Allowance for Credit Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
% of Balance
Current period gross charge-offs, total	$ (91,168)	$ (114,134)	$ (185,165)	$ (210,216)
Current period recoveries, total	11,377	11,695	22,691	23,692
Current period net charge-offs, total	(79,791)	$ (102,439)	(162,474)	$ (186,524)
Consumer Portfolio Segment | Student Loan
% of Balance
Current period gross charge-offs, Year 1			(163)		$ (1,812)
Current period gross charge-offs, Year 2			(8,669)		(31,032)
Current period gross charge-offs, Year 3			(32,873)		(70,331)
Current period gross charge-offs, Year 4			(26,921)		(49,624)
Current period gross charge-offs, Year 5			(18,886)		(50,585)
Current period gross charge-offs, After Year 5			(97,404)		(216,711)
Current period gross charge-offs, total			(184,916)		(420,095)
Current period recoveries, Year 1			0		172
Current period recoveries, Year 2			700		2,342
Current period recoveries, Year 3			3,257		6,496
Current period recoveries, Year 4			2,941		4,923
Current period recoveries, Year 5			2,011		5,260
Current period recoveries, After Year 5			13,782		27,175
Current period recoveries, total			22,691		46,368
Current period net charge-offs, Year 1			(163)		(1,640)
Current period net charge-offs, Year 2			(7,969)		(28,690)
Current period recoveries, Year 3			(29,616)		(63,835)
Current period recoveries, Year 4			(23,980)		(44,701)
Current period recoveries, Year 5			(16,875)		(45,325)
Current period recoveries, After Year 5			(83,622)		(189,536)
Current period net charge-offs, total			(162,225)		(373,727)
Total accrued interest by origination vintage, Year 1	43,576		43,576		177,959
Total accrued interest by origination vintage, Year 2	357,479		357,479		408,800
Total accrued interest by origination vintage, Year 3	329,161		329,161		269,978
Total accrued interest by origination vintage, Year 4	221,437		221,437		152,094
Total accrued interest by origination vintage, Year 5	128,373		128,373		116,618
Total accrued interest by origination vintage, After Year 5	287,456		287,456		229,116
Total accrued interest by origination vintage	1,367,482		1,367,482		1,354,565
Consumer Portfolio Segment | Student Loan | With and without cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,480,348		1,480,348		4,490,119
Year 2	5,423,019		5,423,019		5,088,739
Year 3	3,297,769		3,297,769		2,937,422
Year 4	2,177,720		2,177,720		1,819,089
Year 5	1,472,239		1,472,239		1,632,300
After Year 5	5,768,436		5,768,436		5,058,175
Loan type	$ 19,619,531		$ 19,619,531		$ 21,025,844
% of Balance
Total in percent	100.00%		100.00%		100.00%
Consumer Portfolio Segment | Student Loan | With cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 1,263,118		$ 1,263,118		$ 3,903,676
Year 2	4,792,996		4,792,996		4,428,163
Year 3	2,830,170		2,830,170		2,516,380
Year 4	1,844,136		1,844,136		1,535,308
Year 5	1,229,753		1,229,753		1,378,699
After Year 5	5,084,224		5,084,224		4,529,768
Loan type	$ 17,044,397		$ 17,044,397		$ 18,291,994
% of Balance
Private education loans	87.00%		87.00%		87.00%
Consumer Portfolio Segment | Student Loan | Without cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 217,230		$ 217,230		$ 586,443
Year 2	630,023		630,023		660,576
Year 3	467,599		467,599		421,042
Year 4	333,584		333,584		283,781
Year 5	242,486		242,486		253,601
After Year 5	684,212		684,212		528,407
Loan type	$ 2,575,134		$ 2,575,134		$ 2,733,850
% of Balance
Private education loans	13.00%		13.00%		13.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 1,480,348		$ 1,480,348		$ 4,490,119
Year 2	5,423,019		5,423,019		5,088,739
Year 3	3,297,769		3,297,769		2,937,422
Year 4	2,177,720		2,177,720		1,819,089
Year 5	1,472,239		1,472,239		1,632,300
After Year 5	5,768,436		5,768,436		5,058,175
Loan type	$ 19,619,531		$ 19,619,531		$ 21,025,844
% of Balance
Total in percent	100.00%		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	$ 90,651		$ 90,651		$ 328,199
Year 2	396,047		396,047		395,526
Year 3	265,203		265,203		208,696
Year 4	158,763		158,763		118,935
Year 5	99,015		99,015		137,494
After Year 5	515,518		515,518		451,613
Loan type	$ 1,525,197		$ 1,525,197		$ 1,640,463
% of Balance
Private education loans at origination	8.00%		8.00%		8.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 197,505		$ 197,505		$ 635,642
Year 2	757,395		757,395		704,642
Year 3	457,439		457,439		400,744
Year 4	294,690		294,690		254,762
Year 5	209,037		209,037		257,840
After Year 5	979,895		979,895		868,777
Loan type	$ 2,895,961		$ 2,895,961		$ 3,122,407
% of Balance
Private education loans at origination	15.00%		15.00%		15.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 463,674		$ 463,674		$ 1,383,779
Year 2	1,663,571		1,663,571		1,586,783
Year 3	1,030,601		1,030,601		934,033
Year 4	693,019		693,019		590,401
Year 5	480,075		480,075		545,333
After Year 5	1,948,940		1,948,940		1,709,299
Loan type	$ 6,279,880		$ 6,279,880		$ 6,749,628
% of Balance
Private education loans at origination	32.00%		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	$ 728,518		$ 728,518		$ 2,142,499
Year 2	2,606,006		2,606,006		2,401,788
Year 3	1,544,526		1,544,526		1,393,949
Year 4	1,031,248		1,031,248		854,991
Year 5	684,112		684,112		691,633
After Year 5	2,324,083		2,324,083		2,028,486
Loan type	$ 8,918,493		$ 8,918,493		$ 9,513,346
% of Balance
Private education loans at origination	45.00%		45.00%		45.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 1,480,348		$ 1,480,348		$ 4,490,119
Year 2	5,423,019		5,423,019		5,088,739
Year 3	3,297,769		3,297,769		2,937,422
Year 4	2,177,720		2,177,720		1,819,089
Year 5	1,472,239		1,472,239		1,632,300
After Year 5	5,768,436		5,768,436		5,058,175
Loan type	$ 19,619,531		$ 19,619,531		$ 21,025,844
% of Balance
Total in percent	100.00%		100.00%		100.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | Less than 670
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 139,717		$ 139,717		$ 495,451
Year 2	648,278		648,278		638,381
Year 3	471,147		471,147		379,738
Year 4	307,412		307,412		217,956
Year 5	195,258		195,258		214,665
After Year 5	883,767		883,767		791,875
Loan type	$ 2,645,579		$ 2,645,579		$ 2,738,066
% of Balance
Private education loans at origination	13.00%		13.00%		13.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 198,681		$ 198,681		$ 616,684
Year 2	719,888		719,888		672,777
Year 3	412,143		412,143		365,674
Year 4	262,386		262,386		193,462
Year 5	152,440		152,440		176,963
After Year 5	631,641		631,641		564,245
Loan type	$ 2,377,179		$ 2,377,179		$ 2,589,805
% of Balance
Private education loans at origination	12.00%		12.00%		12.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 449,714		$ 449,714		$ 1,347,094
Year 2	1,559,804		1,559,804		1,477,310
Year 3	919,511		919,511		836,747
Year 4	590,474		590,474		498,414
Year 5	390,302		390,302		445,244
After Year 5	1,538,382		1,538,382		1,361,073
Loan type	$ 5,448,187		$ 5,448,187		$ 5,965,882
% of Balance
Private education loans at origination	28.00%		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	$ 692,236		$ 692,236		$ 2,030,890
Year 2	2,495,049		2,495,049		2,300,271
Year 3	1,494,968		1,494,968		1,355,263
Year 4	1,017,448		1,017,448		909,257
Year 5	734,239		734,239		795,428
After Year 5	2,714,646		2,714,646		2,340,982
Loan type	$ 9,148,586		$ 9,148,586		$ 9,732,091
% of Balance
Private education loans at origination	47.00%		47.00%		47.00%
Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 1,480,348		$ 1,480,348		$ 4,490,119
Year 2	5,423,019		5,423,019		5,088,739
Year 3	3,297,769		3,297,769		2,937,422
Year 4	2,177,720		2,177,720		1,819,089
Year 5	1,472,239		1,472,239		1,632,300
After Year 5	5,768,436		5,768,436		5,058,175
Loan type	$ 19,619,531		$ 19,619,531		$ 21,025,844
% of Balance
Total in percent	100.00%		100.00%		100.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 1-12 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 772,152		$ 772,152		$ 2,514,079
Year 2	2,750,486		2,750,486		740,450
Year 3	448,319		448,319		440,293
Year 4	286,029		286,029		245,631
Year 5	173,738		173,738		208,941
After Year 5	411,072		411,072		332,608
Loan type	$ 4,841,796		$ 4,841,796		$ 4,482,002
% of Balance
Seasoning based on monthly scheduled payments due from 1-12 payments	25.00%		25.00%		21.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 0		$ 0		$ 0
Year 2	418,106		418,106		2,675,956
Year 3	1,635,135		1,635,135		303,045
Year 4	190,611		190,611		167,532
Year 5	135,833		135,833		165,577
After Year 5	412,987		412,987		384,760
Loan type	$ 2,792,672		$ 2,792,672		$ 3,696,870
% of Balance
Seasoning based on monthly scheduled payments due from 13 - 24 payments	14.00%		14.00%		18.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 25-36 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 0		$ 0		$ 0
Year 2	0		0		0
Year 3	258,592		258,592		1,524,834
Year 4	1,094,789		1,094,789		195,091
Year 5	116,401		116,401		129,571
After Year 5	528,339		528,339		456,448
Loan type	$ 1,998,121		$ 1,998,121		$ 2,305,944
% of Balance
Seasoning based on monthly scheduled payments due from 25 - 36 payments	10.00%		10.00%		11.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 37-48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 0		$ 0		$ 0
Year 2	0		0		0
Year 3	0		0		0
Year 4	153,321		153,321		902,938
Year 5	687,962		687,962		208,521
After Year 5	472,389		472,389		446,350
Loan type	$ 1,313,672		$ 1,313,672		$ 1,557,809
% of Balance
Seasoning based on monthly scheduled payments due from 37 - 48 payments	7.00%		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		$ 0
Year 2	0		0		0
Year 3	0		0		0
Year 4	0		0		116
Year 5	126,750		126,750		706,097
After Year 5	3,417,762		3,417,762		2,985,015
Loan type	$ 3,544,512		$ 3,544,512		$ 3,691,228
% of Balance
Seasoning based on monthly scheduled payments due from more than 48 payments	18.00%		18.00%		18.00%
Consumer Portfolio Segment | Student Loan | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 708,196		$ 708,196		$ 1,976,040
Year 2	2,254,427		2,254,427		1,672,333
Year 3	955,723		955,723		669,250
Year 4	452,970		452,970		307,781
Year 5	231,555		231,555		213,593
After Year 5	525,887		525,887		452,994
Loan type	$ 5,128,758		$ 5,128,758		$ 5,291,991
% of Balance
Seasoning based on monthly scheduled payments due from not yet in repayment	26.00%		26.00%		25.00%