Allowance for Credit Losses - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Allowance for Credit Losses
Beginning balance				$ 1,361,723				$ 441,912	$ 1,361,723	$ 441,912	$ 341,121
Transfer from unfunded commitment liability									301,655	320,808
Provision for current period									(232,708)	190,898
Loan sale reduction to provision									(66,460)	(204,709)
Loans transferred to held-for-sale									1,887	(205,669)
Total provisions									(297,281)	(219,480)	353,934
Net charge-offs:
Charge-offs									(230,268)	(245,043)	(284,114)
Recoveries									29,506	29,007	30,971
Net charge-offs									(200,762)	(216,036)	(253,143)
Loan sales										(108,534)
Ending balance	$ 1,165,335				$ 1,361,723				1,165,335	1,361,723	441,912
Allowance:
Ending balance: individually evaluated for impairment	47,712				104,265				47,712	104,265	186,697
Ending balance: collectively evaluated for impairment	1,117,623				1,257,458				1,117,623	1,257,458	255,215
Loans:
Ending balance: individually evaluated for impairment	1,057,665				1,274,590				1,057,665	1,274,590	1,581,966
Ending balance: collectively evaluated for impairment	20,379,428				19,204,578				20,379,428	19,204,578	23,443,822
Provisions for unfunded loan commitments									232,822	311,659	5,937
Less: provisions for credit losses	(15,309)	$ 138,442	$ 69,677	(225,767)	(316,372)	$ (3,640)	$ 351,887	61,258	(32,957)	93,133	354,249
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
Net charge-offs:
Ending balance											1,584,965
Private Education Loans
Allowance for Credit Losses
Beginning balance				1,355,844				374,300	1,355,844	374,300	277,943
Transfer from unfunded commitment liability									301,655	320,808
Provision for current period									(233,852)	148,673
Loan sale reduction to provision									(66,460)	(161,793)
Loans transferred to held-for-sale									1,887	(205,669)
Total provisions									(298,425)	(218,789)	279,570
Net charge-offs:
Charge-offs									(229,591)	(205,326)	(208,978)
Recoveries									29,494	24,021	25,765
Net charge-offs									(200,097)	(181,305)	(183,213)
Loan sales										0
Ending balance	1,158,977				1,355,844				1,158,977	1,355,844	374,300
Allowance:
Ending balance: individually evaluated for impairment	47,712				104,265				47,712	104,265	186,697
Ending balance: collectively evaluated for impairment	1,111,265				1,251,579				1,111,265	1,251,579	187,603
Loans:
Ending balance: individually evaluated for impairment	1,057,665				1,274,590				1,057,665	1,274,590	1,581,966
Ending balance: collectively evaluated for impairment	19,659,198				18,454,747				$ 19,659,198	$ 18,454,747	$ 21,607,625
Net charge-offs as a percentage of average loans in repayment (annualized)									1.33%	1.17%	1.17%
Allowance as a percentage of the ending total loan balance									5.59%	6.87%	1.61%
Allowance as a percentage of the ending loans in repayment									7.47%	9.48%	2.23%
Allowance coverage of net charge-offs (annualized)									5.79	7.48	2.04
Ending total loans, gross	20,716,863				19,729,337				$ 20,716,863	$ 19,729,337	$ 23,189,591
Average loans in repayment									15,019,869	15,518,851	15,605,927
Ending loans in repayment	15,511,212				14,304,821				15,511,212	14,304,821	16,787,670
Provisions for unfunded loan commitments									264,324	312,613
Less: provisions for credit losses									(34,101)	93,824
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
Net charge-offs:
Ending balance											1,435,130
Other Industries, Excluding Private Education Loans
Allowance for Credit Losses
Total provisions									1,144	(691)
FFELP Loans
Allowance for Credit Losses
Beginning balance				4,378				1,633	4,378	1,633	977
Transfer from unfunded commitment liability									0	0
Provision for current period									20	412
Loan sale reduction to provision									0	0
Loans transferred to held-for-sale									0	0
Total provisions									20	412	1,478
Net charge-offs:
Charge-offs									(321)	(519)	(822)
Recoveries									0	0	0
Net charge-offs									(321)	(519)	(822)
Loan sales										0
Ending balance	4,077				4,378				4,077	4,378	1,633
Allowance:
Ending balance: individually evaluated for impairment	0				0				0	0	0
Ending balance: collectively evaluated for impairment	4,077				4,378				4,077	4,378	1,633
Loans:
Ending balance: individually evaluated for impairment	0				0				0	0	0
Ending balance: collectively evaluated for impairment	695,216				737,593				$ 695,216	$ 737,593	$ 783,306
Net charge-offs as a percentage of average loans in repayment (annualized)									0.06%	0.09%	0.13%
Allowance as a percentage of the ending total loan balance									0.59%	0.59%	0.21%
Allowance as a percentage of the ending loans in repayment									0.74%	0.76%	0.26%
Allowance coverage of net charge-offs (annualized)									12.70	8.44	1.99
Ending total loans, gross	695,216				737,593				$ 695,216	$ 737,593	$ 783,306
Average loans in repayment									545,689	549,584	631,029
Ending loans in repayment	553,980				573,361				553,980	573,361	617,646
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
Net charge-offs:
Ending balance											4,485
Credit Cards
Allowance for Credit Losses
Beginning balance				1,501				102	1,501	102	0
Transfer from unfunded commitment liability									0	0
Provision for current period									1,124	1,328
Loan sale reduction to provision									0	0
Loans transferred to held-for-sale									0	0
Total provisions									1,124	1,328	103
Net charge-offs:
Charge-offs									(356)	(119)	(1)
Recoveries									12	2	0
Net charge-offs									(344)	(117)	(1)
Loan sales										0
Ending balance	2,281				1,501				2,281	1,501	102
Allowance:
Ending balance: individually evaluated for impairment	0				0				0	0	0
Ending balance: collectively evaluated for impairment	2,281				1,501				2,281	1,501	102
Loans:
Ending balance: individually evaluated for impairment	0				0				0	0	0
Ending balance: collectively evaluated for impairment	25,014				12,238				$ 25,014	$ 12,238	$ 3,884
Net charge-offs as a percentage of average loans in repayment (annualized)									2.24%	1.26%	0.13%
Allowance as a percentage of the ending total loan balance									9.12%	12.27%	2.63%
Allowance as a percentage of the ending loans in repayment									9.12%	12.27%	2.63%
Allowance coverage of net charge-offs (annualized)									6.63	12.83	102.00
Ending total loans, gross	25,014				12,238				$ 25,014	$ 12,238	$ 3,884
Average loans in repayment									15,343	9,286	786
Ending loans in repayment	$ 25,014				12,238				25,014	12,238	3,884
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
Net charge-offs:
Ending balance											290
Personal Loans
Allowance for Credit Losses
Beginning balance				$ 0				65,877	$ 0	65,877	62,201
Transfer from unfunded commitment liability										0
Provision for current period										40,485
Loan sale reduction to provision										(42,916)
Loans transferred to held-for-sale										0
Total provisions										(2,431)	72,783
Net charge-offs:
Charge-offs										(39,079)	(74,313)
Recoveries										4,984	5,206
Net charge-offs										(34,095)	(69,107)
Loan sales										(108,534)
Ending balance					0					0	65,877
Allowance:
Ending balance: individually evaluated for impairment					0					0	0
Ending balance: collectively evaluated for impairment					0					0	65,877
Loans:
Ending balance: individually evaluated for impairment					0					0	0
Ending balance: collectively evaluated for impairment					0					$ 0	$ 1,049,007
Net charge-offs as a percentage of average loans in repayment (annualized)										0.00%	6.07%
Allowance as a percentage of the ending total loan balance										0.00%	6.28%
Allowance as a percentage of the ending loans in repayment										0.00%	6.28%
Allowance coverage of net charge-offs (annualized)										0	0.95
Ending total loans, gross					0					$ 0	$ 1,049,007
Average loans in repayment										0	1,138,887
Ending loans in repayment					$ 0					0	1,049,007
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
Net charge-offs:
Ending balance											$ 145,060