Allowance for Credit Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
% of Balance
Current period recoveries, total	$ (421,096)	$ (431,244)	$ (230,268)
Current period recoveries, total	46,368	41,742	29,506
Current period net charge-offs, total	(374,728)	(389,502)	(200,762)
Consumer Portfolio Segment | Student Loan
% of Balance
Current period gross charge-offs, Year 1	(1,812)	(2,224)
Current period gross charge-offs, Year 2	(31,032)	(25,698)
Current period gross charge-offs, Year 3	(70,331)	(48,271)
Current period gross charge-offs, Year 4	(49,624)	(62,071)
Current period gross charge-offs, Year 5	(50,585)	(57,505)
Current period gross charge-offs, Year 6	(216,711)	(231,647)
Current period recoveries, total	(420,095)	(427,416)
Current period recoveries, Year 1	172	124
Current period recoveries, Year 2	2,342	1,841
Current period recoveries, Year 3	6,496	4,170
Current period recoveries, Year 4	4,923	5,556
Current period recoveries, Year 5	5,260	5,407
Current period recoveries, Year 6	27,175	24,639
Current period recoveries, total	46,368	41,737
Current period net charge-offs, Year 1	(1,640)	(2,100)
Current period net charge-offs, Year 2	(28,690)	(23,857)
Current period net charge-offs, Year 3	(63,835)	(44,101)
Current period net charge-offs, Year 4	(44,701)	(56,515)
Current period net charge-offs, Year 5	(45,325)	(52,098)
Current period net charge-offs, Year 6	(189,536)	(207,008)
Current period net charge-offs, total	(373,727)	(385,679)
Total accrued interest by origination vintage, Year 1	177,959	142,915
Total accrued interest by origination vintage, Year 2	408,800	315,308
Total accrued interest by origination vintage, Year 3	269,978	207,858
Total accrued interest by origination vintage, Year 4	152,094	184,832
Total accrued interest by origination vintage, Year 5	116,618	116,211
Total accrued interest by origination vintage, Year 6	229,116	210,438
Total accrued interest by origination vintage	1,354,565	1,177,562
Consumer Portfolio Segment | Student Loan | Loan delinquent, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	2,469,817	2,411,441	2,234,876
Year 2	3,254,534	2,991,839	2,786,646
Year 3	2,131,040	1,907,574	2,321,728
Year 4	1,416,069	1,683,986	1,772,651
Year 5	1,323,825	1,301,809	1,570,815
After Year 5	4,213,986	4,262,698	4,319,057
Loan type	14,809,271	14,559,347	15,005,773
Consumer Portfolio Segment | Student Loan | Loan delinquent 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	17,599	14,164	15,148
Year 2	34,627	30,740	29,146
Year 3	37,147	30,877	46,616
Year 4	28,020	35,213	43,197
Year 5	31,432	31,366	41,695
After Year 5	149,926	144,948	132,757
Loan type	298,751	287,308	308,559
Consumer Portfolio Segment | Student Loan | Loan delinquent 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	5,720	5,523	3,194
Year 2	17,227	15,056	7,441
Year 3	20,077	14,433	14,044
Year 4	16,614	18,201	14,310
Year 5	15,482	16,697	16,425
After Year 5	75,897	77,595	61,533
Loan type	151,017	147,505	116,947
Consumer Portfolio Segment | Student Loan | Loan delinquent 90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,678	1,710	642
Year 2	16,939	11,842	3,683
Year 3	21,470	14,872	8,453
Year 4	15,155	16,819	10,632
Year 5	16,150	16,107	11,935
After Year 5	79,383	74,040	44,588
Loan type	150,775	135,390	$ 79,933
Consumer Portfolio Segment | Student Loan | With and without cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	4,490,119	4,276,533
Year 2	5,088,739	4,547,159
Year 3	2,937,422	2,584,622
Year 4	1,819,089	2,172,660
Year 5	1,632,300	1,615,842
After Year 5	5,058,175	5,106,872
Loan type	$ 21,025,844	$ 20,303,688
% of Balance
Total in percent	100.00%	100.00%
Consumer Portfolio Segment | Student Loan | With cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 3,903,676	$ 3,656,111
Year 2	4,428,163	3,941,921
Year 3	2,516,380	2,208,033
Year 4	1,535,308	1,853,619
Year 5	1,378,699	1,402,828
After Year 5	4,529,768	4,626,491
Loan type	$ 18,291,994	$ 17,689,003
% of Balance
Private education loans	87.00%	87.00%
Consumer Portfolio Segment | Student Loan | Without cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 586,443	$ 620,422
Year 2	660,576	605,238
Year 3	421,042	376,589
Year 4	283,781	319,041
Year 5	253,601	213,014
After Year 5	528,407	480,381
Loan type	$ 2,733,850	$ 2,614,685
% of Balance
Private education loans	13.00%	13.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 4,490,119	$ 4,276,533
Year 2	5,088,739	4,547,159
Year 3	2,937,422	2,584,622
Year 4	1,819,089	2,172,660
Year 5	1,632,300	1,615,842
After Year 5	5,058,175	5,106,872
Loan type	$ 21,025,844	$ 20,303,688
% of Balance
Total in percent	100.00%	100.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | Less than 670
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 328,199	$ 326,991
Year 2	395,526	307,646
Year 3	208,696	158,606
Year 4	118,935	177,098
Year 5	137,494	143,674
After Year 5	451,613	439,587
Loan type	$ 1,640,463	$ 1,553,602
% of Balance
Private Education Loans at origination	8.00%	8.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 635,642	$ 593,216
Year 2	704,642	611,649
Year 3	400,744	356,541
Year 4	254,762	339,685
Year 5	257,840	259,142
After Year 5	868,777	878,426
Loan type	$ 3,122,407	$ 3,038,659
% of Balance
Private Education Loans at origination	15.00%	15.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 1,383,779	$ 1,336,765
Year 2	1,586,783	1,440,510
Year 3	934,033	834,819
Year 4	590,401	719,777
Year 5	545,333	537,680
After Year 5	1,709,299	1,722,068
Loan type	$ 6,749,628	$ 6,591,619
% of Balance
Private Education Loans at origination	32.00%	32.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | Greater than or equal to 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 2,142,499	$ 2,019,561
Year 2	2,401,788	2,187,354
Year 3	1,393,949	1,234,656
Year 4	854,991	936,100
Year 5	691,633	675,346
After Year 5	2,028,486	2,066,791
Loan type	$ 9,513,346	$ 9,119,808
% of Balance
Private Education Loans at origination	45.00%	45.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 4,490,119	$ 4,276,533
Year 2	5,088,739	4,547,159
Year 3	2,937,422	2,584,622
Year 4	1,819,089	2,172,660
Year 5	1,632,300	1,615,842
After Year 5	5,058,175	5,106,872
Loan type	$ 21,025,844	$ 20,303,688
% of Balance
Total in percent	100.00%	100.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | Less than 670
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 495,451	$ 443,868
Year 2	638,381	461,589
Year 3	379,738	242,310
Year 4	217,956	237,105
Year 5	214,665	204,894
After Year 5	791,875	773,324
Loan type	$ 2,738,066	$ 2,363,090
% of Balance
Private Education Loans at origination	13.00%	12.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 616,684	$ 594,118
Year 2	672,777	579,784
Year 3	365,674	284,244
Year 4	193,462	240,999
Year 5	176,963	173,754
After Year 5	564,245	564,344
Loan type	$ 2,589,805	$ 2,437,243
% of Balance
Private Education Loans at origination	12.00%	12.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 1,347,094	$ 1,322,558
Year 2	1,477,310	1,378,910
Year 3	836,747	748,368
Year 4	498,414	628,060
Year 5	445,244	449,701
After Year 5	1,361,073	1,388,090
Loan type	$ 5,965,882	$ 5,915,687
% of Balance
Private Education Loans at origination	28.00%	29.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | Greater than or equal to 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 2,030,890	$ 1,915,989
Year 2	2,300,271	2,126,876
Year 3	1,355,263	1,309,700
Year 4	909,257	1,066,496
Year 5	795,428	787,493
After Year 5	2,340,982	2,381,114
Loan type	$ 9,732,091	$ 9,587,668
% of Balance
Private Education Loans at origination	47.00%	47.00%
Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 4,490,119	$ 4,276,533
Year 2	5,088,739	4,547,159
Year 3	2,937,422	2,584,622
Year 4	1,819,089	2,172,660
Year 5	1,632,300	1,615,842
After Year 5	5,058,175	5,106,872
Loan type	$ 21,025,844	$ 20,303,688
% of Balance
Total in percent	100.00%	100.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 1-12 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 2,514,079	$ 2,448,884
Year 2	740,450	636,073
Year 3	440,293	384,334
Year 4	245,631	330,316
Year 5	208,941	235,878
After Year 5	332,608	424,636
Loan type	$ 4,482,002	$ 4,460,121
% of Balance
Seasoning based on monthly scheduled payments due from 1-12 payments	21.00%	22.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 0	$ 0
Year 2	2,675,956	2,477,764
Year 3	303,045	255,510
Year 4	167,532	195,753
Year 5	165,577	166,045
After Year 5	384,760	455,782
Loan type	$ 3,696,870	$ 3,550,854
% of Balance
Seasoning based on monthly scheduled payments due from 13 - 24 payments	18.00%	18.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 25-36 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 0	$ 0
Year 2	0	0
Year 3	1,524,834	1,366,398
Year 4	195,091	257,534
Year 5	129,571	126,223
After Year 5	456,448	489,157
Loan type	$ 2,305,944	$ 2,239,312
% of Balance
Seasoning based on monthly scheduled payments due from 25 - 36 payments	11.00%	11.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 37-48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 0	$ 0
Year 2	0	0
Year 3	0	127
Year 4	902,938	1,008,418
Year 5	208,521	224,805
After Year 5	446,350	451,102
Loan type	$ 1,557,809	$ 1,684,452
% of Balance
Seasoning based on monthly scheduled payments due from 37 - 48 payments	7.00%	8.00%
Consumer Portfolio Segment | Student Loan | Seasoning | More than 48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 0	$ 0
Year 2	0	0
Year 3	0	0
Year 4	116	0
Year 5	706,097	643,611
After Year 5	2,985,015	2,830,285
Loan type	$ 3,691,228	$ 3,473,896
% of Balance
Seasoning based on monthly scheduled payments due from more than 48 payments	18.00%	17.00%
Consumer Portfolio Segment | Student Loan | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 1,976,040	$ 1,827,649
Year 2	1,672,333	1,433,322
Year 3	669,250	578,253
Year 4	307,781	380,639
Year 5	213,593	219,280
After Year 5	452,994	455,910
Loan type	$ 5,291,991	$ 4,895,053
% of Balance
Seasoning based on monthly scheduled payments due from not yet in repayment	25.00%	24.00%