Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,734,559)	$ (1,929,323)	$ (441,912)	$ (441,912)	$ (414,406)	$ (383,997)	$ (341,121)
Loans held for investment, net	$ 21,709,771			$ 24,667,792
Percentage of Private Education Loans in repayment, 2020	58.20%			58.20%
Percentage of Private Education Loans in repayment, 2019	60.20%			65.40%
Percentage of Private Education Loans in repayment, 2018	67.60%			71.20%
Percentage of Private Education Loans in repayment, 2017	71.80%			77.20%
Percentage of Private Education Loans in repayment, 2016	77.40%			82.20%
Percentage of Private Education Loans in repayment, 2015 and prior	83.60%			85.40%
Percentage of Private Education Loans in repayment	70.10%			72.40%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2020	0.30%			0.60%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2019	1.20%			1.20%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2018	2.60%			2.40%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2017	3.50%			3.20%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2016	4.00%			4.00%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2015 and prior	4.90%			5.00%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment	3.00%			2.80%
Delinquencies as a percentage of Private Education Loans in repayment and delinquent forbearance loans, 2020	0.30%			0.60%
Delinquencies as a percentage of Private Education Loans in repayment and delinquent forbearance loans, 2019	1.20%			1.20%
Delinquencies as a percentage of Private Education Loans in repayment and delinquent forbearance loans, 2018	2.60%			2.40%
Delinquencies as a percentage of Private Education Loans in repayment and delinquent forbearance loans, 2017	3.50%			3.20%
Delinquencies as a percentage of Private Education Loans in repayment and delinquent forbearance loans, 2016	4.10%			4.00%
Delinquencies as a percentage of Private Education Loans in repayment and delinquent forbearance loans, 2015 and Prior	4.90%			5.00%
Delinquencies as a percentage of Private Education Loans in repayment and delinquent forbearance loans	3.00%			2.80%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2020	0.50%			0.90%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2019	2.00%			3.30%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2018	4.40%			4.80%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2017	5.60%			5.20%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2016	5.90%			5.20%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2015 and Prior	5.70%			4.70%
Loans in forbearance as a percentage of loans in repayment and forbearance	4.30%			4.10%
Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (114,778)	$ (85,958)			$ (1,967)	$ (755)
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0			$ 1,049,007
Private Education Loans allowance for losses	0		$ (65,877)	(65,877)
Loans held for investment, net	0			983,643
Student Loan | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	1,758,337
2019	3,031,984			2,340,221
2018	2,374,526			3,159,878
2017	2,092,242			2,781,132
2016	1,920,310			2,566,815
2015 and Prior	4,197,607			2,225,721
2014 and Prior				3,241,884
Total	15,375,006			16,315,651
Total Private Education Loans in repayment, 2019	3,068,154			2,355,395
Total Private Education Loans in repayment, 2018	2,437,078			3,199,351
Total Private Education Loans in repayment, 2017	2,169,127			2,850,380
Total Private Education Loans in repayment, 2016	2,001,314			2,651,946
Total Private Education Loans in repayment, 2015	4,413,952			2,317,788
Total Private Education Loans in repayment, 2014 and prior				3,412,810
Total Private Education Loans in repayment	15,853,309			16,787,670
Total Private Education Loans, gross, 2020	3,028,657			4,047,048
Total Private Education Loans, gross, 2019	5,095,700			4,888,373
Total Private Education Loans, gross, 2018	3,605,096			4,003,485
Total Private Education Loans, gross, 2017	3,019,169			3,433,858
Total Private Education Loans, gross, 2016	2,585,078			2,821,172
Total Private Education Loans, gross, 2014 and prior	5,280,627			3,995,655
Total Private Education Loans, gross	22,614,327			23,189,591
Private Education Loans deferred origination costs and unamortized premium/(discount), 2020	16,654			23,661
Private Education Loans deferred origination costs and unamortized premium/(discount), 2019	18,633			17,699
Private Education Loans deferred origination costs and unamortized premium/(discount), 2018	11,151			13,843
Private Education Loans deferred origination costs and unamortized premium/(discount), 2017	8,354			12,304
Private Education Loans deferred origination costs and unamortized premium/(discount), 2016	6,971			8,564
Private Education Loans deferred origination costs and unamortized premium/(discount), 2015	8,643			5,153
Private Education Loans deferred origination costs and unamortized premium/(discount), Total	70,406			81,224
Total Private Education Loans, 2020	3,045,311			4,070,709
Total Private Education Loans, 2019	5,114,333			4,906,072
Total Private Education Loans, 2018	3,616,247			4,017,328
Total Private Education Loans, 2017	3,027,523			3,446,162
Total Private Education Loans, 2016	2,592,049			2,829,736
Total Private Education Loans, 2015 and Prior	5,289,270			4,000,808
Total Private Education Loans	22,684,733			23,270,815
Private Education Loans allowance for losses, 2020	(208,507)			(3,013)
Private Education Loans allowance for losses, 2019	(407,435)			(19,105)
Private Education Loans allowance for losses, 2018	(293,033)			(44,858)
Private Education Loans allowance for losses, 2017	(243,715)			(71,598)
Private Education Loans allowance for losses, 2016	(199,119)			(80,974)
Private Education Loans allowance for losses, 2015 and prior	(377,002)			(154,752)
Private Education Loans allowance for losses	(1,728,811)			(374,300)
Private Education Loans, net, 2020	2,836,804			4,067,696
Private Education Loans, net, 2019	4,706,898			4,886,967
Private Education Loans, net, 2018	3,323,214			3,972,470
Private Education Loans, net, 2017	2,783,808			3,374,564
Private Education Loans, net, 2016	2,392,930			2,748,762
Private Education Loans, net, 2015 and prior	4,912,268			3,846,056
Loans held for investment, net	20,955,922			22,896,515
Student Loan | Consumer Portfolio Segment | Loan delinquency 31-60 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	4,209
2019	24,834			11,152
2018	37,444			26,096
2017	43,055			44,382
2016	43,560			51,656
2015 and Prior	112,149			54,559
2014 and Prior				100,206
Total	265,251			288,051
Student Loan | Consumer Portfolio Segment | Loan delinquency 61-90
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	828
2019	8,421			3,087
2018	17,289			9,527
2017	22,429			17,048
2016	24,515			21,161
2015 and Prior	66,341			24,562
2014 and Prior				45,917
Total	139,823			121,302
Student Loan | Consumer Portfolio Segment | Loan delinquency greater than 90 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	310
2019	2,915			935
2018	7,819			3,850
2017	11,401			7,818
2016	12,929			12,314
2015 and Prior	37,855			12,946
2014 and Prior				24,803
Total	73,229			62,666
Student Loan | Loans In-School/Grace/Deferment | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	1,256,630
2019	1,963,374			1,670,644
2018	1,057,069			1,580,513
2017	722,081			1,010,764
2016	457,475			635,798
2015 and Prior	598,932			375,585
2014 and Prior				414,101
Total	6,055,561			5,687,405
Student Loan | Loans In Forbearance | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	8,343
2019	64,167			21,009
2018	110,935			108,509
2017	127,915			142,341
2016	126,218			146,114
2015 and Prior	267,584			127,799
2014 and Prior				168,744
Total	705,162			714,516
Total Private Education Loans in forbearance, 2020	8,343
Total Private Education Loans in forbearance, 2019	64,172
Total Private Education Loans in forbearance, 2018	110,949
Total Private Education Loans in forbearance, 2017	127,961
Total Private Education Loans in forbearance, 2016	126,289
Total Private Education Loans in forbearance, 2015 and Prior	705,457
2015 and Prior	267,743
Total Private Education Loans in repayment, 2019				21,009
Total Private Education Loans in repayment, 2018				108,509
Total Private Education Loans in repayment, 2017				142,341
Total Private Education Loans in repayment, 2016				146,114
Total Private Education Loans in repayment, 2015				127,799
Total Private Education Loans in repayment, 2014 and prior				168,744
Total Private Education Loans in repayment				714,516
Student Loan | Loans In Forbearance | Consumer Portfolio Segment | Loan delinquency 31-60 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0
2019	5			0
2018	14			0
2017	25			0
2016	71			0
2015 and Prior	159			0
2014 and Prior				0
Total	274			0
Student Loan | Loans In Forbearance | Consumer Portfolio Segment | Loan delinquency 61-90
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0
2019	0			0
2018	0			0
2017	21			0
2016	0			0
2015 and Prior	0			0
2014 and Prior				0
Total	21			0
Student Loan | Loans In Forbearance | Consumer Portfolio Segment | Loan delinquency greater than 90 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	0
2019	0			0
2018	0			0
2017	0			0
2016	0			0
2015 and Prior	0			0
2014 and Prior				0
Total	$ 0			$ 0