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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current period gross charge-offs, total	$ (56,225)	$ (60,666)	$ (161,569)	$ (178,120)
Current period recoveries, total	7,305	6,896	21,987	23,282
Net charge-offs	(48,920)	$ (53,770)	(139,582)	$ (154,838)
Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current period gross charge-offs	(372)
One year before current period gross charge-offs	(4,159)				$ (1,087)
Two years before current period gross charge-offs	(14,794)				(10,940)
Three years before current period gross charge-offs	(23,210)				(27,000)
Four years before current period gross charge-offs	(26,696)				(35,851)
Five years before current period gross charge-offs	(91,808)				(36,416)
Six years before current period gross charge-offs					(94,032)
Current period gross charge-offs, total	(161,039)				(205,326)
Current period recoveries	14
One year before current period recoveries	254				42
Two years before current period recoveries	1,398				636
Three years before current period recoveries	2,679				2,274
Four years before current period recoveries	3,288				3,585
Five years before current period recoveries	14,344				4,284
Six years before current period recoveries					13,200
Current period recoveries, total	21,977				24,021
Current period net charge-offs	(358)
One year before current period net charge-offs	(3,905)				(1,045)
Two years before current period net charge-offs	(13,396)				(10,304)
Three years before current period net charge-offs	(20,531)				(24,726)
Four years before current period net charge-offs	(23,408)				(32,266)
Five years before current period net charge-offs	(77,464)				(32,132)
Six years before current period net charge-offs					(80,832)
Net charge-offs	(139,062)				(181,305)
Total accrued interest by origination vintage, 2021	58,245		58,245
Total accrued interest by origination vintage, 2020	250,412		250,412		90,438
Total accrued interest by origination vintage, 2019	331,670		331,670		265,688
Total accrued interest by origination vintage, 2018	259,988		259,988		252,251
Total accrued interest by origination vintage, 2017	192,350		192,350		209,178
Total accrued interest by origination vintage, 2016 and prior	293,762		293,762		141,094
Total accrued interest by origination vintage, 2015 and prior					210,247
Total accrued interest by origination vintage	1,386,427		1,386,427		1,168,896
Consumer Portfolio Segment | Student Loan | With cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	2,740,609		2,740,609
2020	3,859,697		3,859,697		2,915,328
2019	3,131,422		3,131,422		3,467,219
2018	2,239,265		2,239,265		2,556,400
2017	1,938,613		1,938,613		2,262,635
2016 and prior	5,201,099		5,201,099		1,977,952
2015 and Prior					4,198,748
Total	$ 19,110,705		$ 19,110,705		$ 17,378,282
Private education loans	88.00%		88.00%		88.00%
Consumer Portfolio Segment | Student Loan | Without cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	$ 449,218		$ 449,218
2020	601,172		601,172		$ 527,437
2019	481,391		481,391		559,629
2018	322,867		322,867		384,111
2017	233,512		233,512		277,159
2016 and prior	503,972		503,972		211,270
2015 and Prior					391,449
Total	$ 2,592,132		$ 2,592,132		$ 2,351,055
Private education loans	12.00%		12.00%		12.00%
Consumer Portfolio Segment | Student Loan | Cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	$ 3,189,827		$ 3,189,827
2020	4,460,869		4,460,869		$ 3,442,765
2019	3,612,813		3,612,813		4,026,848
2018	2,562,132		2,562,132		2,940,511
2017	2,172,125		2,172,125		2,539,794
2016 and prior	5,705,071		5,705,071		2,189,222
2015 and Prior					4,590,197
Total	$ 21,702,837		$ 21,702,837		$ 19,729,337
Total in percent	100.00%		100.00%		100.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	$ 3,189,827		$ 3,189,827
2020	4,460,869		4,460,869		$ 3,442,765
2019	3,612,813		3,612,813		4,026,848
2018	2,562,132		2,562,132		2,940,511
2017	2,172,125		2,172,125		2,539,794
2016 and prior	5,705,071		5,705,071		2,189,222
2015 and Prior					4,590,197
Total	$ 21,702,837		$ 21,702,837		$ 19,729,337
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]
2021	$ 206,260		$ 206,260
2020	250,867		250,867		$ 195,214
2019	275,722		275,722		290,711
2018	207,725		207,725		225,276
2017	177,237		177,237		197,948
2016 and prior	461,829		461,829		162,413
2015 and Prior					369,609
Total	$ 1,579,640		$ 1,579,640		$ 1,441,171
Private education loans at origination	7.00%		7.00%		7.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	$ 423,163		$ 423,163
2020	598,066		598,066		$ 464,785
2019	548,962		548,962		594,950
2018	396,081		396,081		441,357
2017	358,145		358,145		407,394
2016 and prior	961,855		961,855		351,303
2015 and Prior					771,477
Total	$ 3,286,272		$ 3,286,272		$ 3,031,266
Private education loans at origination	15.00%		15.00%		16.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	$ 1,010,429		$ 1,010,429
2020	1,439,849		1,439,849		$ 1,111,373
2019	1,184,965		1,184,965		1,310,390
2018	848,391		848,391		967,802
2017	725,425		725,425		846,983
2016 and prior	1,917,552		1,917,552		740,028
2015 and Prior					1,533,517
Total	$ 7,126,611		$ 7,126,611		$ 6,510,093
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]
2021	$ 1,549,975		$ 1,549,975
2020	2,172,087		2,172,087		$ 1,671,393
2019	1,603,164		1,603,164		1,830,797
2018	1,109,935		1,109,935		1,306,076
2017	911,318		911,318		1,087,469
2016 and prior	2,363,835		2,363,835		935,478
2015 and Prior					1,915,594
Total	$ 9,710,314		$ 9,710,314		$ 8,746,807
Private education loans at origination	45.00%		45.00%		44.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	$ 3,189,827		$ 3,189,827
2020	4,460,869		4,460,869		$ 3,442,765
2019	3,612,813		3,612,813		4,026,848
2018	2,562,132		2,562,132		2,940,511
2017	2,172,125		2,172,125		2,539,794
2016 and prior	5,705,071		5,705,071		2,189,222
2015 and Prior					4,590,197
Total	$ 21,702,837		$ 21,702,837		$ 19,729,337
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]
2021	$ 262,997		$ 262,997
2020	271,778		271,778		$ 240,154
2019	282,320		282,320		331,229
2018	247,343		247,343		301,784
2017	249,302		249,302		298,195
2016 and prior	795,264		795,264		293,077
2015 and Prior					734,599
Total	$ 2,109,004		$ 2,109,004		$ 2,199,038
Private education loans at origination	10.00%		10.00%		11.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	$ 425,798		$ 425,798
2020	510,906		510,906		$ 438,665
2019	411,306		411,306		493,135
2018	278,497		278,497		336,966
2017	229,881		229,881		283,906
2016 and prior	622,290		622,290		231,759
2015 and Prior					504,779
Total	$ 2,478,678		$ 2,478,678		$ 2,289,210
Private education loans at origination	11.00%		11.00%		12.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	$ 1,003,524		$ 1,003,524
2020	1,393,710		1,393,710		$ 1,102,666
2019	1,117,756		1,117,756		1,248,806
2018	759,346		759,346		871,677
2017	624,787		624,787		734,222
2016 and prior	1,584,356		1,584,356		603,160
2015 and Prior					1,220,468
Total	$ 6,483,479		$ 6,483,479		$ 5,780,999
Private education loans at origination	30.00%		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]
2021	$ 1,497,508		$ 1,497,508
2020	2,284,475		2,284,475		$ 1,661,280
2019	1,801,431		1,801,431		1,953,678
2018	1,276,946		1,276,946		1,430,084
2017	1,068,155		1,068,155		1,223,471
2016 and prior	2,703,161		2,703,161		1,061,226
2015 and Prior					2,130,351
Total	$ 10,631,676		$ 10,631,676		$ 9,460,090
Private education loans at origination	49.00%		49.00%		48.00%
Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	$ 3,189,827		$ 3,189,827
2020	4,460,869		4,460,869		$ 3,442,765
2019	3,612,813		3,612,813		4,026,848
2018	2,562,132		2,562,132		2,940,511
2017	2,172,125		2,172,125		2,539,794
2016 and prior	5,705,071		5,705,071		2,189,222
2015 and Prior					4,590,197
Total	$ 21,702,837		$ 21,702,837		$ 19,729,337
Seasoning based on monthly scheduled payments due from 1-12 payments	24.00%		24.00%		23.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments	16.00%		16.00%		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%		9.00%
Seasoning based on monthly scheduled payments due from more than 48 payments	14.00%		14.00%		15.00%
Seasoning based on monthly scheduled payments due from not yet in repayment	27.00%		27.00%		24.00%
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]
2021	$ 1,837,837		$ 1,837,837
2020	1,772,385		1,772,385		$ 2,068,517
2019	397,777		397,777		600,038
2018	329,370		329,370		469,143
2017	311,956		311,956		472,258
2016 and prior	543,901		543,901		381,197
2015 and Prior					507,343
Total	5,193,226		5,193,226		4,498,496
Consumer Portfolio Segment | Student Loan | Seasoning | 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	0		0
2020	1,027,243		1,027,243		163
2019	1,487,280		1,487,280		2,096,635
2018	221,972		221,972		383,977
2017	220,104		220,104		223,332
2016 and prior	532,905		532,905		217,379
2015 and Prior					425,345
Total	3,489,504		3,489,504		3,346,831
Consumer Portfolio Segment | Student Loan | Seasoning | 25-36 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	0		0
2020	42		42		0
2019	619,601		619,601		0
2018	996,813		996,813		1,353,567
2017	189,051		189,051		370,250
2016 and prior	527,481		527,481		181,940
2015 and Prior					439,337
Total	2,332,988		2,332,988		2,345,094
Consumer Portfolio Segment | Student Loan | Seasoning | 37-48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	0		0
2020	0		0		0
2019	0		0		0
2018	397,585		397,585		0
2017	777,883		777,883		965,476
2016 and prior	513,718		513,718		351,433
2015 and Prior					402,552
Total	1,689,186		1,689,186		1,719,461
Consumer Portfolio Segment | Student Loan | Seasoning | More than 48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	0		0
2020	0		0		0
2019	0		0		0
2018	0		0		0
2017	250,029		250,029		0
2016 and prior	2,892,624		2,892,624		729,510
2015 and Prior					2,310,905
Total	3,142,653		3,142,653		3,040,415
Consumer Portfolio Segment | Student Loan | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2021	1,351,990		1,351,990
2020	1,661,199		1,661,199		1,374,085
2019	1,108,155		1,108,155		1,330,175
2018	616,392		616,392		733,824
2017	423,102		423,102		508,478
2016 and prior	694,442		694,442		327,763
2015 and Prior					504,715
Total	$ 5,855,280		$ 5,855,280		$ 4,779,040