$ 766,954
Average loans in repayment	554,510	600,534
Ending loans in repayment	540,903	581,997
Private Education Loan provision for credit losses, other impacts	29	37
FFELP Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Beginning balance		4,485	4,485
Transfer from unfunded commitment liability	0
Net charge-offs:
Day 1 adjustment for the adoption of CECL				2,852
Private Education Loans
Allowance for Credit Losses
Beginning balance	1,355,844	374,300	374,300
Transfer from unfunded commitment liability		142,075
Provision for current period	(254,942)	143,862
Loan sale reduction to provision	(8,858)	(161,793)
Loan transfer from held-for-sale	1,887
Total provision	261,913	17,931
Net charge-offs:
Charge-offs	(55,139)	(51,469)
Recoveries	7,703	7,976
Net charge-offs	(47,436)	(43,493)
Loan sales	0
Ending Balance	1,173,375	1,515,781	1,355,844
Allowance
Ending balance: individually evaluated for impairment	95,536	150,822
Ending balance: collectively evaluated for impairment	1,077,839	1,364,959
Loans:
Ending balance: individually evaluated for impairment	1,225,604	1,518,763
Ending balance: collectively evaluated for impairment	$ 19,516,236	$ 20,107,984
Net charge-offs as a percentage of average loans in repayment (annualized)	1.29%	1.05%
Allowance as a percentage of the ending total loan balance	5.66%	7.01%
Allowance as a percentage of the ending loans in repayment	7.94%	10.11%
Allowance coverage of net charge-offs (annualized)	6.18	8.71
Ending total loans, gross	$ 20,741,840	$ 21,626,747
Average loans in repayment	14,743,508	16,521,356
Ending loans in repayment	14,777,939	14,988,345
Private Education Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Beginning balance		1,435,130	1,435,130
Transfer from unfunded commitment liability	126,880
Net charge-offs:
Day 1 adjustment for the adoption of CECL				1,060,830
Personal Loans
Allowance for Credit Losses
Beginning balance		65,877	65,877
Transfer from unfunded commitment liability		0
Provision for current period		25,318
Loan sale reduction to provision		0
Total provision		(25,318)
Net charge-offs:
Charge-offs		(19,247)
Recoveries		1,542
Net charge-offs		(17,705)
Ending Balance		152,673
Allowance
Ending balance: individually evaluated for impairment		0
Ending balance: collectively evaluated for impairment		152,673
Loans:
Ending balance: individually evaluated for impairment		0
Ending balance: collectively evaluated for impairment		$ 899,704
Net charge-offs as a percentage of average loans in repayment (annualized)		7.27%
Allowance as a percentage of the ending total loan balance		16.97%
Allowance as a percentage of the ending loans in repayment		16.97%
Allowance coverage of net charge-offs (annualized)		2.16
Ending total loans, gross		$ 899,704
Average loans in repayment		973,772
Ending loans in repayment		899,704
Private Education Loan provision for credit losses, other impacts		25,318
Personal Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Beginning balance		145,060	145,060
Net charge-offs:
Day 1 adjustment for the adoption of CECL				79,183
Credit Cards
Allowance for Credit Losses
Beginning balance	1,501	102	102
Transfer from unfunded commitment liability		0
Provision for current period	(86)	291
Loan sale reduction to provision	0	0
Loan transfer from held-for-sale	0
Total provision	86	(291)
Net charge-offs:
Charge-offs	(88)	(7)
Recoveries	1	0
Net charge-offs	(87)	(7)
Loan sales	0
Ending Balance	1,328	574	1,501
Allowance
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	1,328	574
Loans:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	$ 11,309	$ 7,234
Net charge-offs as a percentage of average loans in repayment (annualized)	2.92%	0.52%
Allowance as a percentage of the ending total loan balance	11.74%	7.93%
Allowance as a percentage of the ending loans in repayment	11.74%	7.93%
Allowance coverage of net charge-offs (annualized)	3.82	20.50
Ending total loans, gross	$ 11,309	$ 7,234
Average loans in repayment	11,909	5,364
Ending loans in repayment	11,309	7,234
Private Education Loan provision for credit losses, other impacts	(86)	291
Credit Cards | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Beginning balance		$ 290	$ 290
Transfer from unfunded commitment liability	$ 0
Net charge-offs:
Day 1 adjustment for the adoption of CECL				$ 188