Allowance for Loan Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current period gross charge-offs	$ 70,949	$ 55,062
Current period recoveries	9,518	6,606
Current period net charge-offs	61,431	$ 48,456
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	899,704		$ 1,049,007
Total accrued interest by origination vintage, 2020	0		3,431
Total accrued interest by origination vintage, 2019	3,161		4,166
Total accrued interest by origination vintage, 2018	3,443		572
Total accrued interest by origination vintage, 2017	414		5
Total accrued interest by origination vintage, 2016	3
Total accrued interest by origination vintage	7,021		8,174
Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	454,436
2019	3,020,099		2,340,221
2018	2,422,162		3,159,878
2017	2,162,435		2,781,132
2016	1,999,047		2,566,815
2015 and Prior	4,458,032		2,225,721
2014 and Prior			3,241,884
Total	14,516,211		16,315,651
Total accrued interest by origination vintage, 2020	9,927		116,423
Total accrued interest by origination vintage, 2019	181,551		321,568
Total accrued interest by origination vintage, 2018	281,619		327,002
Total accrued interest by origination vintage, 2017	279,039		261,083
Total accrued interest by origination vintage, 2016	217,988		165,764
Total accrued interest by origination vintage, 2015 and prior	288,627		174,318
Total accrued interest by origination vintage	1,258,751		1,366,158
Current period gross charge-offs, 2020	0		1,697
Current period gross charge-offs, 2019	1,597		14,650
Current period gross charge-offs, 2018	5,241		29,119
Current period gross charge-offs, 2017	8,136		40,576
Current period gross charge-offs, 2016	10,284		41,141
Current period gross charge-offs, 2015 and prior	26,211		81,795
Current period gross charge-offs	51,469		208,978
Current period recoveries, 2020	0		69
Current period recoveries, 2019	92		1,016
Current period recoveries, 2018	599		2,622
Current period recoveries, 2017	1,080		4,431
Current period recoveries, 2016	1,505		5,175
Current period recoveries, 2015 and prior	4,700		12,452
Current period recoveries	7,976		25,765
Current period net charge-offs, 2020	0		1,628
Current period net charge-offs, 2019	1,505		13,634
Current period net charge-offs, 2018	4,642		26,497
Current period net charge-offs, 2017	7,056		36,145
Current period net charge-offs, 2016	8,779		35,966
Current period net charge-offs, 2015 and prior	21,511		69,343
Current period net charge-offs	43,493		183,213
Consumer Portfolio Segment | Student Loan | With Cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	698,291
2019	4,527,042		3,475,256
2018	3,311,178		4,303,772
2017	2,869,295		3,575,973
2016	2,515,751		3,112,873
2015 and Prior	5,295,854		2,579,214
2014 and Prior			3,662,547
Total	$ 19,217,411		$ 20,709,635
Private education loans	89.00%		89.00%
Consumer Portfolio Segment | Student Loan | Without Cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 126,138
2019	676,071		$ 571,792
2018	489,070		584,601
2017	353,832		427,512
2016	268,799		320,985
2015 and Prior	495,426		241,958
2014 and Prior			333,108
Total	$ 2,409,336		$ 2,479,956
Private education loans	11.00%		11.00%
Consumer Portfolio Segment | Student Loan | Cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 824,429
2019	5,203,113		$ 4,047,048
2018	3,800,248		4,888,373
2017	3,223,127		4,003,485
2016	2,784,550		3,433,858
2015 and Prior	5,791,280		2,821,172
2014 and Prior			3,995,655
Total	$ 21,626,747		$ 23,189,591
Total in percent	100.00%		100.00%
Consumer Portfolio Segment | Student Loan | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 824,429
2019	5,203,113		$ 4,047,048
2018	3,800,248		4,888,373
2017	3,223,127		4,003,485
2016	2,784,550		3,433,858
2015 and Prior	5,791,280		2,821,172
2014 and Prior			3,995,655
Total	$ 21,626,747		$ 23,189,591
Total in percent	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	$ 62,064
2019	363,345		$ 283,040
2018	274,016		343,613
2017	237,052		285,747
2016	197,390		236,457
2015 and Prior	446,246		203,145
2014 and Prior			313,587
Total	$ 1,580,113		$ 1,665,589
Private Education Loans At Origination	7.00%		7.00%
Consumer Portfolio Segment | Student Loan | FICO at Original Approval | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 123,512
2019	757,806		$ 592,376
2018	555,719		714,779
2017	502,930		617,676
2016	435,075		529,575
2015 and Prior	955,404		439,050
2014 and Prior			676,569
Total	$ 3,330,446		$ 3,570,025
Private Education Loans At Origination	15.00%		16.00%
Consumer Portfolio Segment | Student Loan | FICO at Original Approval | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 268,388
2019	1,688,114		$ 1,319,563
2018	1,243,712		1,601,904
2017	1,068,091		1,325,387
2016	937,643		1,155,253
2015 and Prior	1,933,589		944,135
2014 and Prior			1,324,506
Total	$ 7,139,537		$ 7,670,748
Private Education Loans At Origination	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	$ 370,465
2019	2,393,848		$ 1,852,069
2018	1,726,801		2,228,077
2017	1,415,054		1,774,675
2016	1,214,442		1,512,573
2015 and Prior	2,456,041		1,234,842
2014 and Prior			1,680,993
Total	$ 9,576,651		$ 10,283,229
Private Education Loans At Origination	45.00%		44.00%
Consumer Portfolio Segment | Student Loan | School FICO, Refreshed Amounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 824,429
2019	5,203,113		$ 4,047,048
2018	3,800,248		4,888,373
2017	3,223,127		4,003,485
2016	2,784,550		3,433,858
2015 and Prior	5,791,280		2,821,172
2014 and Prior			3,995,655
Total	$ 21,626,747		$ 23,189,591
Total in percent	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	$ 86,212
2019	544,160		$ 401,979
2018	442,893		515,901
2017	419,466		475,007
2016	400,833		449,568
2015 and Prior	1,029,741		419,308
2014 and Prior			717,674
Total	$ 2,923,305		$ 2,979,437
Private Education Loans At Origination	14.00%		13.00%
Consumer Portfolio Segment | Student Loan | School FICO, Refreshed Amounts | 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 127,503
2019	735,546		$ 582,256
2018	495,808		645,422
2017	400,096		497,497
2016	324,334		397,889
2015 and Prior	651,732		308,607
2014 and Prior			451,451
Total	$ 2,735,019		$ 2,883,122
Private Education Loans At Origination	13.00%		13.00%
Consumer Portfolio Segment | Student Loan | School FICO, Refreshed Amounts | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 261,585
2019	1,633,986		$ 1,284,867
2018	1,145,724		1,506,849
2017	952,467		1,199,564
2016	795,350		994,309
2015 and Prior	1,542,487		772,205
2014 and Prior			1,048,808
Total	$ 6,331,599		$ 6,806,602
Private Education Loans At Origination	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	$ 349,129
2019	2,289,421		$ 1,777,946
2018	1,715,823		2,220,201
2017	1,451,098		1,831,417
2016	1,264,033		1,592,092
2015 and Prior	2,567,320		1,321,052
2014 and Prior			1,777,722
Total	$ 9,636,824		$ 10,520,430
Private Education Loans At Origination	44.00%		45.00%
Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 824,429
2019	5,203,113		$ 4,047,048
2018	3,800,248		4,888,373
2017	3,223,127		4,003,485
2016	2,784,550		3,433,858
2015 and Prior	5,791,280		2,821,172
2014 and Prior			3,995,655
Total	$ 21,626,747		$ 23,189,591
Seasoning based on monthly scheduled payments due from 1-12 payments	25.00%		23.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments	15.00%		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	9.00%		10.00%
Seasoning based on monthly scheduled payments due from more than 48 payments	13.00%		13.00%
Seasoning based on monthly scheduled payments due from not yet in repayment	27.00%		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]
2020	$ 460,330
2019	2,838,628		$ 2,376,404
2018	469,314		719,158
2017	529,441		705,181
2016	460,742		617,174
2015 and Prior	715,809		462,946
2014 and Prior			470,839
Total	5,474,264		5,351,702
Consumer Portfolio Segment | Student Loan | Seasoning | 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0
2019	255,403		0
2018	1,979,761		2,588,702
2017	277,499		424,953
2016	258,606		305,078
2015 and Prior	580,426		285,513
2014 and Prior			399,905
Total	3,351,695		4,004,151
Consumer Portfolio Segment | Student Loan | Seasoning | 25-36 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0
2019	0		0
2018	169,436		0
2017	1,494,597		1,862,587
2016	268,357		418,048
2015 and Prior	538,209		227,391
2014 and Prior			394,339
Total	2,470,599		2,902,365
Consumer Portfolio Segment | Student Loan | Seasoning | 37-48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0
2019	0		0
2018	0		0
2017	117,409		0
2016	1,194,038		1,457,760
2015 and Prior	568,792		413,508
2014 and Prior			342,676
Total	1,880,239		2,213,944
Consumer Portfolio Segment | Student Loan | Seasoning | More than 48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0
2019	0		0
2018	0		0
2017	0		0
2016	87,201		0
2015 and Prior	2,714,272		1,056,229
2014 and Prior			1,973,795
Total	2,801,473		3,030,024
Consumer Portfolio Segment | Student Loan | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	364,099
2019	2,109,082		1,670,644
2018	1,181,737		1,580,513
2017	804,181		1,010,764
2016	515,606		635,798
2015 and Prior	673,772		375,585
2014 and Prior			414,101
Total	5,648,477		5,687,405
Consumer Portfolio Segment | Student Loan | Loans In Forbearance
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	4,370
2019	51,418		21,009
2018	144,691		108,509
2017	180,617		142,341
2016	186,362		146,114
2015 and Prior	422,467		127,799
2014 and Prior			168,744
Total	989,925		714,516
Consumer Portfolio Segment | Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	40
2019	346,123		400,216
2018	433,664		522,778
2017	76,188		99,581
2016	787		942
2015 and Prior	856,802
Consumer Portfolio Segment | Personal Loans | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	40
2019	364,849		404,776
2018	455,692		540,784
2017	78,336		102,463
2016	787		984
Total	$ 899,704		$ 1,049,007
Total in percent	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
2019	7,501		$ 8,315
2018	26,745		32,021
2017	5,072		7,030
2016	0		1
Total	$ 39,318		$ 47,367
Private Education Loans At Origination	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
2019	70,479		$ 77,746
2018	128,797		152,909
2017	21,295		28,384
2016	47		59
Total	$ 220,618		$ 259,098
Private Education Loans At Origination	25.00%		25.00%
Consumer Portfolio Segment | Personal Loans | FICO at Original Approval | 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 40
2019	195,857		$ 217,642
2018	215,424		255,374
2017	37,261		48,254
2016	456		586
Total	$ 449,038		$ 521,856
Private Education Loans At Origination	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
2019	91,012		$ 101,073
2018	84,726		100,480
2017	14,708		18,795
2016	284		338
Total	$ 190,730		$ 220,686
Private Education Loans At Origination	21.00%		21.00%
Consumer Portfolio Segment | Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 40
2019	364,849		$ 404,776
2018	455,692		540,784
2017	78,336		102,463
2016	787		984
Total	$ 899,704		$ 1,049,007
Seasoning based on monthly scheduled payments due from 1-12 payments	37.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments	48.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments	15.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments	0.00%
Seasoning based on monthly scheduled payments due from more than 48 payments	0.00%
Seasoning based on monthly scheduled payments due from not yet in repayment	0.00%
Total in percent	100.00%		100.00%
Consumer Portfolio Segment | Personal Loans | Seasoning | 1-12 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 40
2019	331,474		$ 404,776
2018	400		65,164
2017	0		0
2016	0		0
Total	331,914		$ 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
2019	33,375		$ 0
2018	393,608		475,620
2017	6		29,698
2016	0		0
Total	426,989		$ 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
2019	0		$ 0
2018	61,684		0
2017	76,924		72,765
2016	0		984
Total	138,608		$ 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
2019	0		$ 0
2018	0		0
2017	1,406		0
2016	787		0
Total	2,193		$ 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
2019	0		$ 0
2018	0		0
2017	0		0
2016	0		0
Total	0		$ 0
Seasoning based on monthly scheduled payments due from more than 48 payments			0.00%
Consumer Portfolio Segment | Personal Loans | Seasoning | Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0
2019	0		$ 0
2018	0		0
2017	0		0
2016	0		0
Total	0		$ 0
Seasoning based on monthly scheduled payments due from not yet in repayment			0.00%
Consumer Portfolio Segment | Personal Loans | Loans In Forbearance
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0
2019	12,380		$ 0
2018	7,137		0
2017	0		0
2016	0		0
2015 and Prior	19,517
Total			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)
Current period 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	2,885
Current period gross charge-offs, 2018	13,758
Current period gross charge-offs, 2017	2,563
Current period gross charge-offs, 2016	(41)
Current period gross charge-offs	19,247
Current period recoveries, 2020	0
Current period recoveries, 2019	15
Current period recoveries, 2018	1,127
Current period recoveries, 2017	389
Current period recoveries, 2016	11
Current period recoveries	1,542
Current period net charge-offs, 2020	0
Current period net charge-offs, 2019	2,870
Current period net charge-offs, 2018	12,631
Current period net charge-offs, 2017	2,174
Current period net charge-offs, 2016	(30)
Current period net charge-offs	$ 17,705