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, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
% of Balance
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff	$ 114,134	$ 106,999	$ 210,216	$ 191,065
Recoveries	11,695	10,977	23,692	19,013
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff	102,439	$ 96,022	186,524	$ 172,052
Consumer Portfolio Segment | Student Loan
% of Balance
Current period gross charge-offs, Year 1			(42)		$ (2,224)
Current period gross charge-offs, Year 2			(9,040)		(25,698)
Current period gross charge-offs, Year 3			(35,961)		(48,271)
Current period gross charge-offs, Year 4			(25,581)		(62,071)
Current period gross charge-offs, Year 5			(26,051)		(57,505)
Current period gross charge-offs, After Year 5			(112,960)		(231,647)
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff			209,635		427,416
Current period recoveries, Year 1			0		124
Current period recoveries, Year 2			646		1,841
Current period recoveries, Year 3			3,279		4,170
Current period recoveries, Year 4			2,541		5,556
Current period recoveries, Year 5			2,870		5,407
Current period recoveries, After Year 5			14,356		24,639
Recoveries			23,692		41,737
Current period net charge-offs, Year 1			(42)		(2,100)
Current period net charge-offs, Year 2			(8,394)		(23,857)
Current period recoveries, Year 3			(32,682)		(44,101)
Current period recoveries, Year 4			(23,040)		(56,515)
Current period recoveries, Year 5			(23,181)		(52,098)
Current period recoveries, After Year 5			(98,604)		(207,008)
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss, Writeoff			185,943		385,679
Total accrued interest by origination vintage, Year 1	39,186		39,186		142,915
Total accrued interest by origination vintage, Year 2	320,737		320,737		315,308
Total accrued interest by origination vintage, Year 3	304,663		304,663		207,858
Total accrued interest by origination vintage, Year 4	182,079		182,079		184,832
Total accrued interest by origination vintage, Year 5	153,744		153,744		116,211
Total accrued interest by origination vintage, After Year 5	275,332		275,332		210,438
Total accrued interest by origination vintage	1,275,741		1,275,741		1,177,562
Consumer Portfolio Segment | Student Loan | With and without cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,374,107		1,374,107		4,276,533
Year 2	5,481,115		5,481,115		4,547,159
Year 3	3,455,211		3,455,211		2,584,622
Year 4	2,071,289		2,071,289		2,172,660
Year 5	1,824,409		1,824,409		1,615,842
After Year 5	5,732,232		5,732,232		5,106,872
Loan type	$ 19,938,363		$ 19,938,363		$ 20,303,688
% 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	$ 1,128,289		$ 1,128,289		$ 3,656,111
Year 2	4,760,539		4,760,539		3,941,921
Year 3	2,967,985		2,967,985		2,208,033
Year 4	1,750,797		1,750,797		1,853,619
Year 5	1,542,521		1,542,521		1,402,828
After Year 5	5,134,126		5,134,126		4,626,491
Loan type	$ 17,284,257		$ 17,284,257		$ 17,689,003
% 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	$ 245,818		$ 245,818		$ 620,422
Year 2	720,576		720,576		605,238
Year 3	487,226		487,226		376,589
Year 4	320,492		320,492		319,041
Year 5	281,888		281,888		213,014
After Year 5	598,106		598,106		480,381
Loan type	$ 2,654,106		$ 2,654,106		$ 2,614,685
% 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	$ 1,374,107		$ 1,374,107		$ 4,276,533
Year 2	5,481,115		5,481,115		4,547,159
Year 3	3,455,211		3,455,211		2,584,622
Year 4	2,071,289		2,071,289		2,172,660
Year 5	1,824,409		1,824,409		1,615,842
After Year 5	5,732,232		5,732,232		5,106,872
Loan type	$ 19,938,363		$ 19,938,363		$ 20,303,688
% 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	$ 107,391		$ 107,391		$ 326,991
Year 2	420,793		420,793		307,646
Year 3	240,208		240,208		158,606
Year 4	131,690		131,690		177,098
Year 5	151,540		151,540		143,674
After Year 5	504,422		504,422		439,587
Loan type	$ 1,556,044		$ 1,556,044		$ 1,553,602
% 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	$ 204,117		$ 204,117		$ 593,216
Year 2	755,918		755,918		611,649
Year 3	465,841		465,841		356,541
Year 4	287,047		287,047		339,685
Year 5	285,693		285,693		259,142
After Year 5	977,653		977,653		878,426
Loan type	$ 2,976,269		$ 2,976,269		$ 3,038,659
% 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	$ 438,522		$ 438,522		$ 1,336,765
Year 2	1,706,428		1,706,428		1,440,510
Year 3	1,094,180		1,094,180		834,819
Year 4	669,539		669,539		719,777
Year 5	607,938		607,938		537,680
After Year 5	1,932,404		1,932,404		1,722,068
Loan type	$ 6,449,011		$ 6,449,011		$ 6,591,619
% 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	$ 624,077		$ 624,077		$ 2,019,561
Year 2	2,597,976		2,597,976		2,187,354
Year 3	1,654,982		1,654,982		1,234,656
Year 4	983,013		983,013		936,100
Year 5	779,238		779,238		675,346
After Year 5	2,317,753		2,317,753		2,066,791
Loan type	$ 8,957,039		$ 8,957,039		$ 9,119,808
% 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	$ 1,374,107		$ 1,374,107		$ 4,276,533
Year 2	5,481,115		5,481,115		4,547,159
Year 3	3,455,211		3,455,211		2,584,622
Year 4	2,071,289		2,071,289		2,172,660
Year 5	1,824,409		1,824,409		1,615,842
After Year 5	5,732,232		5,732,232		5,106,872
Loan type	$ 19,938,363		$ 19,938,363		$ 20,303,688
% 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	$ 129,421		$ 129,421		$ 443,868
Year 2	597,699		597,699		461,589
Year 3	383,532		383,532		242,310
Year 4	212,612		212,612		237,105
Year 5	212,598		212,598		204,894
After Year 5	822,750		822,750		773,324
Loan type	$ 2,358,612		$ 2,358,612		$ 2,363,090
% of Balance
Private education loans at origination	12.00%		12.00%		12.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 200,879		$ 200,879		$ 594,118
Year 2	732,796		732,796		579,784
Year 3	421,987		421,987		284,244
Year 4	222,546		222,546		240,999
Year 5	197,829		197,829		173,754
After Year 5	635,210		635,210		564,344
Loan type	$ 2,411,247		$ 2,411,247		$ 2,437,243
% 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	$ 432,512		$ 432,512		$ 1,322,558
Year 2	1,632,162		1,632,162		1,378,910
Year 3	1,017,167		1,017,167		748,368
Year 4	579,151		579,151		628,060
Year 5	509,954		509,954		449,701
After Year 5	1,563,951		1,563,951		1,388,090
Loan type	$ 5,734,897		$ 5,734,897		$ 5,915,687
% of Balance
Private education loans at origination	29.00%		29.00%		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	$ 611,295		$ 611,295		$ 1,915,989
Year 2	2,518,458		2,518,458		2,126,876
Year 3	1,632,525		1,632,525		1,309,700
Year 4	1,056,980		1,056,980		1,066,496
Year 5	904,028		904,028		787,493
After Year 5	2,710,321		2,710,321		2,381,114
Loan type	$ 9,433,607		$ 9,433,607		$ 9,587,668
% 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	$ 1,374,107		$ 1,374,107		$ 4,276,533
Year 2	5,481,115		5,481,115		4,547,159
Year 3	3,455,211		3,455,211		2,584,622
Year 4	2,071,289		2,071,289		2,172,660
Year 5	1,824,409		1,824,409		1,615,842
After Year 5	5,732,232		5,732,232		5,106,872
Loan type	$ 19,938,363		$ 19,938,363		$ 20,303,688
% 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	$ 708,493		$ 708,493		$ 2,448,884
Year 2	2,807,944		2,807,944		636,073
Year 3	444,729		444,729		384,334
Year 4	274,745		274,745		330,316
Year 5	236,836		236,836		235,878
After Year 5	446,584		446,584		424,636
Loan type	$ 4,919,331		$ 4,919,331		$ 4,460,121
% of Balance
Seasoning based on monthly scheduled payments due from 1-12 payments	25.00%		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		$ 0
Year 2	467,523		467,523		2,477,764
Year 3	1,781,078		1,781,078		255,510
Year 4	180,207		180,207		195,753
Year 5	195,356		195,356		166,045
After Year 5	526,126		526,126		455,782
Loan type	$ 3,150,290		$ 3,150,290		$ 3,550,854
% of Balance
Seasoning based on monthly scheduled payments due from 13 - 24 payments	16.00%		16.00%		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	255,314		255,314		1,366,398
Year 4	1,001,721		1,001,721		257,534
Year 5	121,822		121,822		126,223
After Year 5	513,211		513,211		489,157
Loan type	$ 1,892,068		$ 1,892,068		$ 2,239,312
% of Balance
Seasoning based on monthly scheduled payments due from 25 - 36 payments	9.00%		9.00%		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		127
Year 4	186,722		186,722		1,008,418
Year 5	864,585		864,585		224,805
After Year 5	499,560		499,560		451,102
Loan type	$ 1,550,867		$ 1,550,867		$ 1,684,452
% of Balance
Seasoning based on monthly scheduled payments due from 37 - 48 payments	8.00%		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		$ 0
Year 2	0		0		0
Year 3	0		0		0
Year 4	0		0		0
Year 5	112,729		112,729		643,611
After Year 5	3,211,222		3,211,222		2,830,285
Loan type	$ 3,323,951		$ 3,323,951		$ 3,473,896
% of Balance
Seasoning based on monthly scheduled payments due from more than 48 payments	17.00%		17.00%		17.00%
Consumer Portfolio Segment | Student Loan | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	$ 665,614		$ 665,614		$ 1,827,649
Year 2	2,205,648		2,205,648		1,433,322
Year 3	974,090		974,090		578,253
Year 4	427,894		427,894		380,639
Year 5	293,081		293,081		219,280
After Year 5	535,529		535,529		455,910
Loan type	$ 5,101,856		$ 5,101,856		$ 4,895,053
% of Balance
Seasoning based on monthly scheduled payments due from not yet in repayment	25.00%		25.00%		24.00%