Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,361,723)	$ (441,912)	$ (341,121)	$ (251,475)
Loans held for investment, net	$ 19,183,143	$ 24,667,792
Percentage of Private Education Loans in repayment, 2013 and prior			82.30%
Percentage of Private Education Loans in repayment, 2020	59.60%
Percentage of Private Education Loans in repayment, 2019	64.70%	58.20%
Percentage of Private Education Loans in repayment, 2018	71.30%	65.40%	58.20%
Percentage of Private Education Loans in repayment, 2017	75.30%	71.20%	64.00%
Percentage of Private Education Loans in repayment, 2016	80.00%	77.20%	72.00%
Percentage of Private Education Loans in repayment, 2014 and prior	84.70%	85.40%	78.30%
Percentage of Private Education Loans in repayment	72.50%	72.40%	71.50%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2020	0.50%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2019	1.20%	0.60%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2018	2.40%	1.20%	0.60%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2017	3.20%	2.40%	1.20%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2016	3.80%	3.20%	2.30%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2015 and prior	4.80%	4.00%	3.10%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2014 and prior		5.00%	3.80%
Delinquencies As Percentage Of Loans In Repayment, Originated Six Years Before Current Fiscal YearDelinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, 2013 and prior			5.10%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, total	2.80%	2.80%	2.60%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2020	0.80%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2019	3.40%	0.90%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2018	5.00%	3.30%	0.60%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2017	5.90%	4.80%	3.20%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2016	5.90%	5.20%	4.30%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2015		5.20%	4.80%
Loans in forbearance as a percentage of loans in repayment and forbearance, 2014 and prior	4.90%	4.70%	5.00%
Loans in forbearance as a percentage of loans in repayment and forbearance	4.30%	4.10%	3.80%
Student Loan | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	$ 2,043,033
2019	2,573,228	$ 2,340,221
2018	2,045,012	3,159,878	$ 2,233,102
2017	1,850,539	2,781,132	2,800,142
2016	1,685,572	2,566,815	2,769,458
2015 and Prior	3,701,564	2,225,721	2,533,550
2014 and Prior		3,241,884	1,865,094
2013 and Prior			2,088,359
Total	13,898,948	16,315,651	14,289,705
Loans In Repayment, Originated Seven Or More Years Before Current Fiscal Year			2,199,514
Total Private Education Loans in forbearance, 2019	2,603,996	2,355,395
Total Private Education Loans in forbearance, 2018	2,096,368	3,199,351	2,247,526
Total Private Education Loans in forbearance, 2017	1,912,370	2,850,380	2,832,910
Total Private Education Loans in forbearance, 2016	1,752,386	2,651,946	2,835,181
Total Private Education Loans in forbearance, 2015 and Prior	3,887,180	2,317,788	2,613,796
Total Private Education Loans in repayment, 2014 and prior		3,412,810	1,937,929
Private Education Loans in forbearance, total	14,304,821	16,787,670	14,666,856
Total Private Education Loans, gross, 2020	3,442,765
Total Private Education Loans, gross, 2019	4,026,848	4,047,048
Total Private Education Loans, gross, 2018	2,940,511	4,888,373	3,864,545
Total Private Education Loans, gross, 2017	2,539,794	4,003,485	4,423,883
Total Private Education Loans, gross, 2016	2,189,222	3,433,858	3,938,366
Total Private Education Loans, gross, 2015		2,821,172	3,336,377
Total Private Education Loans, gross, 2014			2,353,607
Total Private Education Loans, gross, 2013			2,587,687
Total Private Education Loans, gross	19,729,337	23,189,591	20,504,465
Private Education Loans deferred origination costs and unamortized premium/(discount), 2020	21,129
Private Education Loans deferred origination costs and unamortized premium/(discount), 2019	13,933	23,661
Private Education Loans deferred origination costs and unamortized premium/(discount), 2018	8,671	17,699	20,433
Private Education Loans deferred origination costs and unamortized premium/(discount), 2017	6,708	13,843	16,362
Private Education Loans deferred origination costs and unamortized premium/(discount), 2016	5,721	12,304	14,747
Private Education Loans deferred origination costs and unamortized premium/(discount), 2015		8,564	10,422
Private Education Loans deferred origination costs and unamortized premium/(discount), 2014	7,313	5,153	4,992
Private Education Loans deferred origination costs and unamortized premium/(discount), 2013			1,365
Private Education Loans deferred origination costs and unamortized premium/(discount), Total	63,475	81,224	68,321
Total Private Education Loans, 2020	3,463,894
Total Private Education Loans, 2019	4,040,781	4,070,709
Total Private Education Loans, 2018	2,949,182	4,906,072	3,884,978
Total Private Education Loans, 2017	2,546,502	4,017,328	4,440,245
Total Private Education Loans, 2016	2,194,943	3,446,162	3,953,113
Total Private Education Loans, 2015		2,829,736	3,346,799
Total Private Education Loans, 2014 and Prior	4,597,510	4,000,808	2,358,599
Total Private Education Loans, 2013 and Prior			2,589,052
Loans, Before Allowance For Credit Loss	19,792,812	23,270,815	20,572,786
Financing Receivable, Allowance For Credit Loss, Originated Six Or More Years Before Current Fiscal Year			90,041
Private Education Loans allowance for losses, 2020	(210,875)
Private Education Loans allowance for losses, 2019	(298,776)	(3,013)
Private Education Loans allowance for losses, 2018	(218,136)	(19,105)	(2,670)
Private Education Loans allowance for losses, 2017	(184,265)	(44,858)	(15,469)
Private Education Loans allowance for losses, 2016	(150,150)	(71,598)	(43,469)
Private Education Loans allowance for losses, 2015		80,974	63,442
Private Education Loans allowance for losses, 2015 and prior	(293,642)	(154,752)	(62,852)
Private Education Loans allowance for losses	(1,355,844)	(374,300)	(277,943)
Financing Receivable, after Allowance for Credit Loss, Originated Six Or More Years Before Current Fiscal Year			2,499,011
Private Education Loans, net, 2020	3,253,019
Private Education Loans, net, 2019	3,742,005	4,067,696
Private Education Loans, net, 2018	2,731,046	4,886,967	3,882,308
Private Education Loans, net, 2017	2,362,237	3,972,470	4,424,776
Private Education Loans, net, 2016	2,044,793	3,374,564	3,909,644
Private Education Loans, net, 2015		2,748,762	3,283,357
Private Education Loans, net, 2014 and prior	4,303,868	3,846,056	2,295,747
Loans held for investment, net	18,436,968	22,896,515	20,294,843
Student Loan | Consumer Portfolio Segment | Loans delinquent 31-60 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	6,400
2019	16,983	11,152
2018	26,934	26,096	10,263
2017	30,771	44,382	22,219
2016	33,040	51,656	41,788
2015 and Prior	91,400	54,559	49,762
2014 and Prior		100,206	43,291
2013 and Prior			63,893
Total	205,528	288,051	231,216
Student Loan | Consumer Portfolio Segment | Loans delinquent 61-90 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	2,628
2019	9,143	3,087
2018	15,026	9,527	3,521
2017	18,121	17,048	7,227
2016	19,064	21,161	15,893
2015 and Prior	55,661	24,562	19,935
2014 and Prior		45,917	18,561
2013 and Prior			29,968
Total	119,643	121,302	95,105
Student Loan | Consumer Portfolio Segment | Loans delinquent greater than 90 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	460
2019	4,642	935
2018	9,396	3,850	640
2017	12,939	7,818	3,322
2016	14,710	12,314	8,042
2015 and Prior	38,555	12,946	10,549
2014 and Prior		24,803	10,983
2013 and Prior			17,294
Total	80,702	62,666	50,830
Student Loan | Loans In-School/Grace/Deferment | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
2020	1,374,085
2019	1,330,175	1,670,644
2018	733,824	1,580,513	1,602,476
2017	508,478	1,010,764	1,497,955
2016	327,763	635,798	975,796
2015 and Prior	504,715	375,585	589,558
2014 and Prior		414,101	314,393
2013 and Prior			280,267
Total	4,779,040	5,687,405	5,260,445
Student Loan | Loans In Forbearance | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in forbearance, 2020	16,159
Total Private Education Loans in forbearance, 2019		21,009
Total Private Education Loans in forbearance, 2018		108,509
Total Private Education Loans in forbearance, 2017		142,341
Total Private Education Loans in forbearance, 2016		146,114
Total Private Education Loans in forbearance, 2015 and Prior		127,799
Total Private Education Loans in repayment, 2014 and prior		168,744
Private Education Loans in forbearance, total		714,516
Total Private Education Loans in forbearance, 2018	110,319		14,543
Total Private Education Loans in forbearance, 2017	118,946		93,018
Total Private Education Loans in forbearance, 2016	109,073		127,389
Private Education Loans in forbearance, total	645,476		577,164
Total Private Education Loans in forbearance, 2015 and Prior	198,302		$ 133,023
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	1,049,007
Private Education Loans allowance for losses	0	(65,877)
Loans held for investment, net	$ 0	$ 983,643