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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Transfer of allowance for funded loans	$ (60,666)	$ (76,227)	$ (178,120)	$ (205,916)
Current period recoveries	6,896	8,322	23,282	22,312
Net charge-offs	(53,770)	(67,905)	(154,838)	$ (183,604)
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0		0		$ 1,049,007
Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Transfer of allowance for funded loans	(100,470)	$ (2,170)
Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	1,758,337		1,758,337
2019	3,031,984		3,031,984		2,340,221
2018	2,374,526		2,374,526		3,159,878
2017	2,092,242		2,092,242		2,781,132
2016	1,920,310		1,920,310		2,566,815
2015 and Prior	4,197,607		4,197,607		2,225,721
2014 and Prior					3,241,884
Total	15,375,006		15,375,006		16,315,651
Current period gross charge-offs, 2020			(458)		(1,697)
Current period gross charge-offs, 2019			(6,040)		(14,650)
Current period gross charge-offs, 2018			(18,124)		(29,119)
Current period gross charge-offs, 2017			(25,359)		(40,576)
Current period gross charge-offs, 2016			(24,979)		(41,141)
Current period gross charge-offs, 2015 and prior			(63,586)		(81,795)
Transfer of allowance for funded loans			(138,546)		(208,978)
Current period recoveries, 2020			5		69
Current period recoveries, 2019			381		1,016
Current period recoveries, 2018			1,672		2,622
Current period recoveries, 2017			2,778		4,431
Current period recoveries, 2016			3,277		5,175
Current period recoveries, 2015 and prior			10,185		12,452
Current period recoveries			18,298		25,765
Current period net charge-offs, 2020			(453)		(1,628)
Current period net charge-offs, 2019			(5,659)		(13,634)
Current period net charge-offs, 2018			(16,452)		(26,497)
Current period net charge-offs, 2017			(22,581)		(36,145)
Current period net charge-offs, 2016			(21,702)		(35,966)
Current period net charge-offs, 2015 and prior			(53,401)		(69,343)
Net charge-offs			(120,248)		(183,213)
Total accrued interest by origination vintage, 2020	54,276		54,276		116,423
Total accrued interest by origination vintage, 2019	308,207		308,207		321,568
Total accrued interest by origination vintage, 2018	325,240		325,240		327,002
Total accrued interest by origination vintage, 2017	289,104		289,104		261,083
Total accrued interest by origination vintage, 2016	209,478		209,478		165,764
Total accrued interest by origination vintage, 2015 and prior	267,871		267,871		174,318
Total accrued interest by origination vintage	1,454,176		1,454,176		1,366,158
Consumer Portfolio Segment | Student Loan | With Cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	2,583,111		2,583,111
2019	4,444,417		4,444,417		3,475,256
2018	3,156,414		3,156,414		4,303,772
2017	2,695,281		2,695,281		3,575,973
2016	2,338,980		2,338,980		3,112,873
2015 and Prior	4,833,371		4,833,371		2,579,214
2014 and Prior					3,662,547
Total	$ 20,051,574		$ 20,051,574		$ 20,709,635
Private education loans	89.00%		89.00%		89.00%
Consumer Portfolio Segment | Student Loan | Without Cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 445,546		$ 445,546
2019	651,283		651,283		$ 571,792
2018	448,682		448,682		584,601
2017	323,888		323,888		427,512
2016	246,098		246,098		320,985
2015 and Prior	447,256		447,256		241,958
2014 and Prior					333,108
Total	$ 2,562,753		$ 2,562,753		$ 2,479,956
Private education loans	11.00%		11.00%		11.00%
Consumer Portfolio Segment | Student Loan | Cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 3,028,657		$ 3,028,657
2019	5,095,700		5,095,700		$ 4,047,048
2018	3,605,096		3,605,096		4,888,373
2017	3,019,169		3,019,169		4,003,485
2016	2,585,078		2,585,078		3,433,858
2015 and Prior	5,280,627		5,280,627		2,821,172
2014 and Prior					3,995,655
Total	$ 22,614,327		$ 22,614,327		$ 23,189,591
Total in percent	100.00%		100.00%		100.00%
Consumer Portfolio Segment | Student Loan | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 3,028,657		$ 3,028,657
2019	5,095,700		5,095,700		$ 4,047,048
2018	3,605,096		3,605,096		4,888,373
2017	3,019,169		3,019,169		4,003,485
2016	2,585,078		2,585,078		3,433,858
2015 and Prior	5,280,627		5,280,627		2,821,172
2014 and Prior					3,995,655
Total	$ 22,614,327		$ 22,614,327		$ 23,189,591
Total in percent	100.00%		100.00%		100.00%
Consumer Portfolio Segment | Student Loan | FICO at Original Approval | Less than 670
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 177,986		$ 177,986
2019	361,387		361,387		$ 283,040
2018	267,945		267,945		343,613
2017	228,293		228,293		285,747
2016	187,659		187,659		236,457
2015 and Prior	414,785		414,785		203,145
2014 and Prior					313,587
Total	$ 1,638,055		$ 1,638,055		$ 1,665,589
Private education loans at origination	7.00%		7.00%		7.00%
Consumer Portfolio Segment | Student Loan | FICO at Original Approval | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 411,825		$ 411,825
2019	751,145		751,145		$ 592,376
2018	537,957		537,957		714,779
2017	478,944		478,944		617,676
2016	410,372		410,372		529,575
2015 and Prior	883,813		883,813		439,050
2014 and Prior					676,569
Total	$ 3,474,056		$ 3,474,056		$ 3,570,025
Private education loans at origination	15.00%		15.00%		16.00%
Consumer Portfolio Segment | Student Loan | FICO at Original Approval | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 975,514		$ 975,514
2019	1,661,005		1,661,005		$ 1,319,563
2018	1,183,935		1,183,935		1,601,904
2017	1,004,662		1,004,662		1,325,387
2016	871,989		871,989		1,155,253
2015 and Prior	1,767,363		1,767,363		944,135
2014 and Prior					1,324,506
Total	$ 7,464,468		$ 7,464,468		$ 7,670,748
Private education loans at origination	33.00%		33.00%		33.00%
Consumer Portfolio Segment | Student Loan | FICO at Original Approval | Greater than or equal to 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 1,463,332		$ 1,463,332
2019	2,322,163		2,322,163		$ 1,852,069
2018	1,615,259		1,615,259		2,228,077
2017	1,307,270		1,307,270		1,774,675
2016	1,115,058		1,115,058		1,512,573
2015 and Prior	2,214,666		2,214,666		1,234,842
2014 and Prior					1,680,993
Total	$ 10,037,748		$ 10,037,748		$ 10,283,229
Private education loans at origination	45.00%		45.00%		44.00%
Consumer Portfolio Segment | Student Loan | School FICO, Refreshed Amounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 3,028,657		$ 3,028,657
2019	5,095,700		5,095,700		$ 4,047,048
2018	3,605,096		3,605,096		4,888,373
2017	3,019,169		3,019,169		4,003,485
2016	2,585,078		2,585,078		3,433,858
2015 and Prior	5,280,627		5,280,627		2,821,172
2014 and Prior					3,995,655
Total	$ 22,614,327		$ 22,614,327		$ 23,189,591
Total in percent	100.00%		100.00%		100.00%
Consumer Portfolio Segment | Student Loan | School FICO, Refreshed Amounts | Less than 670
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 217,218		$ 217,218
2019	395,572		395,572		$ 401,979
2018	344,242		344,242		515,901
2017	336,857		336,857		475,007
2016	332,360		332,360		449,568
2015 and Prior	837,360		837,360		419,308
2014 and Prior					717,674
Total	$ 2,463,609		$ 2,463,609		$ 2,979,437
Private education loans at origination	11.00%		11.00%		13.00%
Consumer Portfolio Segment | Student Loan | School FICO, Refreshed Amounts | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 397,492		$ 397,492
2019	642,016		642,016		$ 582,256
2018	426,549		426,549		645,422
2017	338,010		338,010		497,497
2016	276,258		276,258		397,889
2015 and Prior	578,132		578,132		308,607
2014 and Prior					451,451
Total	$ 2,658,457		$ 2,658,457		$ 2,883,122
Private education loans at origination	12.00%		12.00%		13.00%
Consumer Portfolio Segment | Student Loan | School FICO, Refreshed Amounts | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 963,875		$ 963,875
2019	1,598,763		1,598,763		$ 1,284,867
2018	1,081,419		1,081,419		1,506,849
2017	883,872		883,872		1,199,564
2016	723,213		723,213		994,309
2015 and Prior	1,413,526		1,413,526		772,205
2014 and Prior					1,048,808
Total	$ 6,664,668		$ 6,664,668		$ 6,806,602
Private education loans at origination	29.00%		29.00%		29.00%
Consumer Portfolio Segment | Student Loan | School FICO, Refreshed Amounts | Greater than or equal to 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 1,450,072		$ 1,450,072
2019	2,459,349		2,459,349		$ 1,777,946
2018	1,752,886		1,752,886		2,220,201
2017	1,460,430		1,460,430		1,831,417
2016	1,253,247		1,253,247		1,592,092
2015 and Prior	2,451,609		2,451,609		1,321,052
2014 and Prior					1,777,722
Total	$ 10,827,593		$ 10,827,593		$ 10,520,430
Private education loans at origination	48.00%		48.00%		45.00%
Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 3,028,657		$ 3,028,657
2019	5,095,700		5,095,700		$ 4,047,048
2018	3,605,096		3,605,096		4,888,373
2017	3,019,169		3,019,169		4,003,485
2016	2,585,078		2,585,078		3,433,858
2015 and Prior	5,280,627		5,280,627		2,821,172
2014 and Prior					3,995,655
Total	$ 22,614,327		$ 22,614,327		$ 23,189,591
Seasoning based on monthly scheduled payments due from 1-12 payments	26.00%		26.00%		23.00%
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%		10.00%
Seasoning based on monthly scheduled payments due from more than 48 payments	13.00%		13.00%		13.00%
Seasoning based on monthly scheduled payments due from not yet in repayment	27.00%		27.00%		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]
2020	$ 1,772,006		$ 1,772,006
2019	2,163,584		2,163,584		$ 2,376,404
2018	479,231		479,231		719,158
2017	482,319		482,319		705,181
2016	409,027		409,027		617,174
2015 and Prior	603,765		603,765		462,946
2014 and Prior					470,839
Total	5,909,932		5,909,932		5,351,702
Consumer Portfolio Segment | Student Loan | Seasoning | 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	21		21
2019	968,742		968,742		0
2018	1,437,060		1,437,060		2,588,702
2017	261,628		261,628		424,953
2016	247,112		247,112		305,078
2015 and Prior	502,320		502,320		285,513
2014 and Prior					399,905
Total	3,416,883		3,416,883		4,004,151
Consumer Portfolio Segment | Student Loan | Seasoning | 25-36 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0		0
2019	0		0		0
2018	631,736		631,736		0
2017	1,150,053		1,150,053		1,862,587
2016	225,369		225,369		418,048
2015 and Prior	502,923		502,923		227,391
2014 and Prior					394,339
Total	2,510,081		2,510,081		2,902,365
Consumer Portfolio Segment | Student Loan | Seasoning | 37-48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0		0
2019	0		0		0
2018	0		0		0
2017	403,088		403,088		0
2016	941,217		941,217		1,457,760
2015 and Prior	482,116		482,116		413,508
2014 and Prior					342,676
Total	1,826,421		1,826,421		2,213,944
Consumer Portfolio Segment | Student Loan | Seasoning | More than 48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0		0
2019	0		0		0
2018	0		0		0
2017	0		0		0
2016	304,878		304,878		0
2015 and Prior	2,590,571		2,590,571		1,056,229
2014 and Prior					1,973,795
Total	2,895,449		2,895,449		3,030,024
Consumer Portfolio Segment | Student Loan | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	1,256,630		1,256,630
2019	1,963,374		1,963,374		1,670,644
2018	1,057,069		1,057,069		1,580,513
2017	722,081		722,081		1,010,764
2016	457,475		457,475		635,798
2015 and Prior	598,932		598,932		375,585
2014 and Prior					414,101
Total	6,055,561		6,055,561		5,687,405
Consumer Portfolio Segment | Student Loan | Loans In Forbearance
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	8,343		8,343
2019	64,167		64,167		21,009
2018	110,935		110,935		108,509
2017	127,915		127,915		142,341
2016	126,218		126,218		146,114
2015 and Prior	267,584		267,584		127,799
2014 and Prior					168,744
Total	$ 705,162		$ 705,162		$ 714,516