Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,165,335)	$ (1,361,723)	$ (441,912)	$ (341,121)
Loans held for investment, net	$ 20,341,283	$ 19,183,143
Percentage of Private Education Loans in repayment, Year 1	59.00%	59.60%	58.20%
Percentage of Private Education Loans in repayment, Year 2	67.90%	64.70%	65.40%
Percentage of Private Education Loans in repayment, Year 3	74.30%	71.30%	71.20%
Percentage of Private Education Loans in repayment, Year 4	79.30%	75.30%	77.20%
Percentage of Private Education Loans in repayment, Year 5	82.90%	80.00%	82.20%
Percentage of Private Education Loans in repayment, Year 6	87.40%	84.70%	85.40%
Percentage of Private Education Loans in repayment	74.90%	72.50%	72.40%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 1	0.80%	0.50%	0.60%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 2	1.40%	1.20%	1.20%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3	2.90%	2.40%	2.40%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4	3.70%	3.20%	3.20%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5	4.30%	3.80%	4.00%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 6	5.20%	4.80%	5.00%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, total	3.30%	2.80%	2.80%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 1	0.50%	0.80%	0.90%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 2	1.90%	3.40%	3.30%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3	2.10%	5.00%	4.80%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 4	2.30%	5.90%	5.20%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 5	2.50%	5.90%	5.20%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 6	2.10%	4.90%	4.70%
Loans in forbearance as a percentage of loans in repayment and forbearance	1.90%	4.30%	4.10%
Student Loan | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in forbearance, Year 1	$ 2,253,860	$ 2,052,521	$ 2,355,395
Total Private Education Loans in forbearance, Year 2	2,826,916	2,603,996	3,199,351
Total Private Education Loans in forbearance, Year 3	2,390,841	2,096,368	2,850,380
Total Private Education Loans in forbearance, Year 4	1,840,790	1,912,370	2,651,946
Total Private Education Loans in forbearance, Year 5	1,640,870	1,752,386	2,317,788
Total Private Education Loans in forbearance, Year 6	4,557,935	3,887,180	3,412,810
Private Education Loans in forbearance, total	15,511,212	14,304,821	16,787,670
Total Private Education Loans, gross, Year 1	3,822,361	3,442,765	4,047,048
Total Private Education Loans, gross, Year 2	4,166,283	4,026,848	4,888,373
Total Private Education Loans, gross, Year 3	3,216,525	2,940,511	4,003,485
Total Private Education Loans, gross, Year 4	2,320,060	2,539,794	3,433,858
Total Private Education Loans, gross, Year 5	1,978,233	2,189,222	2,821,172
Total Private Education Loans, gross, Year 6	5,213,401	4,590,197	3,995,655
Total Private Education Loans, gross	20,716,863	19,729,337	23,189,591
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1	22,169	21,129	23,661
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2	16,067	13,933	17,699
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3	9,575	8,671	13,843
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4	5,918	6,708	12,304
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5	4,588	5,721	8,564
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 6	9,171	7,313	5,153
Private Education Loans deferred origination costs and unamortized premium/(discount), Total	67,488	63,475	81,224
Total Private Education Loans, Year 1	3,844,530	3,463,894	4,070,709
Total Private Education Loans, Year 2	4,182,350	4,040,781	4,906,072
Total Private Education Loans, Year 3	3,226,100	2,949,182	4,017,328
Total Private Education Loans, Year 4	2,325,978	2,546,502	3,446,162
Total Private Education Loans, Year 5	1,982,821	2,194,943	2,829,736
Total Private Education Loans, Year 6	5,222,572	4,597,510	4,000,808
Total Private Education Loans	20,784,351	19,792,812	23,270,815
Private Education Loans allowance for losses, Year 1	(248,102)	(210,875)	(3,013)
Private Education Loans allowance for losses, Year 2	(239,507)	(298,776)	(19,105)
Private Education Loans allowance for losses, Year 3	(195,223)	(218,136)	(44,858)
Private Education Loans allowance for losses, Year 4	(129,678)	(184,265)	(71,598)
Private Education Loans allowance for losses, Year 5	(99,982)	(150,150)	(80,974)
Private Education Loans allowance for losses, Year 6	(246,485)	(293,642)	(154,752)
Private Education Loans allowance for losses	(1,158,977)	(1,355,844)	(374,300)
Private Education Loans, net, Year 1	3,596,428	3,253,019	4,067,696
Private Education Loans, net, Year 2	3,942,843	3,742,005	4,886,967
Private Education Loans, net, Year 3	3,030,877	2,731,046	3,972,470
Private Education Loans, net, Year 4	2,196,300	2,362,237	3,374,564
Private Education Loans, net, Year 5	1,882,839	2,044,793	2,748,762
Private Education Loans, net, Year 6	4,976,087	4,303,868	3,846,056
Loans held for investment, net	19,625,374	18,436,968	22,896,515
Student Loan | Consumer Portfolio Segment | Loan delinquent, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	2,234,876	2,043,033	2,340,221
Year 2	2,786,646	2,573,228	3,159,878
Year 3	2,321,728	2,045,012	2,781,132
Year 4	1,772,651	1,850,539	2,566,815
Year 5	1,570,815	1,685,572	2,225,721
Year 6	4,319,057	3,701,564	3,241,884
Total	15,005,773	13,898,948	16,315,651
Student Loan | Consumer Portfolio Segment | Loan delinquent 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	15,148	6,400	11,152
Year 2	29,146	16,983	26,096
Year 3	46,616	26,934	44,382
Year 4	43,197	30,771	51,656
Year 5	41,695	33,040	54,559
Year 6	132,757	91,400	100,206
Total	308,559	205,528	288,051
Student Loan | Consumer Portfolio Segment | Loan delinquent 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	3,194	2,628	3,087
Year 2	7,441	9,143	9,527
Year 3	14,044	15,026	17,048
Year 4	14,310	18,121	21,161
Year 5	16,425	19,064	24,562
Year 6	61,533	55,661	45,917
Total	116,947	119,643	121,302
Student Loan | Consumer Portfolio Segment | Loan delinquent 90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	642	460	935
Year 2	3,683	4,642	3,850
Year 3	8,453	9,396	7,818
Year 4	10,632	12,939	12,314
Year 5	11,935	14,710	12,946
Year 6	44,588	38,555	24,803
Total	79,933	80,702	62,666
Student Loan | Loans In-School/Grace/Deferment | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,556,550	1,374,085	1,670,644
Year 2	1,283,523	1,330,175	1,580,513
Year 3	773,320	733,824	1,010,764
Year 4	435,657	508,478	635,798
Year 5	296,008	327,763	375,585
Year 6	559,356	504,715	414,101
Total	4,904,414	4,779,040	5,687,405
Student Loan | Loans In Forbearance | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in forbearance, Year 1	11,951	16,159	21,009
Total Private Education Loans in forbearance, Year 2	55,844	92,677	108,509
Total Private Education Loans in forbearance, Year 3	52,364	110,319	142,341
Total Private Education Loans in forbearance, Year 4	43,613	118,946	146,114
Total Private Education Loans in forbearance, Year 5	41,355	109,073	127,799
Total Private Education Loans in forbearance, Year 6	96,110	198,302	168,744
Private Education Loans in forbearance, total	301,237	645,476	714,516
Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses			$ (65,877)
Student Loan, Borrowers Who Did Not Return To School And Received Extenstion | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in forbearance, Year 1	$ 609,000
Student Loan, Borrowers Who Did Not Return To School And Received Extenstion | Loans In-School/Grace/Deferment | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1		401,000
Student Loan, Borrowers Who Did Not Return To School And Received Extenstion | Loans In Forbearance | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in forbearance, Year 1		$ 30,000