Allowance for Credit Losses and Unfunded Loan Commitments - Loan Portfolio Stratified by Key Credit Quality Indicators (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024
% of Balance
Current period gross charge-offs, total		$ (93,997)
Current period recoveries, total		11,314
Current period net charge-offs, total		$ (82,683)
Consumer Portfolio Segment | Student Loan
% of Balance
Current period gross charge-offs, Year 1	$ (30)		$ (1,826)
Current period gross charge-offs, Year 2	(2,204)		(29,094)
Current period gross charge-offs, Year 3	(11,879)		(68,454)
Current period gross charge-offs, Year 4	(14,517)		(53,697)
Current period gross charge-offs, Year 5	(11,192)		(37,318)
Current period gross charge-offs, After Year 5	(47,081)		(186,451)
Current period gross charge-offs, total	(86,903)		(376,840)
Current period recoveries, Year 1	0		117
Current period recoveries, Year 2	167		2,191
Current period recoveries, Year 3	1,045		6,487
Current period recoveries, Year 4	1,515		5,771
Current period recoveries, Year 5	1,135		3,840
Current period recoveries, After Year 5	6,872		26,350
Current period recoveries, total	10,734		44,756
Current period net charge-offs, Year 1	(30)		(1,709)
Current period net charge-offs, Year 2	(2,037)		(26,903)
Current period recoveries, Year 3	(10,834)		(61,967)
Current period recoveries, Year 4	(13,002)		(47,926)
Current period recoveries, Year 5	(10,057)		(33,478)
Current period recoveries, After Year 5	(40,209)		(160,101)
Current period net charge-offs, total	(76,169)		(332,084)
Total accrued interest by origination vintage, Year 1	14,681		195,291
Total accrued interest by origination vintage, Year 2	304,006		484,531
Total accrued interest by origination vintage, Year 3	427,589		325,962
Total accrued interest by origination vintage, Year 4	300,194		197,504
Total accrued interest by origination vintage, Year 5	186,056		106,565
Total accrued interest by origination vintage, After Year 5	325,939		239,562
Total accrued interest by origination vintage	1,558,465		1,549,415
Consumer Portfolio Segment | Student Loan | With and without cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	942,378		5,024,592
Year 2	6,461,366		5,321,510
Year 3	4,141,150		3,185,247
Year 4	2,784,645		2,069,436
Year 5	1,890,758		1,373,996
After Year 5	6,211,828		5,260,227
Loan type	$ 22,432,125		$ 22,235,008
% 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	$ 859,190		$ 4,519,952
Year 2	5,895,269		4,707,685
Year 3	3,629,844		2,741,871
Year 4	2,386,878		1,759,261
Year 5	1,604,095		1,151,751
After Year 5	5,421,272		4,642,019
Loan type	$ 19,796,548		$ 19,522,539
% of Balance
Private education loans	88.00%		88.00%
Consumer Portfolio Segment | Student Loan | Without cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 83,188		$ 504,640
Year 2	566,097		613,825
Year 3	511,306		443,376
Year 4	397,767		310,175
Year 5	286,663		222,245
After Year 5	790,556		618,208
Loan type	$ 2,635,577		$ 2,712,469
% of Balance
Private education loans	12.00%		12.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 942,378		$ 5,024,592
Year 2	6,461,366		5,321,510
Year 3	4,141,150		3,185,247
Year 4	2,784,645		2,069,436
Year 5	1,890,758		1,373,996
After Year 5	6,211,828		5,260,227
Loan type	$ 22,432,125		$ 22,235,008
% 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	$ 51,495		$ 293,025
Year 2	379,320		394,962
Year 3	310,698		261,589
Year 4	231,818		155,661
Year 5	144,364		94,355
After Year 5	537,354		475,186
Loan type	$ 1,655,049		$ 1,674,778
% of Balance
Private education loans at origination	7.00%		8.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 124,044		$ 615,617
Year 2	783,165		753,548
Year 3	590,368		449,214
Year 4	394,851		285,181
Year 5	260,696		197,205
After Year 5	1,029,177		898,535
Loan type	$ 3,182,301		$ 3,199,300
% of Balance
Private education loans at origination	14.00%		14.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 294,174		$ 1,525,547
Year 2	1,955,019		1,641,641
Year 3	1,278,201		998,834
Year 4	872,195		660,373
Year 5	603,290		451,695
After Year 5	2,093,039		1,782,121
Loan type	$ 7,095,918		$ 7,060,211
% 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	$ 472,665		$ 2,590,403
Year 2	3,343,862		2,531,359
Year 3	1,961,883		1,475,610
Year 4	1,285,781		968,221
Year 5	882,408		630,741
After Year 5	2,552,258		2,104,385
Loan type	$ 10,498,857		$ 10,300,719
% of Balance
Private education loans at origination	47.00%		46.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 942,378		$ 5,024,592
Year 2	6,461,366		5,321,510
Year 3	4,141,150		3,185,247
Year 4	2,784,645		2,069,436
Year 5	1,890,758		1,373,996
After Year 5	6,211,828		5,260,227
Loan type	$ 22,432,125		$ 22,235,008
% 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	$ 78,272		$ 453,705
Year 2	616,695		666,049
Year 3	578,313		467,562
Year 4	449,560		301,367
Year 5	302,294		194,124
After Year 5	1,037,950		831,053
Loan type	$ 3,063,084		$ 2,913,860
% of Balance
Private education loans at origination	14.00%		13.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 126,040		$ 633,749
Year 2	800,321		710,546
Year 3	530,393		409,808
Year 4	344,692		248,325
Year 5	222,205		138,730
After Year 5	664,969		578,639
Loan type	$ 2,688,620		$ 2,719,797
% 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	$ 286,159		$ 1,485,771
Year 2	1,863,056		1,512,643
Year 3	1,148,737		879,450
Year 4	742,163		563,941
Year 5	494,666		362,715
After Year 5	1,590,227		1,398,737
Loan type	$ 6,125,008		$ 6,203,257
% of Balance
Private education loans at origination	27.00%		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	$ 451,907		$ 2,451,367
Year 2	3,181,294		2,432,272
Year 3	1,883,707		1,428,427
Year 4	1,248,230		955,803
Year 5	871,593		678,427
After Year 5	2,918,682		2,451,798
Loan type	$ 10,555,413		$ 10,398,094
% 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	$ 942,378		$ 5,024,592
Year 2	6,461,366		5,321,510
Year 3	4,141,150		3,185,247
Year 4	2,784,645		2,069,436
Year 5	1,890,758		1,373,996
After Year 5	6,211,828		5,260,227
Loan type	$ 22,432,125		$ 22,235,008
% 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	$ 506,319		$ 2,860,113
Year 2	3,438,840		774,471
Year 3	544,214		499,812
Year 4	398,355		280,154
Year 5	227,135		159,762
After Year 5	389,019		324,506
Loan type	$ 5,503,882		$ 4,898,818
% of Balance
Seasoning based on monthly scheduled payments due from 1-12 payments	25.00%		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	295,985		2,729,334
Year 3	2,093,179		372,496
Year 4	263,144		191,989
Year 5	169,556		122,938
After Year 5	406,222		340,556
Loan type	$ 3,228,086		$ 3,757,313
% of Balance
Seasoning based on monthly scheduled payments due from 13 - 24 payments	14.00%		17.00%
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	161,894		1,564,157
Year 4	1,384,100		254,068
Year 5	172,968		110,952
After Year 5	483,553		429,127
Loan type	$ 2,202,515		$ 2,358,304
% 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	103,094		987,977
Year 5	947,463		170,051
After Year 5	547,483		451,494
Loan type	$ 1,598,040		$ 1,609,522
% 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		0
Year 5	53,621		625,916
After Year 5	3,782,254		3,262,308
Loan type	$ 3,835,875		$ 3,888,224
% of Balance
Seasoning based on monthly scheduled payments due from more than 48 payments	17.00%		17.00%
Consumer Portfolio Segment | Student Loan | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 436,059		$ 2,164,479
Year 2	2,726,541		1,817,705
Year 3	1,341,863		748,782
Year 4	635,952		355,248
Year 5	320,015		184,377
After Year 5	603,297		452,236
Loan type	$ 6,063,727		$ 5,722,827
% of Balance
Seasoning based on monthly scheduled payments due from not yet in repayment	27.00%		26.00%