Allowance for Credit Losses - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Allowance for Credit Losses
Beginning Balance	$ 1,165,335	$ 1,361,723	$ 441,912
Transfer from unfunded commitment liability	344,310	301,655	320,808
Provision for current period	413,535	(232,708)	190,898
Loan sale reduction to provision	(174,231)	(66,460)	(204,709)
Loans transferred to held-for-sale	2,372	1,887	205,669
Total provisions	236,932	(297,281)	(219,480)
Net charge-offs
Charge-offs	(431,244)	(230,268)	(245,043)
Recoveries	41,742	29,506	29,007
Net charge-offs	(389,502)	(200,762)	(216,036)
Loan sales			(108,534)
Ending Balance	1,357,075	1,165,335	1,361,723
Allowance [Abstract]
Ending balance: individually evaluated for impairment		47,712	104,265
Ending balance: collectively evaluated for impairment	1,357,075	1,117,623	1,257,458
Loans [Abstract]
Ending balance: individually evaluated for impairment		1,057,665	1,274,590
Ending balance: collectively evaluated for impairment	20,912,738	20,379,428	19,204,578
Accrued interest to be capitalized
Ending balance: individually evaluated for impairment		0	0
Ending balance: collectively evaluated for impairment	936,837	947,391	973,201
Private Education Loan provision for credit losses, other impacts	909	1,144	(691)
Provisions for credit losses reported in consolidated statements of income	633,453	(32,957)	93,133
Adjustments
Allowance for Credit Losses
Beginning Balance		1,143,053
Net charge-offs
Ending Balance			1,143,053
Remaining Adjusted Transition Amounts to be Phased-In
Allowance for Credit Losses
Beginning Balance			1,584,965
FFELP Loans
Allowance for Credit Losses
Beginning Balance	4,077	4,378	1,633
Transfer from unfunded commitment liability	0	0	0
Provision for current period	(20)	20	412
Loan sale reduction to provision	0	0	0
Loans transferred to held-for-sale	0	0	0
Total provisions	(20)	20	412
Net charge-offs
Charge-offs	(613)	(321)	(519)
Recoveries	0	0	0
Net charge-offs	(613)	(321)	(519)
Loan sales			0
Ending Balance	3,444	4,077	4,378
Allowance [Abstract]
Ending balance: individually evaluated for impairment		0	0
Ending balance: collectively evaluated for impairment	3,444	4,077	4,378
Loans [Abstract]
Ending balance: individually evaluated for impairment		0	0
Ending balance: collectively evaluated for impairment	609,050	695,216	737,593
Accrued interest to be capitalized
Ending balance: individually evaluated for impairment		0	0
Ending balance: collectively evaluated for impairment	$ 0	$ 0	$ 0
Net charge-offs as a percentage of average loans in repayment (annualized)	0.12%	0.06%	0.09%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized	0.57%	0.59%	0.59%
Allowance as a percentage of the ending loans in repayment	0.76%	0.74%	0.76%
Allowance coverage of net charge-offs	5.62	12.70	8.44
Ending total loans, gross	$ 609,050	$ 695,216	$ 737,593
Average loans in repayment	517,139	545,689	549,584
Ending loans in repayment	453,915	553,980	573,361
Accrued interest to be capitalized on loans in repayment	0	0	0
Private Education Loan provision for credit losses, other impacts	(20)	20	412
FFELP Loans | Adjustments
Allowance for Credit Losses
Beginning Balance		2,852
Net charge-offs
Ending Balance			2,852
FFELP Loans | Remaining Adjusted Transition Amounts to be Phased-In
Allowance for Credit Losses
Beginning Balance			4,485
Private Education Loans
Allowance for Credit Losses
Beginning Balance	1,158,977	1,355,844	374,300
Transfer from unfunded commitment liability	344,310	301,655	320,808
Provision for current period	410,254	(233,852)	148,673
Loan sale reduction to provision	(174,231)	(66,460)	(161,793)
Loans transferred to held-for-sale	0	1,887	205,669
Total provisions	236,023	(298,425)	(218,789)
Net charge-offs
Charge-offs	(427,416)	(229,591)	(205,326)
Recoveries	41,737	29,494	24,021
Net charge-offs	(385,679)	(200,097)	(181,305)
Loan sales			0
Ending Balance	1,353,631	1,158,977	1,355,844
Allowance [Abstract]
Ending balance: individually evaluated for impairment		47,712	104,265
Ending balance: collectively evaluated for impairment	1,353,631	1,111,265	1,251,579
Loans [Abstract]
Ending balance: individually evaluated for impairment		1,057,665	1,274,590
Ending balance: collectively evaluated for impairment	20,303,688	19,659,198	18,454,747
Accrued interest to be capitalized
Ending balance: individually evaluated for impairment		0	0
Ending balance: collectively evaluated for impairment	$ 936,837	$ 947,391	$ 973,201
Net charge-offs as a percentage of average loans in repayment (annualized)	2.55%	1.33%	1.17%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized	6.37%	5.35%	6.55%
Allowance as a percentage of the ending loans in repayment	8.76%	7.32%	9.28%
Allowance coverage of net charge-offs	3.51	5.79	7.48
Ending total loans, gross	$ 20,303,688	$ 20,716,863	$ 19,729,337
Average loans in repayment	15,103,123	15,019,869	15,518,851
Ending loans in repayment	15,129,550	15,511,212	14,304,821
Accrued interest to be capitalized on loans in repayment	324,384	312,537	308,655
Provisions for loan losses	236,023	(298,425)	(218,789)
Provisions for unfunded loan commitments	396,521	264,324	312,613
Total Private Education Loan provisions for credit losses	632,544	(34,101)	93,824
Private Education Loans | Adjustments
Allowance for Credit Losses
Beginning Balance		1,060,830
Net charge-offs
Ending Balance			1,060,830
Private Education Loans | Remaining Adjusted Transition Amounts to be Phased-In
Allowance for Credit Losses
Beginning Balance			1,435,130
Personal Loans
Allowance for Credit Losses
Beginning Balance		0	65,877
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)
Net charge-offs
Charge-offs			(39,079)
Recoveries			4,984
Net charge-offs			(34,095)
Loan sales			(108,534)
Ending Balance			0
Allowance [Abstract]
Ending balance: individually evaluated for impairment			0
Ending balance: collectively evaluated for impairment			0
Loans [Abstract]
Ending balance: individually evaluated for impairment			0
Ending balance: collectively evaluated for impairment			0
Accrued interest to be capitalized
Ending balance: individually evaluated for impairment			0
Ending balance: collectively evaluated for impairment			$ 0
Net charge-offs as a percentage of average loans in repayment (annualized)			0.00%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized			0.00%
Allowance as a percentage of the ending loans in repayment			0.00%
Allowance coverage of net charge-offs			0
Ending total loans, gross			$ 0
Average loans in repayment			0
Ending loans in repayment			0
Accrued interest to be capitalized on loans in repayment			0
Private Education Loan provision for credit losses, other impacts			(2,431)
Personal Loans | Adjustments
Allowance for Credit Losses
Beginning Balance		79,183
Net charge-offs
Ending Balance			79,183
Personal Loans | Remaining Adjusted Transition Amounts to be Phased-In
Allowance for Credit Losses
Beginning Balance			145,060
Credit Cards
Allowance for Credit Losses
Beginning Balance	2,281	1,501	102
Transfer from unfunded commitment liability	0	0	0
Provision for current period	3,301	1,124	1,328
Loan sale reduction to provision	0	0	0
Loans transferred to held-for-sale	2,372	0	0
Total provisions	929	1,124	1,328
Net charge-offs
Charge-offs	(3,215)	(356)	(119)
Recoveries	5	12	2
Net charge-offs	(3,210)	(344)	(117)
Loan sales			0
Ending Balance	0	2,281	1,501
Allowance [Abstract]
Ending balance: individually evaluated for impairment		0	0
Ending balance: collectively evaluated for impairment	0	2,281	1,501
Loans [Abstract]
Ending balance: individually evaluated for impairment		0	0
Ending balance: collectively evaluated for impairment	0	25,014	12,238
Accrued interest to be capitalized
Ending balance: individually evaluated for impairment		0	0
Ending balance: collectively evaluated for impairment	$ 0	$ 0	$ 0
Net charge-offs as a percentage of average loans in repayment (annualized)	0.00%	2.24%	1.26%
Allowance as a percentage of the ending total loan balance and accrued interest to be capitalized	0.00%	9.12%	12.27%
Allowance as a percentage of the ending loans in repayment	0.00%	9.12%	12.27%
Allowance coverage of net charge-offs	0	6.63	12.83
Ending total loans, gross	$ 0	$ 25,014	$ 12,238
Average loans in repayment	0	15,343	9,286
Ending loans in repayment	0	25,014	12,238
Accrued interest to be capitalized on loans in repayment	0	0	0
Private Education Loan provision for credit losses, other impacts	$ 929	1,124	1,328
Credit Cards | Adjustments
Allowance for Credit Losses
Beginning Balance		$ 188
Net charge-offs
Ending Balance			188
Credit Cards | Remaining Adjusted Transition Amounts to be Phased-In
Allowance for Credit Losses
Beginning Balance			$ 290