Allowance for Credit Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of FFELP loans insured and guaranteed (at least)	97.00%
Current period gross charge-offs, total	$ (84,066)	$ (55,316)
Current period recoveries, total	8,036	7,704
Net charge-offs	(76,030)	$ (47,612)
Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current period gross charge-offs	0
One year before current period gross charge-offs	(1,618)		$ (1,183)
Two years before current period gross charge-offs	(7,397)		(8,604)
Three years before current period gross charge-offs	(12,931)		(23,866)
Four years before current period gross charge-offs	(12,579)		(32,741)
Five years before current period gross charge-offs	(49,331)		(37,186)
Six years before current period gross charge-offs			(126,011)
Current period gross charge-offs, total	(83,856)		(229,591)
Current period recoveries	0
One year before current period recoveries	107		35
Two years before current period recoveries	507		540
Three years before current period recoveries	989		2,092
Four years before current period recoveries	1,030		3,693
Five years before current period recoveries	5,400		4,450
Six years before current period recoveries			18,684
Current period recoveries, total	8,033		29,494
Current period net charge-offs	0
One year before current period net charge-offs	(1,511)		(1,148)
Two years before current period net charge-offs	(6,890)		(8,064)
Three years before current period net charge-offs	(11,942)		(21,774)
Four years before current period net charge-offs	(11,549)		(29,048)
Five years before current period net charge-offs	(43,931)		(32,736)
Six years before current period net charge-offs			(107,327)
Net charge-offs	(75,823)		(200,097)
Total accrued interest by origination vintage, 2022	9,778
Total accrued interest by origination vintage, 2021	180,992		109,233
Total accrued interest by origination vintage, 2020	269,425		247,418
Total accrued interest by origination vintage, 2019	271,083		270,242
Total accrued interest by origination vintage, 2018	190,288		198,816
Total accrued interest by origination vintage, 2017 and prior	320,007		131,685
Total accrued interest by origination vintage, 2016 and prior			229,729
Total accrued interest by origination vintage	1,241,573		1,187,123
Consumer Portfolio Segment | Student Loan | With cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	664,611
2021	4,368,829		3,263,892
2020	3,450,387		3,604,553
2019	2,626,952		2,778,262
2018	1,898,677		2,025,463
2017 and Prior	6,087,363		1,765,719
2016 and Prior			4,753,775
Total	$ 19,096,819		$ 18,191,664
Private education loans	88.00%		88.00%
Consumer Portfolio Segment | Student Loan | Without cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 117,001
2021	671,963		$ 558,469
2020	533,219		561,730
2019	411,385		438,263
2018	275,611		294,597
2017 and Prior	629,371		212,514
2016 and Prior			459,626
Total	$ 2,638,550		$ 2,525,199
Private education loans	12.00%		12.00%
Consumer Portfolio Segment | Student Loan | With and without cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 781,612
2021	5,040,792		$ 3,822,361
2020	3,983,606		4,166,283
2019	3,038,337		3,216,525
2018	2,174,288		2,320,060
2017 and Prior	6,716,734		1,978,233
2016 and Prior			5,213,401
Total	$ 21,735,369		$ 20,716,863
Total in percent	100.00%		100.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 781,612
2021	5,040,792		$ 3,822,361
2020	3,983,606		4,166,283
2019	3,038,337		3,216,525
2018	2,174,288		2,320,060
2017 and Prior	6,716,734		1,978,233
2016 and Prior			5,213,401
Total	$ 21,735,369		$ 20,716,863
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]
2022	$ 58,976
2021	333,682		$ 248,368
2020	231,772		238,005
2019	241,643		251,157
2018	183,391		193,123
2017 and Prior	561,766		166,048
2016 and Prior			428,416
Total	$ 1,611,230		$ 1,525,117
Private education loans at origination	7.00%		7.00%
Consumer Portfolio Segment | Student Loan | FICO at Origination | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 111,251
2021	669,450		$ 508,264
2020	543,884		564,497
2019	468,705		493,237
2018	343,601		363,313
2017 and Prior	1,141,747		329,807
2016 and Prior			884,981
Total	$ 3,278,638		$ 3,144,099
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]
2022	$ 251,606
2021	1,591,862		$ 1,210,833
2020	1,290,542		1,348,269
2019	999,922		1,057,001
2018	723,591		770,452
2017 and Prior	2,257,304		660,270
2016 and Prior			1,753,709
Total	$ 7,114,827		$ 6,800,534
Private education loans at origination	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	$ 359,779
2021	2,445,798		$ 1,854,896
2020	1,917,408		2,015,512
2019	1,328,067		1,415,130
2018	923,705		993,172
2017 and Prior	2,755,917		822,108
2016 and Prior			2,146,295
Total	$ 9,730,674		$ 9,247,113
Private education loans at origination	45.00%		45.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 781,612
2021	5,040,792		$ 3,822,361
2020	3,983,606		4,166,283
2019	3,038,337		3,216,525
2018	2,174,288		2,320,060
2017 and Prior	6,716,734		1,978,233
2016 and Prior			5,213,401
Total	$ 21,735,369		$ 20,716,863
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]
2022	$ 72,244
2021	472,669		$ 326,613
2020	310,819		279,578
2019	292,706		273,652
2018	245,690		235,684
2017 and Prior	964,669		233,022
2016 and Prior			739,268
Total	$ 2,358,797		$ 2,087,817
Private education loans at origination	11.00%		10.00%
Consumer Portfolio Segment | Student Loan | FICO Refreshed | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 115,446
2021	670,147		$ 506,021
2020	453,846		475,674
2019	343,831		365,133
2018	236,812		256,400
2017 and Prior	738,916		209,536
2016 and Prior			570,605
Total	$ 2,558,998		$ 2,383,369
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]
2022	$ 249,052
2021	1,561,528		$ 1,209,493
2020	1,203,084		1,285,015
2019	911,247		978,763
2018	631,484		682,024
2017 and Prior	1,855,009		568,766
2016 and Prior			1,448,692
Total	$ 6,411,404		$ 6,172,753
Private education loans at origination	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	$ 344,870
2021	2,336,448		$ 1,780,234
2020	2,015,857		2,126,016
2019	1,490,553		1,598,977
2018	1,060,302		1,145,952
2017 and Prior	3,158,140		966,909
2016 and Prior			2,454,836
Total	$ 10,406,170		$ 10,072,924
Private education loans at origination	48.00%		48.00%
Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 781,612
2021	5,040,792		$ 3,822,361
2020	3,983,606		4,166,283
2019	3,038,337		3,216,525
2018	2,174,288		2,320,060
2017 and Prior	6,716,734		1,978,233
2016 and Prior			5,213,401
Total	$ 21,735,369		$ 20,716,863
Seasoning based on monthly scheduled payments due from 1-12 payments	27.00%		22.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments	16.00%		17.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments	11.00%		12.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments	7.00%		8.00%
Seasoning based on monthly scheduled payments due from more than 48 payments	14.00%		16.00%
Seasoning based on monthly scheduled payments due from not yet in repayment	25.00%		25.00%
Total in percent	100.00%		100.00%
Consumer Portfolio Segment | Student Loan | Seasoning | 1-12 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	$ 417,858
2021	3,015,407		$ 2,265,811
2020	597,296		594,850
2019	506,047		515,328
2018	384,057		385,246
2017 and Prior	873,302		340,242
2016 and Prior			501,269
Total	5,793,967		4,602,746
Consumer Portfolio Segment | Student Loan | Seasoning | 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	0
2021	0		0
2020	2,190,940		2,287,737
2019	331,049		362,674
2018	188,777		203,674
2017 and Prior	663,245		211,064
2016 and Prior			479,540
Total	3,374,011		3,544,689
Consumer Portfolio Segment | Student Loan | Seasoning | 25-36 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	0
2021	0		0
2020	188		173
2019	1,483,980		1,565,203
2018	285,486		312,049
2017 and Prior	613,350		164,575
2016 and Prior			482,369
Total	2,383,004		2,524,369
Consumer Portfolio Segment | Student Loan | Seasoning | 37-48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	0
2021	0		0
2020	0		0
2019	0		0
2018	921,378		983,434
2017 and Prior	708,204		295,206
2016 and Prior			464,563
Total	1,629,582		1,743,203
Consumer Portfolio Segment | Student Loan | Seasoning | More than 48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	0
2021	0		0
2020	0		0
2019	0		0
2018	0		0
2017 and Prior	3,148,853		671,138
2016 and Prior			2,726,304
Total	3,148,853		3,397,442
Consumer Portfolio Segment | Student Loan | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2022	363,754
2021	2,025,385		1,556,550
2020	1,195,182		1,283,523
2019	717,261		773,320
2018	394,590		435,657
2017 and Prior	709,780		296,008
2016 and Prior			559,356
Total	$ 5,405,952		$ 4,904,414