Allowance for Loan Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current period gross charge-offs	$ 46,505		$ 74,627	$ 117,454	$ 129,689
Current period recoveries	6,868		7,384	16,386	13,990
Net charge-offs	(39,637)		(67,243)	(101,068)	$ (115,699)
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	772,086			772,086		$ 1,049,007
Total accrued interest by origination vintage, 2020	0			0		3,431
Total accrued interest by origination vintage, 2019	3,850			3,850		4,166
Total accrued interest by origination vintage, 2018	3,977			3,977		572
Total accrued interest by origination vintage, 2017	397			397		5
Total accrued interest by origination vintage, 2016	3			3
Total accrued interest by origination vintage	8,227			8,227		8,174
Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current period gross charge-offs	42,490		$ 308
Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	664,058			664,058
2019	3,004,489			3,004,489		2,340,221
2018	2,346,348			2,346,348		3,159,878
2017	2,076,951			2,076,951		2,781,132
2016	1,907,320			1,907,320		2,566,815
2015 and Prior	4,191,163			4,191,163		2,225,721
2014 and Prior						3,241,884
Total	14,190,329			14,190,329		16,315,651
Current period gross charge-offs, 2020	(99)	$ (1,697)
Current period gross charge-offs, 2019	(3,325)	(14,650)
Current period gross charge-offs, 2018	(10,513)	(29,119)
Current period gross charge-offs, 2017	(14,602)	(40,576)
Current period gross charge-offs, 2016	(15,397)	(41,141)
Current period gross charge-offs, 2015 and prior	(39,312)	(81,795)
Current period gross charge-offs	83,248	208,978
Current period recoveries, 2020	0	69
Current period recoveries, 2019	221	1,016
Current period recoveries, 2018	1,178	2,622
Current period recoveries, 2017	1,988	4,431
Current period recoveries, 2016	2,506	5,175
Current period recoveries, 2015 and prior	7,615	12,452
Current period recoveries	13,508	25,765
Current period net charge-offs, 2020	(99)	(1,628)
Current period net charge-offs, 2019	(3,104)	(13,634)
Current period net charge-offs, 2018	(9,335)	(26,497)
Current period net charge-offs, 2017	(12,614)	(36,145)
Current period net charge-offs, 2016	(12,891)	(35,966)
Current period net charge-offs, 2015 and prior	(31,697)	(69,343)
Net charge-offs	(69,740)	$ (183,213)
Total accrued interest by origination vintage, 2020	24,602			24,602		116,423
Total accrued interest by origination vintage, 2019	248,904			248,904		321,568
Total accrued interest by origination vintage, 2018	307,765			307,765		327,002
Total accrued interest by origination vintage, 2017	287,159			287,159		261,083
Total accrued interest by origination vintage, 2016	215,989			215,989		165,764
Total accrued interest by origination vintage, 2015 and prior	281,753			281,753		174,318
Total accrued interest by origination vintage	1,366,172			1,366,172		1,366,158
Consumer Portfolio Segment | Student Loan | With Cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	985,808			985,808
2019	4,513,841			4,513,841		3,475,256
2018	3,244,267			3,244,267		4,303,772
2017	2,790,768			2,790,768		3,575,973
2016	2,434,473			2,434,473		3,112,873
2015 and Prior	5,072,593			5,072,593		2,579,214
2014 and Prior						3,662,547
Total	$ 19,041,750			$ 19,041,750		$ 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	$ 231,032			$ 231,032
2019	671,312			671,312		$ 571,792
2018	470,339			470,339		584,601
2017	340,709			340,709		427,512
2016	258,451			258,451		320,985
2015 and Prior	471,872			471,872		241,958
2014 and Prior						333,108
Total	$ 2,443,715			$ 2,443,715		$ 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	$ 1,216,840			$ 1,216,840
2019	5,185,153			5,185,153		$ 4,047,048
2018	3,714,606			3,714,606		4,888,373
2017	3,131,477			3,131,477		4,003,485
2016	2,692,924			2,692,924		3,433,858
2015 and Prior	5,544,465			5,544,465		2,821,172
2014 and Prior						3,995,655
Total	$ 21,485,465			$ 21,485,465		$ 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	$ 1,216,840			$ 1,216,840
2019	5,185,153			5,185,153		$ 4,047,048
2018	3,714,606			3,714,606		4,888,373
2017	3,131,477			3,131,477		4,003,485
2016	2,692,924			2,692,924		3,433,858
2015 and Prior	5,544,465			5,544,465		2,821,172
2014 and Prior						3,995,655
Total	$ 21,485,465			$ 21,485,465		$ 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	$ 86,008			$ 86,008
2019	363,637			363,637		$ 283,040
2018	271,480			271,480		343,613
2017	233,371			233,371		285,747
2016	193,171			193,171		236,457
2015 and Prior	431,144			431,144		203,145
2014 and Prior						313,587
Total	$ 1,578,811			$ 1,578,811		$ 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	$ 181,178			$ 181,178
2019	758,383			758,383		$ 592,376
2018	547,683			547,683		714,779
2017	492,944			492,944		617,676
2016	423,603			423,603		529,575
2015 and Prior	920,255			920,255		439,050
2014 and Prior						676,569
Total	$ 3,324,046			$ 3,324,046		$ 3,570,025
Private education loans at origination	16.00%			16.00%		16.00%
Consumer Portfolio Segment | Student Loan | FICO at Original Approval | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 398,370			$ 398,370
2019	1,685,282			1,685,282		$ 1,319,563
2018	1,218,105			1,218,105		1,601,904
2017	1,039,210			1,039,210		1,325,387
2016	907,291			907,291		1,155,253
2015 and Prior	1,854,579			1,854,579		944,135
2014 and Prior						1,324,506
Total	$ 7,102,837			$ 7,102,837		$ 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	$ 551,284			$ 551,284
2019	2,377,851			2,377,851		$ 1,852,069
2018	1,677,338			1,677,338		2,228,077
2017	1,365,952			1,365,952		1,774,675
2016	1,168,859			1,168,859		1,512,573
2015 and Prior	2,338,487			2,338,487		1,234,842
2014 and Prior						1,680,993
Total	$ 9,479,771			$ 9,479,771		$ 10,283,229
Private education loans at origination	44.00%			44.00%		44.00%
Consumer Portfolio Segment | Student Loan | School FICO, Refreshed Amounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 1,216,840			$ 1,216,840
2019	5,185,153			5,185,153		$ 4,047,048
2018	3,714,606			3,714,606		4,888,373
2017	3,131,477			3,131,477		4,003,485
2016	2,692,924			2,692,924		3,433,858
2015 and Prior	5,544,465			5,544,465		2,821,172
2014 and Prior						3,995,655
Total	$ 21,485,465			$ 21,485,465		$ 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	$ 102,547			$ 102,547
2019	476,798			476,798		$ 401,979
2018	398,671			398,671		515,901
2017	379,970			379,970		475,007
2016	366,824			366,824		449,568
2015 and Prior	927,051			927,051		419,308
2014 and Prior						717,674
Total	$ 2,651,861			$ 2,651,861		$ 2,979,437
Private education loans at origination	12.00%			12.00%		13.00%
Consumer Portfolio Segment | Student Loan | School FICO, Refreshed Amounts | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 169,846			$ 169,846
2019	673,935			673,935		$ 582,256
2018	449,316			449,316		645,422
2017	365,351			365,351		497,497
2016	296,192			296,192		397,889
2015 and Prior	613,518			613,518		308,607
2014 and Prior						451,451
Total	$ 2,568,158			$ 2,568,158		$ 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	$ 396,337			$ 396,337
2019	1,623,733			1,623,733		$ 1,284,867
2018	1,108,642			1,108,642		1,506,849
2017	911,600			911,600		1,199,564
2016	754,672			754,672		994,309
2015 and Prior	1,466,464			1,466,464		772,205
2014 and Prior						1,048,808
Total	$ 6,261,448			$ 6,261,448		$ 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	$ 548,110			$ 548,110
2019	2,410,687			2,410,687		$ 1,777,946
2018	1,757,977			1,757,977		2,220,201
2017	1,474,556			1,474,556		1,831,417
2016	1,275,236			1,275,236		1,592,092
2015 and Prior	2,537,432			2,537,432		1,321,052
2014 and Prior						1,777,722
Total	$ 10,003,998			$ 10,003,998		$ 10,520,430
Private education loans at origination	47.00%			47.00%		45.00%
Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 1,216,840			$ 1,216,840
2019	5,185,153			5,185,153		$ 4,047,048
2018	3,714,606			3,714,606		4,888,373
2017	3,131,477			3,131,477		4,003,485
2016	2,692,924			2,692,924		3,433,858
2015 and Prior	5,544,465			5,544,465		2,821,172
2014 and Prior						3,995,655
Total	$ 21,485,465			$ 21,485,465		$ 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	9.00%			9.00%		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	26.00%			26.00%		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	$ 684,650			$ 684,650
2019	2,718,782			2,718,782		$ 2,376,404
2018	506,754			506,754		719,158
2017	542,026			542,026		705,181
2016	466,704			466,704		617,174
2015 and Prior	695,822			695,822		462,946
2014 and Prior						470,839
Total	5,614,738			5,614,738		5,351,702
Consumer Portfolio Segment | Student Loan | Seasoning | 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0			0
2019	428,312			428,312		0
2018	1,826,430			1,826,430		2,588,702
2017	264,902			264,902		424,953
2016	250,678			250,678		305,078
2015 and Prior	544,044			544,044		285,513
2014 and Prior						399,905
Total	3,314,366			3,314,366		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	280,873			280,873		0
2017	1,394,821			1,394,821		1,862,587
2016	243,690			243,690		418,048
2015 and Prior	528,094			528,094		227,391
2014 and Prior						394,339
Total	2,447,478			2,447,478		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	189,174			189,174		0
2016	1,123,215			1,123,215		1,457,760
2015 and Prior	522,030			522,030		413,508
2014 and Prior						342,676
Total	1,834,419			1,834,419		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	142,975			142,975		0
2015 and Prior	2,651,442			2,651,442		1,056,229
2014 and Prior						1,973,795
Total	2,794,417			2,794,417		3,030,024
Consumer Portfolio Segment | Student Loan | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	532,190			532,190
2019	2,038,059			2,038,059		1,670,644
2018	1,100,549			1,100,549		1,580,513
2017	740,554			740,554		1,010,764
2016	465,662			465,662		635,798
2015 and Prior	603,033			603,033		375,585
2014 and Prior						414,101
Total	5,480,047			5,480,047		5,687,405
Consumer Portfolio Segment | Student Loan | Loans In Forbearance
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	17,801			17,801
2019	123,106			123,106		21,009
2018	222,065			222,065		108,509
2017	248,986			248,986		142,341
2016	248,467			248,467		146,114
2015 and Prior	560,160			560,160		127,799
2014 and Prior						168,744
Total	1,420,585			1,420,585		714,516
Consumer Portfolio Segment | Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	39			39
2019	305,862			305,862		400,216
2018	353,667			353,667		522,778
2017	54,815			54,815		99,581
2016	613			613		942
Total	714,996			714,996		1,023,517
Consumer Portfolio Segment | Personal Loans | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	39			39
2019	327,076			327,076		404,776
2018	384,970			384,970		540,784
2017	59,363			59,363		102,463
2016	638			638		984
Total	$ 772,086			$ 772,086		$ 1,049,007
Total in percent	100.00%			100.00%		100.00%
Consumer Portfolio Segment | Personal Loans | FICO at Original Approval | Less than 670
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 0			$ 0
2019	6,736			6,736		$ 8,315
2018	22,258			22,258		32,021
2017	3,647			3,647		7,030
2016	0			0		1
Total	$ 32,641			$ 32,641		$ 47,367
Private education loans at origination	4.00%			4.00%		4.00%
Consumer Portfolio Segment | Personal Loans | FICO at Original Approval | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 0			$ 0
2019	63,228			63,228		$ 77,746
2018	109,253			109,253		152,909
2017	15,818			15,818		28,384
2016	33			33		59
Total	$ 188,332			$ 188,332		$ 259,098
Private education loans at origination	25.00%			25.00%		25.00%
Consumer Portfolio Segment | Personal Loans | FICO at Original Approval | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 39			$ 39
2019	176,293			176,293		$ 217,642
2018	181,925			181,925		255,374
2017	28,432			28,432		48,254
2016	368			368		586
Total	$ 387,057			$ 387,057		$ 521,856
Private education loans at origination	50.00%			50.00%		50.00%
Consumer Portfolio Segment | Personal Loans | FICO at Original Approval | Greater than or equal to 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 0			$ 0
2019	80,819			80,819		$ 101,073
2018	71,534			71,534		100,480
2017	11,466			11,466		18,795
2016	237			237		338
Total	$ 164,056			$ 164,056		$ 220,686
Private education loans at origination	21.00%			21.00%		21.00%
Consumer Portfolio Segment | Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 39			$ 39
2019	327,076			327,076		$ 404,776
2018	384,970			384,970		540,784
2017	59,363			59,363		102,463
2016	638			638		984
Total	$ 772,086			$ 772,086		$ 1,049,007
Seasoning based on monthly scheduled payments due from 1-12 payments	30.00%			30.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments	44.00%			44.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments	26.00%			26.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments	0.00%			0.00%
Seasoning based on monthly scheduled payments due from more than 48 payments	0.00%			0.00%
Total in percent	100.00%			100.00%		100.00%
Consumer Portfolio Segment | Personal Loans | Seasoning | 1-12 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 39			$ 39
2019	230,341			230,341		$ 404,776
2018	251			251		65,164
2017	0			0		0
2016	0			0		0
Total	230,631			230,631		$ 469,940
Seasoning based on monthly scheduled payments due from 1-12 payments						45.00%
Consumer Portfolio Segment | Personal Loans | Seasoning | 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0			0
2019	96,735			96,735		$ 0
2018	243,781			243,781		475,620
2017	0			0		29,698
2016	0			0		0
Total	340,516			340,516		$ 505,318
Seasoning based on monthly scheduled payments due from 13 - 24 payments						48.00%
Consumer Portfolio Segment | Personal Loans | Seasoning | 25-36 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0			0
2019	0			0		$ 0
2018	140,938			140,938		0
2017	56,281			56,281		72,765
2016	0			0		984
Total	197,219			197,219		$ 73,749
Seasoning based on monthly scheduled payments due from 25 - 36 payments						7.00%
Consumer Portfolio Segment | Personal Loans | Seasoning | 37-48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0			0
2019	0			0		$ 0
2018	0			0		0
2017	3,082			3,082		0
2016	638			638		0
Total	3,720			3,720		$ 0
Seasoning based on monthly scheduled payments due from 37 - 48 payments						0.00%
Consumer Portfolio Segment | Personal Loans | 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	0			0		0
Total	0			0		$ 0
Seasoning based on monthly scheduled payments due from more than 48 payments						0.00%
Consumer Portfolio Segment | Personal Loans | Loans In Forbearance
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0			0
2019	17,037			17,037		$ 0
2018	23,214			23,214		0
2017	3,646			3,646		0
2016	25			25		0
Total	$ 43,922			43,922		0
Commercial Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current period gross charge-offs, 2020						(2,350)
Current period gross charge-offs, 2019						(58,134)
Current period gross charge-offs, 2018						(13,693)
Current period gross charge-offs, 2017						(136)
Current period gross charge-offs						74,313
Current period recoveries, 2020						48
Current period recoveries, 2019						3,397
Current period recoveries, 2018						1,722
Current period recoveries, 2017						39
Current period recoveries						5,206
Current period net charge-offs, 2020						(2,302)
Current period net charge-offs, 2019						(54,737)
Current period net charge-offs, 2018						(11,971)
Current period net charge-offs, 2017						(97)
Net charge-offs						$ (69,107)
Commercial Portfolio Segment | Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current period gross charge-offs, 2020				0
Current period gross charge-offs, 2019				(6,623)
Current period gross charge-offs, 2018				(22,881)
Current period gross charge-offs, 2017				(4,294)
Current period gross charge-offs, 2016				(50)
Current period gross charge-offs				33,848
Current period recoveries, 2020				0
Current period recoveries, 2019				67
Current period recoveries, 2018				2,091
Current period recoveries, 2017				701
Current period recoveries, 2016				19
Current period recoveries				2,878
Current period net charge-offs, 2020				0
Current period net charge-offs, 2019				(6,556)
Current period net charge-offs, 2018				(20,790)
Current period net charge-offs, 2017				(3,593)
Current period net charge-offs, 2016				(31)
Net charge-offs				$ (30,970)