Allowance for Credit Losses and Unfunded Loan Commitments - Loan Portfolio Stratified by Key Credit Quality Indicators (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
% of Balance
Current period recoveries, total	$ (377,220)	$ (421,096)	$ (431,244)
Current period recoveries, total	44,756	46,368	41,742
Current period net charge-offs, total	(332,464)	(374,728)	(389,502)
Consumer Portfolio Segment | Student Loan
% of Balance
Current period gross charge-offs, Year 1	(1,826)	(1,812)
Current period gross charge-offs, Year 2	(29,094)	(31,032)
Current period gross charge-offs, Year 3	(68,454)	(70,331)
Current period gross charge-offs, Year 4	(53,697)	(49,624)
Current period gross charge-offs, Year 5	(37,318)	(50,585)
Current period gross charge-offs, Year 6	(186,451)	(216,711)
Current period recoveries, total	(376,840)	(420,095)
Current period recoveries, Year 1	117	172
Current period recoveries, Year 2	2,191	2,342
Current period recoveries, Year 3	6,487	6,496
Current period recoveries, Year 4	5,771	4,923
Current period recoveries, Year 5	3,840	5,260
Current period recoveries, Year 6	26,350	27,175
Current period recoveries, total	44,756	46,368
Current period net charge-offs, Year 1	(1,709)	(1,640)
Current period net charge-offs, Year 2	(26,903)	(28,690)
Current period net charge-offs, Year 3	(61,967)	(63,835)
Current period net charge-offs, Year 4	(47,926)	(44,701)
Current period net charge-offs, Year 5	(33,478)	(45,325)
Current period net charge-offs, Year 6	(160,101)	(189,536)
Current period net charge-offs, total	(332,084)	(373,727)
Total accrued interest by origination vintage, Year 1	195,291	177,959
Total accrued interest by origination vintage, Year 2	484,531	408,800
Total accrued interest by origination vintage, Year 3	325,962	269,978
Total accrued interest by origination vintage, Year 4	197,504	152,094
Total accrued interest by origination vintage, Year 5	106,565	116,618
Total accrued interest by origination vintage, Year 6	239,562	229,116
Total accrued interest by origination vintage	1,549,415	1,354,565
Consumer Portfolio Segment | Student Loan | Loan delinquent, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	2,820,940	2,469,817	2,411,441
Year 2	3,312,916	3,254,534	2,991,839
Year 3	2,259,455	2,131,040	1,907,574
Year 4	1,590,812	1,416,069	1,683,986
Year 5	1,107,189	1,323,825	1,301,809
After Year 5	4,422,021	4,213,986	4,262,698
Loan type	15,513,333	14,809,271	14,559,347
Consumer Portfolio Segment | Student Loan | Loan delinquent 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	13,533	17,599	14,164
Year 2	36,441	34,627	30,740
Year 3	45,543	37,147	30,877
Year 4	35,245	28,020	35,213
Year 5	27,302	31,432	31,366
After Year 5	152,684	149,926	144,948
Loan type	310,748	298,751	287,308
Consumer Portfolio Segment | Student Loan | Loan delinquent 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	3,973	5,720	5,523
Year 2	15,239	17,227	15,056
Year 3	23,359	20,077	14,433
Year 4	18,365	16,614	18,201
Year 5	10,921	15,482	16,697
After Year 5	68,878	75,897	77,595
Loan type	140,735	151,017	147,505
Consumer Portfolio Segment | Student Loan | Loan delinquent 90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,683	1,678	1,710
Year 2	14,481	16,939	11,842
Year 3	20,147	21,470	14,872
Year 4	17,080	15,155	16,819
Year 5	12,632	16,150	16,107
After Year 5	75,912	79,383	74,040
Loan type	141,935	150,775	$ 135,390
Consumer Portfolio Segment | Student Loan | With and without cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	5,024,592	4,490,119
Year 2	5,321,510	5,088,739
Year 3	3,185,247	2,937,422
Year 4	2,069,436	1,819,089
Year 5	1,373,996	1,632,300
After Year 5	5,260,227	5,058,175
Loan type	$ 22,235,008	$ 21,025,844
% 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	$ 4,519,952	$ 3,903,676
Year 2	4,707,685	4,428,163
Year 3	2,741,871	2,516,380
Year 4	1,759,261	1,535,308
Year 5	1,151,751	1,378,699
After Year 5	4,642,019	4,529,768
Loan type	$ 19,522,539	$ 18,291,994
% of Balance
Private education loans	88.00%	87.00%
Consumer Portfolio Segment | Student Loan | Without cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 504,640	$ 586,443
Year 2	613,825	660,576
Year 3	443,376	421,042
Year 4	310,175	283,781
Year 5	222,245	253,601
After Year 5	618,208	528,407
Loan type	$ 2,712,469	$ 2,733,850
% of Balance
Private education loans	12.00%	13.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 5,024,592	$ 4,490,119
Year 2	5,321,510	5,088,739
Year 3	3,185,247	2,937,422
Year 4	2,069,436	1,819,089
Year 5	1,373,996	1,632,300
After Year 5	5,260,227	5,058,175
Loan type	$ 22,235,008	$ 21,025,844
% 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	$ 293,025	$ 328,199
Year 2	394,962	395,526
Year 3	261,589	208,696
Year 4	155,661	118,935
Year 5	94,355	137,494
After Year 5	475,186	451,613
Loan type	$ 1,674,778	$ 1,640,463
% 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	$ 615,617	$ 635,642
Year 2	753,548	704,642
Year 3	449,214	400,744
Year 4	285,181	254,762
Year 5	197,205	257,840
After Year 5	898,535	868,777
Loan type	$ 3,199,300	$ 3,122,407
% of Balance
Private Education Loans at origination	14.00%	15.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 1,525,547	$ 1,383,779
Year 2	1,641,641	1,586,783
Year 3	998,834	934,033
Year 4	660,373	590,401
Year 5	451,695	545,333
After Year 5	1,782,121	1,709,299
Loan type	$ 7,060,211	$ 6,749,628
% 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	$ 2,590,403	$ 2,142,499
Year 2	2,531,359	2,401,788
Year 3	1,475,610	1,393,949
Year 4	968,221	854,991
Year 5	630,741	691,633
After Year 5	2,104,385	2,028,486
Loan type	$ 10,300,719	$ 9,513,346
% of Balance
Private Education Loans at origination	46.00%	45.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 5,024,592	$ 4,490,119
Year 2	5,321,510	5,088,739
Year 3	3,185,247	2,937,422
Year 4	2,069,436	1,819,089
Year 5	1,373,996	1,632,300
After Year 5	5,260,227	5,058,175
Loan type	$ 22,235,008	$ 21,025,844
% 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	$ 453,705	$ 495,451
Year 2	666,049	638,381
Year 3	467,562	379,738
Year 4	301,367	217,956
Year 5	194,124	214,665
After Year 5	831,053	791,875
Loan type	$ 2,913,860	$ 2,738,066
% of Balance
Private Education Loans at origination	13.00%	13.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 633,749	$ 616,684
Year 2	710,546	672,777
Year 3	409,808	365,674
Year 4	248,325	193,462
Year 5	138,730	176,963
After Year 5	578,639	564,245
Loan type	$ 2,719,797	$ 2,589,805
% 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	$ 1,485,771	$ 1,347,094
Year 2	1,512,643	1,477,310
Year 3	879,450	836,747
Year 4	563,941	498,414
Year 5	362,715	445,244
After Year 5	1,398,737	1,361,073
Loan type	$ 6,203,257	$ 5,965,882
% of Balance
Private Education Loans at origination	28.00%	28.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | Greater than or equal to 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 2,451,367	$ 2,030,890
Year 2	2,432,272	2,300,271
Year 3	1,428,427	1,355,263
Year 4	955,803	909,257
Year 5	678,427	795,428
After Year 5	2,451,798	2,340,982
Loan type	$ 10,398,094	$ 9,732,091
% 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	$ 5,024,592	$ 4,490,119
Year 2	5,321,510	5,088,739
Year 3	3,185,247	2,937,422
Year 4	2,069,436	1,819,089
Year 5	1,373,996	1,632,300
After Year 5	5,260,227	5,058,175
Loan type	$ 22,235,008	$ 21,025,844
% 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	$ 2,860,113	$ 2,514,079
Year 2	774,471	740,450
Year 3	499,812	440,293
Year 4	280,154	245,631
Year 5	159,762	208,941
After Year 5	324,506	332,608
Loan type	$ 4,898,818	$ 4,482,002
% of Balance
Seasoning based on monthly scheduled payments due from 1-12 payments	22.00%	21.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 0	$ 0
Year 2	2,729,334	2,675,956
Year 3	372,496	303,045
Year 4	191,989	167,532
Year 5	122,938	165,577
After Year 5	340,556	384,760
Loan type	$ 3,757,313	$ 3,696,870
% of Balance
Seasoning based on monthly scheduled payments due from 13 - 24 payments	17.00%	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	1,564,157	1,524,834
Year 4	254,068	195,091
Year 5	110,952	129,571
After Year 5	429,127	456,448
Loan type	$ 2,358,304	$ 2,305,944
% 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	0
Year 4	987,977	902,938
Year 5	170,051	208,521
After Year 5	451,494	446,350
Loan type	$ 1,609,522	$ 1,557,809
% of Balance
Seasoning based on monthly scheduled payments due from 37 - 48 payments	7.00%	7.00%
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	0	116
Year 5	625,916	706,097
After Year 5	3,262,308	2,985,015
Loan type	$ 3,888,224	$ 3,691,228
% of Balance
Seasoning based on monthly scheduled payments due from more than 48 payments	17.00%	18.00%
Consumer Portfolio Segment | Student Loan | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 2,164,479	$ 1,976,040
Year 2	1,817,705	1,672,333
Year 3	748,782	669,250
Year 4	355,248	307,781
Year 5	184,377	213,593
After Year 5	452,236	452,994
Loan type	$ 5,722,827	$ 5,291,991
% of Balance
Seasoning based on monthly scheduled payments due from not yet in repayment	26.00%	25.00%