Allowance for Loan Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Jan. 01, 2020	Mar. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ 1,673,324	$ 441,912	$ 441,912	$ 358,325	$ 341,121
Loans held for investment, net	$ 21,695,851	$ 24,667,792
Percentage of Private Education Loans in repayment, 2020	55.30%	58.20%
Percentage of Private Education Loans in repayment, 2019	58.50%	65.40%
Percentage of Private Education Loans in repayment, 2018	65.10%	71.20%
Percentage of Private Education Loans in repayment, 2017	69.40%	77.20%
Percentage of Private Education Loans in repayment, 2016	74.80%	82.20%
Percentage of Private Education Loans in repayment, 2015 and prior	81.10%	85.40%
Percentage of Private Education Loans in repayment	69.30%	72.40%
Delinquencies as a percentage of Private Education Loans in repayment, 2020	0.30%	0.60%
Delinquencies as a percentage of Private Education Loans in repayment, 2019	0.70%	1.20%
Delinquencies as a percentage of Private Education Loans in repayment, 2018	2.10%	2.40%
Delinquencies as a percentage of Private Education Loans in repayment, 2017	3.40%	3.20%
Delinquencies as a percentage of Private Education Loans in repayment, 2016	4.00%	4.00%
Delinquencies as a percentage of Private Education Loans in repayment, 2015 and prior	5.00%	5.00%
Delinquencies as a percentage of Private Education Loans in repayment	3.20%	2.80%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2020	0.90%	0.90%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2019	1.70%	3.30%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2018	5.50%	4.80%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2017	7.50%	5.20%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2016	8.20%	5.20%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2015 and prior	8.30%	4.70%
Loans in forbearance as a percentage of loans in repayment and forbearance	6.20%	4.10%
Student Loan | Consumer Portfolio Segment
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 Private Education Loans in repayment, 2020	455,960	2,355,395
Total Private Education Loans in repayment, 2019	3,042,613	3,199,351
Total Private Education Loans in repayment, 2018	2,473,820	2,850,380
Total Private Education Loans in repayment, 2017	2,238,329	2,651,946
Total Private Education Loans in repayment, 2016	2,082,582	2,317,788
Total Private Education Loans in repayment, 2015 and prior	4,695,041	3,412,810
Total Private Education Loans in repayment	14,988,345	16,787,670
Total Private Education Loans, gross, 2020	824,430	4,047,048
Total Private Education Loans, gross, 2019	5,203,123	4,888,373
Total Private Education Loans, gross, 2018	3,800,248	4,003,485
Total Private Education Loans, gross, 2017	3,223,127	3,433,858
Total Private Education Loans, gross, 2016	2,784,550	2,821,172
Total Private Education Loans, gross, 2015 and prior	5,791,269	3,995,655
Total Private Education Loans, gross	21,626,747	23,189,591
Private Education Loans deferred origination costs and unamortized premium/(discount), 2020	4,958	23,661
Private Education Loans deferred origination costs and unamortized premium/(discount), 2019	20,484	17,699
Private Education Loans deferred origination costs and unamortized premium/(discount), 2018	12,515	13,843
Private Education Loans deferred origination costs and unamortized premium/(discount), 2017	9,491	12,304
Private Education Loans deferred origination costs and unamortized premium/(discount), 2016	7,960	8,564
Private Education Loans deferred origination costs and unamortized premium/(discount), 2015	9,859	5,153
Private Education Loans deferred origination costs and unamortized premium/(discount), Total	65,267	81,224
Total Private Education Loans, 2020	829,388	4,070,709
Total Private Education Loans, 2019	5,223,607	4,906,072
Total Private Education Loans, 2018	3,812,763	4,017,328
Total Private Education Loans, 2017	3,232,618	3,446,162
Total Private Education Loans, 2016	2,792,510	2,829,736
Total Private Education Loans, 2015 and Prior	5,801,128	4,000,808
Total Private Education Loans	21,692,014	23,270,815
Private Education Loans allowance for losses, 2020	59,208	3,013
Private Education Loans allowance for losses, 2019	369,890	19,105
Private Education Loans allowance for losses, 2018	275,259	44,858
Private Education Loans allowance for losses, 2017	234,085	71,598
Private Education Loans allowance for losses, 2016	195,324	80,974
Private Education Loans allowance for losses, 2015 and prior	382,015	154,752
Private Education Loans allowance for losses	1,515,781	374,300
Private Education Loans, net, 2020	770,180	4,067,696
Private Education Loans, net, 2019	4,853,717	4,886,967
Private Education Loans, net, 2018	3,537,504	3,972,470
Private Education Loans, net, 2017	2,998,533	3,374,564
Private Education Loans, net, 2016	2,597,186	2,748,762
Private Education Loans, net, 2015 and prior	5,419,113	3,846,056
Loans held for investment, net	20,176,233	22,896,515
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, 2015 and prior	0	69
Financing Receivable, Allowance for Credit Loss, Recovery, Originated In Fiscal Year Before Latest Fiscal Year	92	1,016
Current period recoveries, 2018	599	2,622
Financing Receivable, Allowance for Credit Loss, Recovery, Originated Three Years Before Current Fiscal Year	1,080	4,431
Financing Receivable, Allowance for Credit Loss, Writeoff, after Recovery, Originated In Current Fiscal Year	0	1,628
Financing Receivable, Allowance for Credit Loss, Writeoff, after Recovery, Originated In Fiscal Year Before Latest Fiscal Year	1,505	13,634
Financing Receivable, Allowance for Credit Loss, Writeoff, after Recovery, Originated Two Years Before Current Fiscal Year	4,642	26,497
Financing Receivable, Allowance for Credit Loss, Writeoff, after Recovery, Originated Three Years Before Current Fiscal Year	7,056	36,145
Current period gross charge-offs, 2016	10,284	41,141
Financing Receivable, Allowance for Credit Loss, Recovery, Originated Four Years Before Current Fiscal Year	1,505	5,175
Financing Receivable, Allowance for Credit Loss, Writeoff, after Recovery, Originated Four Years Before Current Fiscal Year	8,779	35,966
Student Loan | Consumer Portfolio Segment | Loan delinquency 31-60 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	1,524
2019	15,771	11,152
2018	27,690	26,096
2017	40,349	44,382
2016	43,630	51,656
2015 and Prior	125,661	54,559
2014 and Prior		100,206
Total	254,625	288,051
Student Loan | Consumer Portfolio Segment | Loan delinquency 61-90
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0
2019	4,760	3,087
2018	15,016	9,527
2017	22,286	17,048
2016	24,645	21,161
2015 and Prior	69,189	24,562
2014 and Prior		45,917
Total	135,896	121,302
Student Loan | Consumer Portfolio Segment | Loan delinquency greater than 90 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0
2019	1,983	935
2018	8,952	3,850
2017	13,259	7,818
2016	15,260	12,314
2015 and Prior	42,159	12,946
2014 and Prior		24,803
Total	81,613	62,666
Student Loan | Loans In-School/Grace/Deferment | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	364,100
2019	2,109,092	1,670,644
2018	1,181,737	1,580,513
2017	804,181	1,010,764
2016	515,606	635,798
2015 and Prior	673,761	375,585
2014 and Prior		414,101
Total	5,648,477	5,687,405
Student Loan | Loans In Forbearance | Consumer Portfolio Segment
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
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	899,704	1,049,007
Private Education Loans allowance for losses	152,673	65,877	$ 65,877
Loans held for investment, net	747,444	983,643
Personal Loans | Consumer Portfolio Segment
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
Total Private Education Loans in repayment, 2020	40	404,776
Total Private Education Loans in repayment, 2019	352,469	540,784
Total Private Education Loans in repayment, 2018	448,556	102,464
Total Private Education Loans in repayment, 2017	78,335	983
Total Private Education Loans in repayment, 2016	787
Total Private Education Loans in repayment	880,187	1,049,007
Total Private Education Loans, gross, 2020	40	404,776
Total Private Education Loans, gross, 2019	364,849	540,784
Total Private Education Loans, gross, 2018	455,693	102,464
Total Private Education Loans, gross, 2017	78,335	983
Total Private Education Loans, gross, 2016	787
Total Private Education Loans, gross	899,704	1,049,007
Private Education Loans deferred origination costs and unamortized premium/(discount), 2020	0	380
Private Education Loans deferred origination costs and unamortized premium/(discount), 2019	335	133
Private Education Loans deferred origination costs and unamortized premium/(discount), 2018	78	0
Private Education Loans deferred origination costs and unamortized premium/(discount), 2017	0	0
Private Education Loans deferred origination costs and unamortized premium/(discount), 2016	0
Private Education Loans deferred origination costs and unamortized premium/(discount), Total	413	513
Total Private Education Loans, 2020	40	405,156
Total Private Education Loans, 2019	365,184	540,917
Total Private Education Loans, 2018	455,771	102,464
Total Private Education Loans, 2017	78,335	983
Total Private Education Loans, 2016	787
Total Private Education Loans	900,117	1,049,520
Private Education Loans allowance for losses, 2020	6	21,589
Private Education Loans allowance for losses, 2019	51,280	37,492
Private Education Loans allowance for losses, 2018	89,194	6,722
Private Education Loans allowance for losses, 2017	12,110	74
Private Education Loans allowance for losses, 2016	83
Private Education Loans allowance for losses	152,673	65,877
Private Education Loans, net, 2020	34	383,567
Private Education Loans, net, 2019	313,904	503,425
Private Education Loans, net, 2018	366,577	95,742
Private Education Loans, net, 2017	66,225	909
Private Education Loans, net, 2016	704
Loans held for investment, net	$ 747,444	$ 983,643
Percentage of Private Education Loans in repayment, 2020	0.00%	1.10%
Percentage of Private Education Loans in repayment, 2019	1.80%	3.30%
Percentage of Private Education Loans in repayment, 2018	3.30%	2.80%
Percentage of Private Education Loans in repayment, 2017	2.70%	4.20%
Percentage of Private Education Loans in repayment, 2016	0.00%
Percentage of Private Education Loans in repayment	2.60%	2.40%
Loans current, percentage	97.40%	97.60%
Loans In Repayment In Percentage	100.00%	100.00%
Loans current		$ 1,023,517
Personal Loans | Consumer Portfolio Segment | Loan delinquency 31-60 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 0
2019	3,321	2,164
2018	7,448	6,213
2017	810	1,045
2016	0	$ 13
2015 and Prior	$ 11,579
Loans delinquent	1.30%	0.90%
Loans delinquent		$ 9,435
Personal Loans | Consumer Portfolio Segment | Loan delinquency 61-90
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 0
2019	991	1,074
2018	2,663	5,148
2017	701	943
2016	0	$ 7
2015 and Prior	$ 4,355
Loans delinquent	0.50%	0.70%
Loans delinquent		$ 7,172
Personal Loans | Consumer Portfolio Segment | Loan delinquency greater than 90 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 0
2019	2,034	1,322
2018	4,781	6,645
2017	636	895
2016	0	$ 21
2015 and Prior	$ 7,451
Loans delinquent	0.80%	0.80%
Loans delinquent		$ 8,883
Personal Loans | Loans In Forbearance | Consumer Portfolio Segment
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