Allowance for Credit Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
% of Balance
Current period gross charge-offs, total	$ 93,997	$ 96,082
Current period recoveries, total	11,314	11,997
Current period net charge-offs, total	82,683	$ 84,085
Consumer Portfolio Segment | Student Loan
% of Balance
Current period gross charge-offs, Year 1	(7)		$ (1,812)
Current period gross charge-offs, Year 2	(2,683)		(31,032)
Current period gross charge-offs, Year 3	(13,167)		(70,331)
Current period gross charge-offs, Year 4	(14,125)		(49,624)
Current period gross charge-offs, Year 5	(9,561)		(50,585)
Current period gross charge-offs, After Year 5	(54,331)		(216,711)
Current period gross charge-offs, total	93,874		420,095
Current period recoveries, Year 1	0		172
Current period recoveries, Year 2	212		2,342
Current period recoveries, Year 3	1,397		6,496
Current period recoveries, Year 4	1,518		4,923
Current period recoveries, Year 5	1,033		5,260
Current period recoveries, After Year 5	7,154		27,175
Current period recoveries, total	11,314		46,368
Current period net charge-offs, Year 1	(7)		(1,640)
Current period net charge-offs, Year 2	(2,471)		(28,690)
Current period recoveries, Year 3	(11,770)		(63,835)
Current period recoveries, Year 4	(12,607)		(44,701)
Current period recoveries, Year 5	(8,528)		(45,325)
Current period recoveries, After Year 5	(47,177)		(189,536)
Current period net charge-offs, total	82,560		373,727
Total accrued interest by origination vintage, Year 1	15,497		177,959
Total accrued interest by origination vintage, Year 2	281,098		408,800
Total accrued interest by origination vintage, Year 3	365,340		269,978
Total accrued interest by origination vintage, Year 4	240,093		152,094
Total accrued interest by origination vintage, Year 5	138,852		116,618
Total accrued interest by origination vintage, After Year 5	317,107		229,116
Total accrued interest by origination vintage	1,357,987		1,354,565
Consumer Portfolio Segment | Student Loan | With and without cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	926,224		4,490,119
Year 2	5,803,342		5,088,739
Year 3	3,994,361		2,937,422
Year 4	2,454,339		1,819,089
Year 5	1,606,219		1,632,300
After Year 5	6,167,861		5,058,175
Loan type	$ 20,952,346		$ 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	$ 814,592		$ 3,903,676
Year 2	5,130,203		4,428,163
Year 3	3,451,511		2,516,380
Year 4	2,089,098		1,535,308
Year 5	1,347,339		1,378,699
After Year 5	5,440,233		4,529,768
Loan type	$ 18,272,976		$ 18,291,994
% 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	$ 111,632		$ 586,443
Year 2	673,139		660,576
Year 3	542,850		421,042
Year 4	365,241		283,781
Year 5	258,880		253,601
After Year 5	727,628		528,407
Loan type	$ 2,679,370		$ 2,733,850
% 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	$ 926,224		$ 4,490,119
Year 2	5,803,342		5,088,739
Year 3	3,994,361		2,937,422
Year 4	2,454,339		1,819,089
Year 5	1,606,219		1,632,300
After Year 5	6,167,861		5,058,175
Loan type	$ 20,952,346		$ 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	$ 61,789		$ 328,199
Year 2	424,857		395,526
Year 3	316,552		208,696
Year 4	176,921		118,935
Year 5	105,953		137,494
After Year 5	546,958		451,613
Loan type	$ 1,633,030		$ 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	$ 129,467		$ 635,642
Year 2	816,049		704,642
Year 3	553,845		400,744
Year 4	332,430		254,762
Year 5	226,123		257,840
After Year 5	1,041,796		868,777
Loan type	$ 3,099,710		$ 3,122,407
% 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	$ 288,936		$ 1,383,779
Year 2	1,783,230		1,586,783
Year 3	1,243,850		934,033
Year 4	779,798		590,401
Year 5	522,793		545,333
After Year 5	2,082,333		1,709,299
Loan type	$ 6,700,940		$ 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	$ 446,032		$ 2,142,499
Year 2	2,779,206		2,401,788
Year 3	1,880,114		1,393,949
Year 4	1,165,190		854,991
Year 5	751,350		691,633
After Year 5	2,496,774		2,028,486
Loan type	$ 9,518,666		$ 9,513,346
% 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	$ 926,224		$ 4,490,119
Year 2	5,803,342		5,088,739
Year 3	3,994,361		2,937,422
Year 4	2,454,339		1,819,089
Year 5	1,606,219		1,632,300
After Year 5	6,167,861		5,058,175
Loan type	$ 20,952,346		$ 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	$ 84,355		$ 495,451
Year 2	669,427		638,381
Year 3	541,494		379,738
Year 4	335,868		217,956
Year 5	205,407		214,665
After Year 5	951,407		791,875
Loan type	$ 2,787,958		$ 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	$ 129,421		$ 616,684
Year 2	789,950		672,777
Year 3	514,154		365,674
Year 4	299,737		193,462
Year 5	167,203		176,963
After Year 5	671,160		564,245
Loan type	$ 2,571,625		$ 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	$ 286,786		$ 1,347,094
Year 2	1,700,049		1,477,310
Year 3	1,127,465		836,747
Year 4	678,404		498,414
Year 5	430,414		445,244
After Year 5	1,643,152		1,361,073
Loan type	$ 5,866,270		$ 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	$ 425,662		$ 2,030,890
Year 2	2,643,916		2,300,271
Year 3	1,811,248		1,355,263
Year 4	1,140,330		909,257
Year 5	803,195		795,428
After Year 5	2,902,142		2,340,982
Loan type	$ 9,726,493		$ 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	$ 926,224		$ 4,490,119
Year 2	5,803,342		5,088,739
Year 3	3,994,361		2,937,422
Year 4	2,454,339		1,819,089
Year 5	1,606,219		1,632,300
After Year 5	6,167,861		5,058,175
Loan type	$ 20,952,346		$ 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	$ 484,203		$ 2,514,079
Year 2	3,024,087		740,450
Year 3	508,182		440,293
Year 4	323,857		245,631
Year 5	191,566		208,941
After Year 5	433,762		332,608
Loan type	$ 4,965,657		$ 4,482,002
% of Balance
Seasoning based on monthly scheduled payments due from 1-12 payments	24.00%		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	277,441		2,675,956
Year 3	2,095,039		303,045
Year 4	206,882		167,532
Year 5	147,749		165,577
After Year 5	467,866		384,760
Loan type	$ 3,194,977		$ 3,696,870
% of Balance
Seasoning based on monthly scheduled payments due from 13 - 24 payments	15.00%		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	156,087		1,524,834
Year 4	1,301,243		195,091
Year 5	128,581		129,571
After Year 5	559,089		456,448
Loan type	$ 2,145,000		$ 2,305,944
% of Balance
Seasoning based on monthly scheduled payments due from 25 - 36 payments	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
Year 2	0		0
Year 3	0		0
Year 4	80,631		902,938
Year 5	798,248		208,521
After Year 5	529,743		446,350
Loan type	$ 1,408,622		$ 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	71,116		706,097
After Year 5	3,564,277		2,985,015
Loan type	$ 3,635,393		$ 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	$ 442,021		$ 1,976,040
Year 2	2,501,814		1,672,333
Year 3	1,235,053		669,250
Year 4	541,726		307,781
Year 5	268,959		213,593
After Year 5	613,124		452,994
Loan type	$ 5,602,697		$ 5,291,991
% of Balance
Seasoning based on monthly scheduled payments due from not yet in repayment	27.00%		25.00%