Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,479,306)	$ (1,357,075)	$ (1,227,362)	$ (1,165,335)
Loans held for investment, net	$ 21,087,563	$ 19,626,868
Percentage of Private Education Loans in repayment, Year 1	51.90%	56.90%
Percentage of Private Education Loans in repayment, Year 2	57.90%	67.10%
Percentage of Private Education Loans in repayment, Year 3	68.70%	76.10%
Percentage of Private Education Loans in repayment, Year 4	77.10%	80.70%
Percentage of Private Education Loans in repayment, Year 5	81.40%	84.50%
Percentage of Private Education Loans in repayment, After Year 5	88.10%	89.30%
Percentage of Private Education Loans in repayment	73.00%	74.50%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 1	0.30%	0.90%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 2	0.90%	1.90%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3	2.70%	3.10%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4	3.20%	4.00%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5	3.90%	4.70%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, After Year 5	5.50%	6.50%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, total	3.40%	3.80%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 1	0.30%	0.70%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 2	0.60%	2.10%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3	1.40%	1.90%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 4	1.40%	2.10%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 5	1.60%	2.20%
Loans in forbearance as a percentage of loans in repayment and forbearance, After Year 5	1.80%	2.00%
Loans in forbearance as a percentage of loans in repayment and forbearance	1.40%	1.80%
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,475,379)	$ (1,353,631)	$ (1,221,053)	$ (1,158,977)
Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in forbearance, Year 1	442,307	2,432,838
Total Private Education Loans in forbearance, Year 2	3,292,806	3,049,477
Total Private Education Loans in forbearance, Year 3	3,054,063	1,967,756
Total Private Education Loans in forbearance, Year 4	1,920,515	1,754,219
Total Private Education Loans in forbearance, Year 5	1,699,601	1,365,979
Total Private Education Loans in forbearance, After Year 5	5,581,167	4,559,281
Private Education Loans in forbearance, total	15,990,459	15,129,550
Total Private Education Loans, gross, Year 1	852,405	4,276,533
Total Private Education Loans, gross, Year 2	5,688,118	4,547,159
Total Private Education Loans, gross, Year 3	4,444,654	2,584,622
Total Private Education Loans, gross, Year 4	2,491,251	2,172,660
Total Private Education Loans, gross, Year 5	2,087,454	1,615,842
Total Private Education Loans, gross, After Year 5	6,334,121	5,106,872
Total Private Education Loans, gross	21,898,003	20,303,688
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1	5,089	26,714
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2	29,718	15,933
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3	15,151	9,062
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4	8,476	5,496
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5	5,109	3,575
Private Education Loans deferred origination costs and unamortized premium/(discount), After Year 5	11,508	8,876
Private Education Loans deferred origination costs and unamortized premium/(discount), Total	75,051	69,656
Total Private Education Loans, Year 1	857,494	4,303,247
Total Private Education Loans, Year 2	5,717,836	4,563,092
Total Private Education Loans, Year 3	4,459,805	2,593,684
Total Private Education Loans, Year 4	2,499,727	2,178,156
Total Private Education Loans, Year 5	2,092,563	1,619,417
Total Private Education Loans, After Year 5	6,345,629	5,115,748
Total Private Education Loans	21,973,054	20,373,344
Private Education Loans allowance for losses, Year 1	(61,767)	(304,943)
Private Education Loans allowance for losses, Year 2	(372,947)	(323,506)
Private Education Loans allowance for losses, Year 3	(296,851)	(181,915)
Private Education Loans allowance for losses, Year 4	(170,497)	(141,424)
Private Education Loans allowance for losses, Year 5	(138,625)	(101,023)
Private Education Loans allowance for losses, After Year 5	(434,692)	(300,820)
Private Education Loans allowance for losses	(1,475,379)	(1,353,631)
Private Education Loans, net, Year 1	795,727	3,998,304
Private Education Loans, net, Year 2	5,344,889	4,239,586
Private Education Loans, net, Year 3	4,162,954	2,411,769
Private Education Loans, net, Year 4	2,329,230	2,036,732
Private Education Loans, net, Year 5	1,953,938	1,518,394
Private Education Loans, net, After Year 5	5,910,937	4,814,928
Loans held for investment, net	20,497,675	19,019,713
Consumer Portfolio Segment | Student Loan | Loan delinquent, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	441,192	2,411,441
Year 2	3,263,917	2,991,839
Year 3	2,972,969	1,907,574
Year 4	1,859,452	1,683,986
Year 5	1,632,668	1,301,809
After Year 5	5,275,984	4,262,698
Loan type	15,446,182	14,559,347
Consumer Portfolio Segment | Student Loan | Loan delinquent 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,115	14,164
Year 2	17,902	30,740
Year 3	38,446	30,877
Year 4	28,762	35,213
Year 5	31,442	31,366
After Year 5	149,333	144,948
Loan type	267,000	287,308
Consumer Portfolio Segment | Student Loan | Loan delinquent 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	0	5,523
Year 2	6,395	15,056
Year 3	22,104	14,433
Year 4	17,136	18,201
Year 5	18,428	16,697
After Year 5	76,723	77,595
Loan type	140,786	147,505
Consumer Portfolio Segment | Student Loan | Loan delinquent 90 days or greater past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	0	1,710
Year 2	4,592	11,842
Year 3	20,544	14,872
Year 4	15,165	16,819
Year 5	17,063	16,107
After Year 5	79,127	74,040
Loan type	136,491	135,390
Loans In-School/Grace/Deferment | Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	408,842	1,827,649
Year 2	2,374,557	1,433,322
Year 3	1,348,206	578,253
Year 4	543,621	380,639
Year 5	359,759	219,280
After Year 5	651,401	455,910
Loan type	5,686,386	4,895,053
Loans In Forbearance | Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in forbearance, Year 1	1,256	16,046
Total Private Education Loans in forbearance, Year 2	20,755	64,360
Total Private Education Loans in forbearance, Year 3	42,385	38,613
Total Private Education Loans in forbearance, Year 4	27,115	37,802
Total Private Education Loans in forbearance, Year 5	28,094	30,583
Total Private Education Loans in forbearance, After Year 5	101,553	91,681
Total Private Education Loans in forbearance	$ 221,158	$ 279,085