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, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current period gross charge-offs, total	$ (106,999)	$ (50,028)	$ (191,065)	$ (105,344)
Current period recoveries, total	10,977	6,978	19,013	14,682
Net charge-offs	(96,022)	$ (43,050)	(172,052)	$ (90,662)
Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current period gross charge-offs			(79)
One year before current period gross charge-offs			(6,193)		$ (1,183)
Two years before current period gross charge-offs			(20,703)		(8,604)
Three years before current period gross charge-offs			(28,869)		(23,866)
Four years before current period gross charge-offs			(27,542)		(32,741)
Five years before current period gross charge-offs			(106,963)		(37,186)
Six years before current period gross charge-offs					(126,011)
Current period gross charge-offs, total			(190,349)		(229,591)
Current period recoveries			0
One year before current period recoveries			448		35
Two years before current period recoveries			1,765		540
Three years before current period recoveries			2,630		2,092
Four years before current period recoveries			2,501		3,693
Five years before current period recoveries			11,666		4,450
Six years before current period recoveries					18,684
Current period recoveries, total			19,010		29,494
Current period net charge-offs			(79)
One year before current period net charge-offs			(5,745)		(1,148)
Two years before current period net charge-offs			(18,938)		(8,064)
Three years before current period net charge-offs			(26,239)		(21,774)
Four years before current period net charge-offs			(25,041)		(29,048)
Five years before current period net charge-offs			(95,297)		(32,736)
Six years before current period net charge-offs					(107,327)
Net charge-offs			(171,339)		(200,097)
Total accrued interest by origination vintage, 2022	28,511		28,511
Total accrued interest by origination vintage, 2021	238,678		238,678		109,233
Total accrued interest by origination vintage, 2020	235,273		235,273		247,418
Total accrued interest by origination vintage, 2019	234,761		234,761		270,242
Total accrued interest by origination vintage, 2018	163,681		163,681		198,816
Total accrued interest by origination vintage, 2017 and prior	271,006		271,006		131,685
Total accrued interest by origination vintage, 2016 and prior					229,729
Total accrued interest by origination vintage	1,171,910		1,171,910		1,187,123
Consumer Portfolio Segment | Student Loan | With and without cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	1,279,752		1,279,752
2021	4,854,861		4,854,861		3,822,361
2020	3,120,690		3,120,690		4,166,283
2019	2,513,490		2,513,490		3,216,525
2018	1,848,355		1,848,355		2,320,060
2017 and Prior	5,902,037		5,902,037		1,978,233
2016 and Prior					5,213,401
Total	$ 19,519,185		$ 19,519,185		$ 20,716,863
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	$ 1,036,651		$ 1,036,651
2021	4,200,604		4,200,604		$ 3,263,892
2020	2,677,507		2,677,507		3,604,553
2019	2,150,355		2,150,355		2,778,262
2018	1,603,266		1,603,266		2,025,463
2017 and Prior	5,343,522		5,343,522		1,765,719
2016 and Prior					4,753,775
Total	$ 17,011,905		$ 17,011,905		$ 18,191,664
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	$ 243,101		$ 243,101
2021	654,257		654,257		$ 558,469
2020	443,183		443,183		561,730
2019	363,135		363,135		438,263
2018	245,089		245,089		294,597
2017 and Prior	558,515		558,515		212,514
2016 and Prior					459,626
Total	$ 2,507,280		$ 2,507,280		$ 2,525,199
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	$ 1,279,752		$ 1,279,752
2021	4,854,861		4,854,861		$ 3,822,361
2020	3,120,690		3,120,690		4,166,283
2019	2,513,490		2,513,490		3,216,525
2018	1,848,355		1,848,355		2,320,060
2017 and Prior	5,902,037		5,902,037		1,978,233
2016 and Prior					5,213,401
Total	$ 19,519,185		$ 19,519,185		$ 20,716,863
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	$ 94,943		$ 94,943
2021	321,567		321,567		$ 248,368
2020	186,085		186,085		238,005
2019	201,734		201,734		251,157
2018	160,071		160,071		193,123
2017 and Prior	499,905		499,905		166,048
2016 and Prior					428,416
Total	$ 1,464,305		$ 1,464,305		$ 1,525,117
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	$ 182,941		$ 182,941
2021	644,166		644,166		$ 508,264
2020	426,415		426,415		564,497
2019	389,318		389,318		493,237
2018	295,350		295,350		363,313
2017 and Prior	1,008,825		1,008,825		329,807
2016 and Prior					884,981
Total	$ 2,947,015		$ 2,947,015		$ 3,144,099
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	$ 412,341		$ 412,341
2021	1,533,720		1,533,720		$ 1,210,833
2020	1,007,488		1,007,488		1,348,269
2019	830,148		830,148		1,057,001
2018	614,831		614,831		770,452
2017 and Prior	1,985,185		1,985,185		660,270
2016 and Prior					1,753,709
Total	$ 6,383,713		$ 6,383,713		$ 6,800,534
Private education loans at origination	33.00%		33.00%		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	$ 589,527		$ 589,527
2021	2,355,408		2,355,408		$ 1,854,896
2020	1,500,702		1,500,702		2,015,512
2019	1,092,290		1,092,290		1,415,130
2018	778,103		778,103		993,172
2017 and Prior	2,408,122		2,408,122		822,108
2016 and Prior					2,146,295
Total	$ 8,724,152		$ 8,724,152		$ 9,247,113
Private education loans at origination	44.00%		44.00%		45.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 1,279,752		$ 1,279,752
2021	4,854,861		4,854,861		$ 3,822,361
2020	3,120,690		3,120,690		4,166,283
2019	2,513,490		2,513,490		3,216,525
2018	1,848,355		1,848,355		2,320,060
2017 and Prior	5,902,037		5,902,037		1,978,233
2016 and Prior					5,213,401
Total	$ 19,519,185		$ 19,519,185		$ 20,716,863
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	$ 122,222		$ 122,222
2021	459,158		459,158		$ 326,613
2020	261,366		261,366		279,578
2019	257,888		257,888		273,652
2018	219,132		219,132		235,684
2017 and Prior	858,301		858,301		233,022
2016 and Prior					739,268
Total	$ 2,178,067		$ 2,178,067		$ 2,087,817
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	$ 183,128		$ 183,128
2021	620,098		620,098		$ 506,021
2020	340,183		340,183		475,674
2019	276,534		276,534		365,133
2018	198,033		198,033		256,400
2017 and Prior	646,324		646,324		209,536
2016 and Prior					570,605
Total	$ 2,264,300		$ 2,264,300		$ 2,383,369
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	$ 403,721		$ 403,721
2021	1,483,958		1,483,958		$ 1,209,493
2020	923,047		923,047		1,285,015
2019	736,502		736,502		978,763
2018	527,323		527,323		682,024
2017 and Prior	1,615,314		1,615,314		568,766
2016 and Prior					1,448,692
Total	$ 5,689,865		$ 5,689,865		$ 6,172,753
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	$ 570,681		$ 570,681
2021	2,291,647		2,291,647		$ 1,780,234
2020	1,596,094		1,596,094		2,126,016
2019	1,242,566		1,242,566		1,598,977
2018	903,867		903,867		1,145,952
2017 and Prior	2,782,098		2,782,098		966,909
2016 and Prior					2,454,836
Total	$ 9,386,953		$ 9,386,953		$ 10,072,924
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	$ 1,279,752		$ 1,279,752
2021	4,854,861		4,854,861		$ 3,822,361
2020	3,120,690		3,120,690		4,166,283
2019	2,513,490		2,513,490		3,216,525
2018	1,848,355		1,848,355		2,320,060
2017 and Prior	5,902,037		5,902,037		1,978,233
2016 and Prior					5,213,401
Total	$ 19,519,185		$ 19,519,185		$ 20,716,863
Seasoning based on monthly scheduled payments due from 1-12 payments	25.00%		25.00%		22.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments	15.00%		15.00%		17.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments	11.00%		11.00%		12.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments	8.00%		8.00%		8.00%
Seasoning based on monthly scheduled payments due from more than 48 payments	16.00%		16.00%		16.00%
Seasoning based on monthly scheduled payments due from not yet in repayment	25.00%		25.00%		25.00%
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	$ 685,445		$ 685,445
2021	2,532,502		2,532,502		$ 2,265,811
2020	391,306		391,306		594,850
2019	378,038		378,038		515,328
2018	284,209		284,209		385,246
2017 and Prior	617,732		617,732		340,242
2016 and Prior					501,269
Total	4,889,232		4,889,232		4,602,746
Consumer Portfolio Segment | Student Loan | Seasoning | 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	0		0
2021	434,578		434,578		0
2020	1,564,224		1,564,224		2,287,737
2019	179,111		179,111		362,674
2018	163,997		163,997		203,674
2017 and Prior	550,106		550,106		211,064
2016 and Prior					479,540
Total	2,892,016		2,892,016		3,544,689
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	286,503		286,503		173
2019	1,234,821		1,234,821		1,565,203
2018	144,585		144,585		312,049
2017 and Prior	555,690		555,690		164,575
2016 and Prior					482,369
Total	2,221,599		2,221,599		2,524,369
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	0		0		0
2019	163,552		163,552		0
2018	836,832		836,832		983,434
2017 and Prior	533,301		533,301		295,206
2016 and Prior					464,563
Total	1,533,685		1,533,685		1,743,203
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	102,734		102,734		0
2017 and Prior	3,069,896		3,069,896		671,138
2016 and Prior					2,726,304
Total	3,172,630		3,172,630		3,397,442
Consumer Portfolio Segment | Student Loan | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	594,307		594,307
2021	1,887,781		1,887,781		1,556,550
2020	878,657		878,657		1,283,523
2019	557,968		557,968		773,320
2018	315,998		315,998		435,657
2017 and Prior	575,312		575,312		296,008
2016 and Prior					559,356
Total	$ 4,810,023		$ 4,810,023		$ 4,904,414