Allowance for Credit Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022
% of Balance
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff	$ 96,082	$ 84,066
Recoveries	11,997	8,036
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff	84,085	$ 76,030
Consumer Portfolio Segment | Student Loan
% of Balance
Current period gross charge-offs, Year 1	0		$ (2,224)
Current period gross charge-offs, Year 2	(2,262)		(25,698)
Current period gross charge-offs, Year 3	(12,072)		(48,271)
Current period gross charge-offs, Year 4	(10,615)		(62,071)
Current period gross charge-offs, Year 5	(11,773)		(57,505)
Current period gross charge-offs, After Year 5	(58,363)		(231,647)
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff	95,085		427,416
Current period recoveries, Year 1	0		124
Current period recoveries, Year 2	207		1,841
Current period recoveries, Year 3	1,428		4,170
Current period recoveries, Year 4	1,167		5,556
Current period recoveries, Year 5	1,460		5,407
Current period recoveries, After Year 5	7,723		24,639
Recoveries	11,985		41,737
Current period net charge-offs, Year 1	0		(2,100)
Current period net charge-offs, Year 2	(2,055)		(23,857)
Current period recoveries, Year 3	(10,644)		(44,101)
Current period recoveries, Year 4	(9,448)		(56,515)
Current period recoveries, Year 5	(10,313)		(52,098)
Current period recoveries, After Year 5	(50,640)		(207,008)
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff	83,100		385,679
Total accrued interest by origination vintage, Year 1	13,564		142,915
Total accrued interest by origination vintage, Year 2	241,786		315,308
Total accrued interest by origination vintage, Year 3	351,527		207,858
Total accrued interest by origination vintage, Year 4	209,889		184,832
Total accrued interest by origination vintage, Year 5	177,587		116,211
Total accrued interest by origination vintage, After Year 5	310,373		210,438
Total accrued interest by origination vintage	1,304,726		1,177,562
Consumer Portfolio Segment | Student Loan | With and without cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	852,405		4,276,533
Year 2	5,688,118		4,547,159
Year 3	4,444,654		2,584,622
Year 4	2,491,251		2,172,660
Year 5	2,087,454		1,615,842
After Year 5	6,334,121		5,106,872
Loan type	$ 21,898,003		$ 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	$ 728,950		$ 3,656,111
Year 2	4,951,808		3,941,921
Year 3	3,858,219		2,208,033
Year 4	2,130,947		1,853,619
Year 5	1,782,069		1,402,828
After Year 5	5,681,899		4,626,491
Loan type	$ 19,133,892		$ 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	$ 123,455		$ 620,422
Year 2	736,310		605,238
Year 3	586,435		376,589
Year 4	360,304		319,041
Year 5	305,385		213,014
After Year 5	652,222		480,381
Loan type	$ 2,764,111		$ 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	$ 852,405		$ 4,276,533
Year 2	5,688,118		4,547,159
Year 3	4,444,654		2,584,622
Year 4	2,491,251		2,172,660
Year 5	2,087,454		1,615,842
After Year 5	6,334,121		5,106,872
Loan type	$ 21,898,003		$ 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	$ 72,432		$ 326,991
Year 2	438,029		307,646
Year 3	304,216		158,606
Year 4	154,665		177,098
Year 5	171,983		143,674
After Year 5	554,184		439,587
Loan type	$ 1,695,509		$ 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	$ 125,846		$ 593,216
Year 2	785,900		611,649
Year 3	602,706		356,541
Year 4	346,297		339,685
Year 5	327,433		259,142
After Year 5	1,077,557		878,426
Loan type	$ 3,265,739		$ 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	$ 270,689		$ 1,336,765
Year 2	1,771,760		1,440,510
Year 3	1,410,157		834,819
Year 4	806,546		719,777
Year 5	692,615		537,680
After Year 5	2,132,757		1,722,068
Loan type	$ 7,084,524		$ 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	$ 383,438		$ 2,019,561
Year 2	2,692,429		2,187,354
Year 3	2,127,575		1,234,656
Year 4	1,183,743		936,100
Year 5	895,423		675,346
After Year 5	2,569,623		2,066,791
Loan type	$ 9,852,231		$ 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	$ 852,405		$ 4,276,533
Year 2	5,688,118		4,547,159
Year 3	4,444,654		2,584,622
Year 4	2,491,251		2,172,660
Year 5	2,087,454		1,615,842
After Year 5	6,334,121		5,106,872
Loan type	$ 21,898,003		$ 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	$ 88,378		$ 443,868
Year 2	659,414		461,589
Year 3	498,641		242,310
Year 4	256,177		237,105
Year 5	249,547		204,894
After Year 5	969,883		773,324
Loan type	$ 2,722,040		$ 2,363,090
% of Balance
Private education loans at origination	12.00%		12.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 128,620		$ 594,118
Year 2	792,523		579,784
Year 3	571,683		284,244
Year 4	277,061		240,999
Year 5	235,805		173,754
After Year 5	706,213		564,344
Loan type	$ 2,711,905		$ 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	$ 267,500		$ 1,322,558
Year 2	1,715,364		1,378,910
Year 3	1,324,781		748,368
Year 4	710,463		628,060
Year 5	591,355		449,701
After Year 5	1,717,774		1,388,090
Loan type	$ 6,327,237		$ 5,915,687
% of Balance
Private education loans at origination	30.00%		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	$ 367,907		$ 1,915,989
Year 2	2,520,817		2,126,876
Year 3	2,049,549		1,309,700
Year 4	1,247,550		1,066,496
Year 5	1,010,747		787,493
After Year 5	2,940,251		2,381,114
Loan type	$ 10,136,821		$ 9,587,668
% of Balance
Private education loans at origination	46.00%		47.00%
Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 852,405		$ 4,276,533
Year 2	5,688,118		4,547,159
Year 3	4,444,654		2,584,622
Year 4	2,491,251		2,172,660
Year 5	2,087,454		1,615,842
After Year 5	6,334,121		5,106,872
Loan type	$ 21,898,003		$ 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	$ 443,563		$ 2,448,884
Year 2	2,988,688		636,073
Year 3	522,305		384,334
Year 4	327,647		330,316
Year 5	271,788		235,878
After Year 5	507,620		424,636
Loan type	$ 5,061,611		$ 4,460,121
% of Balance
Seasoning based on monthly scheduled payments due from 1-12 payments	23.00%		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	324,873		2,477,764
Year 3	2,414,470		255,510
Year 4	201,750		195,753
Year 5	211,025		166,045
After Year 5	586,952		455,782
Loan type	$ 3,739,070		$ 3,550,854
% 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	159,673		1,366,398
Year 4	1,299,993		257,534
Year 5	158,750		126,223
After Year 5	570,770		489,157
Loan type	$ 2,189,186		$ 2,239,312
% 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		127
Year 4	118,240		1,008,418
Year 5	1,023,883		224,805
After Year 5	568,996		451,102
Loan type	$ 1,711,119		$ 1,684,452
% of Balance
Seasoning based on monthly scheduled payments due from 37 - 48 payments	8.00%		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	0		0
Year 5	62,249		643,611
After Year 5	3,448,382		2,830,285
Loan type	$ 3,510,631		$ 3,473,896
% of Balance
Seasoning based on monthly scheduled payments due from more than 48 payments	16.00%		17.00%
Consumer Portfolio Segment | Student Loan | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 408,842		$ 1,827,649
Year 2	2,374,557		1,433,322
Year 3	1,348,206		578,253
Year 4	543,621		380,639
Year 5	359,759		219,280
After Year 5	651,401		455,910
Loan type	$ 5,686,386		$ 4,895,053
% of Balance
Seasoning based on monthly scheduled payments due from not yet in repayment	26.00%		24.00%