Allowance for Credit Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
% of Balance
Current period gross charge-offs, total	$ (111,559)	$ (56,225)	$ (302,624)	$ (161,569)
Current period recoveries, total	11,402	7,305	30,415	21,987
Net charge-offs	(100,157)	$ (48,920)	(272,209)	$ (139,582)
Consumer Portfolio Segment | Student Loan
% of Balance
Current period gross charge-offs			(696)
One year before current period gross charge-offs			(13,397)		$ (1,183)
Two years before current period gross charge-offs			(33,009)		(8,604)
Three years before current period gross charge-offs			(44,122)		(23,866)
Four years before current period gross charge-offs			(42,589)		(32,741)
Five years before current period gross charge-offs			(165,886)		(37,186)
Six years before current period gross charge-offs					(126,011)
Current period gross charge-offs, total			(299,699)		(229,591)
Current period recoveries			40
One year before current period recoveries			928		35
Two years before current period recoveries			2,901		540
Three years before current period recoveries			4,092		2,092
Four years before current period recoveries			4,049		3,693
Five years before current period recoveries			18,400		4,450
Six years before current period recoveries					18,684
Current period recoveries, total			30,410		29,494
Current period net charge-offs			(656)
One year before current period net charge-offs			(12,469)		(1,148)
Two years before current period net charge-offs			(30,108)		(8,064)
Three years before current period net charge-offs			(40,030)		(21,774)
Four years before current period net charge-offs			(38,540)		(29,048)
Five years before current period net charge-offs			(147,486)		(32,736)
Six years before current period net charge-offs					(107,327)
Net charge-offs			(269,289)		(200,097)
Total accrued interest by origination vintage, 2022	72,446		72,446
Total accrued interest by origination vintage, 2021	292,512		292,512		109,233
Total accrued interest by origination vintage, 2020	222,265		222,265		247,418
Total accrued interest by origination vintage, 2019	217,871		217,871		270,242
Total accrued interest by origination vintage, 2018	149,759		149,759		198,816
Total accrued interest by origination vintage, 2017 (and prior for current year)	246,305		246,305		131,685
Total accrued interest by origination vintage, 2016 and prior					229,729
Total accrued interest by origination vintage	1,201,158		1,201,158		1,187,123
Consumer Portfolio Segment | Student Loan | With and without cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	3,538,717		3,538,717
2021	4,613,297		4,613,297		3,822,361
2020	2,665,608		2,665,608		4,166,283
2019	2,237,381		2,237,381		3,216,525
2018	1,667,150		1,667,150		2,320,060
2017 (and Prior for current year)	5,382,310		5,382,310		1,978,233
2016 and Prior					5,213,401
Total	$ 20,104,463		$ 20,104,463		$ 20,716,863
% 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]
2022	$ 3,047,398		$ 3,047,398
2021	3,994,466		3,994,466		$ 3,263,892
2020	2,273,147		2,273,147		3,604,553
2019	1,903,962		1,903,962		2,778,262
2018	1,443,011		1,443,011		2,025,463
2017 (and Prior for current year)	4,871,685		4,871,685		1,765,719
2016 and Prior					4,753,775
Total	$ 17,533,669		$ 17,533,669		$ 18,191,664
% of Balance
Private education loans	87.00%		87.00%		88.00%
Consumer Portfolio Segment | Student Loan | Without cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 491,319		$ 491,319
2021	618,831		618,831		$ 558,469
2020	392,461		392,461		561,730
2019	333,419		333,419		438,263
2018	224,139		224,139		294,597
2017 (and Prior for current year)	510,625		510,625		212,514
2016 and Prior					459,626
Total	$ 2,570,794		$ 2,570,794		$ 2,525,199
% of Balance
Private education loans	13.00%		13.00%		12.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 3,538,717		$ 3,538,717
2021	4,613,297		4,613,297		$ 3,822,361
2020	2,665,608		2,665,608		4,166,283
2019	2,237,381		2,237,381		3,216,525
2018	1,667,150		1,667,150		2,320,060
2017 (and Prior for current year)	5,382,310		5,382,310		1,978,233
2016 and Prior					5,213,401
Total	$ 20,104,463		$ 20,104,463		$ 20,716,863
% 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]
2022	$ 271,510		$ 271,510
2021	308,184		308,184		$ 248,368
2020	161,273		161,273		238,005
2019	180,403		180,403		251,157
2018	146,085		146,085		193,123
2017 (and Prior for current year)	460,074		460,074		166,048
2016 and Prior					428,416
Total	$ 1,527,529		$ 1,527,529		$ 1,525,117
% of Balance
Private education loans at origination	8.00%		8.00%		7.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 490,170		$ 490,170
2021	615,019		615,019		$ 508,264
2020	364,369		364,369		564,497
2019	348,359		348,359		493,237
2018	266,501		266,501		363,313
2017 (and Prior for current year)	922,131		922,131		329,807
2016 and Prior					884,981
Total	$ 3,006,549		$ 3,006,549		$ 3,144,099
% 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]
2022	$ 1,106,786		$ 1,106,786
2021	1,457,396		1,457,396		$ 1,210,833
2020	859,924		859,924		1,348,269
2019	739,722		739,722		1,057,001
2018	554,847		554,847		770,452
2017 (and Prior for current year)	1,813,627		1,813,627		660,270
2016 and Prior					1,753,709
Total	$ 6,532,302		$ 6,532,302		$ 6,800,534
% of Balance
Private education loans at origination	32.00%		32.00%		33.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | Greater than or equal to 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 1,670,251		$ 1,670,251
2021	2,232,698		2,232,698		$ 1,854,896
2020	1,280,042		1,280,042		2,015,512
2019	968,897		968,897		1,415,130
2018	699,717		699,717		993,172
2017 (and Prior for current year)	2,186,478		2,186,478		822,108
2016 and Prior					2,146,295
Total	$ 9,038,083		$ 9,038,083		$ 9,247,113
% 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]
2022	$ 3,538,717		$ 3,538,717
2021	4,613,297		4,613,297		$ 3,822,361
2020	2,665,608		2,665,608		4,166,283
2019	2,237,381		2,237,381		3,216,525
2018	1,667,150		1,667,150		2,320,060
2017 (and Prior for current year)	5,382,310		5,382,310		1,978,233
2016 and Prior					5,213,401
Total	$ 20,104,463		$ 20,104,463		$ 20,716,863
% 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]
2022	$ 330,446		$ 330,446
2021	431,998		431,998		$ 326,613
2020	233,480		233,480		279,578
2019	239,180		239,180		273,652
2018	206,833		206,833		235,684
2017 (and Prior for current year)	811,604		811,604		233,022
2016 and Prior					739,268
Total	$ 2,253,541		$ 2,253,541		$ 2,087,817
% of Balance
Private education loans at origination	11.00%		11.00%		10.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 494,948		$ 494,948
2021	594,535		594,535		$ 506,021
2020	292,414		292,414		475,674
2019	246,158		246,158		365,133
2018	179,979		179,979		256,400
2017 (and Prior for current year)	589,077		589,077		209,536
2016 and Prior					570,605
Total	$ 2,397,111		$ 2,397,111		$ 2,383,369
% 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]
2022	$ 1,092,230		$ 1,092,230
2021	1,402,767		1,402,767		$ 1,209,493
2020	778,630		778,630		1,285,015
2019	649,713		649,713		978,763
2018	466,824		466,824		682,024
2017 (and Prior for current year)	1,458,193		1,458,193		568,766
2016 and Prior					1,448,692
Total	$ 5,848,357		$ 5,848,357		$ 6,172,753
% of Balance
Private education loans at origination	29.00%		29.00%		30.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | Greater than or equal to 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 1,621,093		$ 1,621,093
2021	2,183,997		2,183,997		$ 1,780,234
2020	1,361,084		1,361,084		2,126,016
2019	1,102,330		1,102,330		1,598,977
2018	813,514		813,514		1,145,952
2017 (and Prior for current year)	2,523,436		2,523,436		966,909
2016 and Prior					2,454,836
Total	$ 9,605,454		$ 9,605,454		$ 10,072,924
% of Balance
Private education loans at origination	48.00%		48.00%		48.00%
Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 3,538,717		$ 3,538,717
2021	4,613,297		4,613,297		$ 3,822,361
2020	2,665,608		2,665,608		4,166,283
2019	2,237,381		2,237,381		3,216,525
2018	1,667,150		1,667,150		2,320,060
2017 (and Prior for current year)	5,382,310		5,382,310		1,978,233
2016 and Prior					5,213,401
Total	$ 20,104,463		$ 20,104,463		$ 20,716,863
% 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]
2022	$ 1,961,724		$ 1,961,724
2021	1,921,089		1,921,089		$ 2,265,811
2020	323,754		323,754		594,850
2019	305,985		305,985		515,328
2018	224,662		224,662		385,246
2017 (and Prior for current year)	482,794		482,794		340,242
2016 and Prior					501,269
Total	$ 5,220,008		$ 5,220,008		$ 4,602,746
% of Balance
Seasoning based on monthly scheduled payments due from 1-12 payments	26.00%		26.00%		22.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 0		$ 0
2021	946,963		946,963		$ 0
2020	1,028,413		1,028,413		2,287,737
2019	169,959		169,959		362,674
2018	156,320		156,320		203,674
2017 (and Prior for current year)	489,470		489,470		211,064
2016 and Prior					479,540
Total	$ 2,791,125		$ 2,791,125		$ 3,544,689
% of Balance
Seasoning based on monthly scheduled payments due from 13 - 24 payments	14.00%		14.00%		17.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 25-36 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 0		$ 0
2021	0		0		$ 0
2020	583,318		583,318		173
2019	909,963		909,963		1,565,203
2018	130,692		130,692		312,049
2017 (and Prior for current year)	508,953		508,953		164,575
2016 and Prior					482,369
Total	$ 2,132,926		$ 2,132,926		$ 2,524,369
% of Balance
Seasoning based on monthly scheduled payments due from 25 - 36 payments	11.00%		11.00%		12.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 37-48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 0		$ 0
2021	0		0		$ 0
2020	42		42		0
2019	362,762		362,762		0
2018	639,349		639,349		983,434
2017 (and Prior for current year)	483,212		483,212		295,206
2016 and Prior					464,563
Total	$ 1,485,365		$ 1,485,365		$ 1,743,203
% of Balance
Seasoning based on monthly scheduled payments due from 37 - 48 payments	7.00%		7.00%		8.00%
Consumer Portfolio Segment | Student Loan | Seasoning | More than 48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 0		$ 0
2021	0		0		$ 0
2020	0		0		0
2019	0		0		0
2018	231,269		231,269		0
2017 (and Prior for current year)	2,886,910		2,886,910		671,138
2016 and Prior					2,726,304
Total	$ 3,118,179		$ 3,118,179		$ 3,397,442
% of Balance
Seasoning based on monthly scheduled payments due from more than 48 payments	16.00%		16.00%		16.00%
Consumer Portfolio Segment | Student Loan | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 1,576,993		$ 1,576,993
2021	1,745,245		1,745,245		$ 1,556,550
2020	730,081		730,081		1,283,523
2019	488,712		488,712		773,320
2018	284,858		284,858		435,657
2017 (and Prior for current year)	530,971		530,971		296,008
2016 and Prior					559,356
Total	$ 5,356,860		$ 5,356,860		$ 4,904,414
% of Balance
Seasoning based on monthly scheduled payments due from not yet in repayment	26.00%		26.00%		25.00%