Allowance for Credit Losses - Age Analysis of Past Due Loans Delinquencies (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans allowance for losses	$ (1,416,048)	$ (1,364,716)	$ (1,357,075)	$ (1,194,238)	$ (1,081,066)	$ (1,165,335)
Private Education Loans, net	$ 20,899,181		$ 19,626,868
Percentage of Private Education Loans in repayment, Year 1	53.90%		56.90%
Percentage of Private Education Loans in repayment, Year 2	60.40%		67.10%
Percentage of Private Education Loans in repayment, Year 3	71.60%		76.10%
Percentage of Private Education Loans in repayment, Year 4	78.30%		80.70%
Percentage of Private Education Loans in repayment, Year 5	82.50%		84.50%
Percentage of Private Education Loans in repayment, After Year 5	88.60%		89.30%
Percentage of Private Education Loans in repayment	71.50%		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	1.70%		1.90%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 3	3.40%		3.10%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 4	3.70%		4.00%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, Year 5	4.20%		4.70%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment, After Year 5	6.30%		6.50%
Delinquent Private Education Loans in repayment as a percentage of Private Education Loans in repayment	3.70%		3.80%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 1	0.20%		0.70%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 2	0.90%		2.10%
Loans in forbearance as a percentage of loans in repayment and forbearance, Year 3	1.20%		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	1.70%		2.20%
Loans in forbearance as a percentage of loans in repayment and forbearance, After Year 5	2.10%		2.00%
Loans in forbearance as a percentage of loans in repayment and forbearance	1.40%		1.80%
Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Private Education Loans in Repayment, Year 1	$ 2,012,005		$ 2,432,838
Total Private Education Loans in Repayment, Year 2	3,273,815		3,049,477
Total Private Education Loans in Repayment, Year 3	2,408,846		1,967,756
Total Private Education Loans in Repayment, Year 4	1,559,204		1,754,219
Total Private Education Loans in Repayment, Year 5	1,443,549		1,365,979
Total Private Education Loans in Repayment, After Year 5	4,807,726		4,559,281
Total Private Education Loans in repayment	15,505,145		15,129,550
Total Private Education Loans, gross, Year 1	3,730,485		4,276,533
Total Private Education Loans, gross, Year 2	5,421,538		4,547,159
Total Private Education Loans, gross, Year 3	3,363,703		2,584,622
Total Private Education Loans, gross, Year 4	1,991,433		2,172,660
Total Private Education Loans, gross, Year 5	1,749,067		1,615,842
Total Private Education Loans, gross, After Year 5	5,424,641		5,106,872
Total Private Education Loans, gross	21,680,867		20,303,688
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 1	26,268		26,714
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 2	21,367		15,933
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 3	11,090		9,062
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 4	6,555		5,496
Private Education Loans deferred origination costs and unamortized premium/(discount), Year 5	3,993		3,575
Private Education Loans deferred origination costs and unamortized premium/(discount), After Year 5	9,400		8,876
Private Education Loans deferred origination costs and unamortized premium/(discount)	78,673		69,656
Total Private Education Loans, Year 1	3,756,753		4,303,247
Total Private Education Loans, Year 2	5,442,905		4,563,092
Total Private Education Loans, Year 3	3,374,793		2,593,684
Total Private Education Loans, Year 4	1,997,988		2,178,156
Total Private Education Loans, Year 5	1,753,060		1,619,417
Total Private Education Loans, After Year 5	5,434,041		5,115,748
Total Private Education Loans	21,759,540		20,373,344
Private Education Loans allowance for losses, Year 1	(220,113)		(304,943)
Private Education Loans allowance for losses, Year 2	(361,902)		(323,506)
Private Education Loans allowance for losses, Year 3	(227,021)		(181,915)
Private Education Loans allowance for losses, Year 4	(134,487)		(141,424)
Private Education Loans allowance for losses, Year 5	(111,946)		(101,023)
Private Education Loans allowance for losses, After Year 5	(355,763)		(300,820)
Private Education Loans allowance for losses	(1,411,232)		(1,353,631)
Private Education Loans, net, Year 1	3,536,640		3,998,304
Private Education Loans, net, Year 2	5,081,003		4,239,586
Private Education Loans, net, Year 3	3,147,772		2,411,769
Private Education Loans, net, Year 4	1,863,501		2,036,732
Private Education Loans, net, Year 5	1,641,114		1,518,394
Private Education Loans, net, After Year 5	5,078,278		4,814,928
Private Education Loans, net	20,348,308		19,019,713
Consumer Portfolio Segment | Student Loan | Loan delinquent, current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	2,005,363		2,411,441
Year 2	3,218,301		2,991,839
Year 3	2,327,629		1,907,574
Year 4	1,500,801		1,683,986
Year 5	1,382,249		1,301,809
After Year 5	4,504,119		4,262,698
Loan type	14,938,462		14,559,347
Consumer Portfolio Segment | Student Loan | Loan delinquent 30-59 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	4,272		14,164
Year 2	32,282		30,740
Year 3	41,945		30,877
Year 4	28,901		35,213
Year 5	30,468		31,366
After Year 5	145,753		144,948
Loan type	283,621		287,308
Consumer Portfolio Segment | Student Loan | Loan delinquent 60-89 days
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,788		5,523
Year 2	15,013		15,056
Year 3	22,612		14,433
Year 4	16,566		18,201
Year 5	16,311		16,697
After Year 5	81,159		77,595
Loan type	153,449		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	582		1,710
Year 2	8,219		11,842
Year 3	16,660		14,872
Year 4	12,936		16,819
Year 5	14,521		16,107
After Year 5	76,695		74,040
Loan type	129,613		135,390
Loans In-School/Grace/Deferment | Consumer Portfolio Segment | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Year 1	1,713,896		1,827,649
Year 2	2,118,976		1,433,322
Year 3	925,728		578,253
Year 4	410,375		380,639
Year 5	280,963		219,280
After Year 5	511,941		455,910
Loan type	5,961,879		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	4,584		16,046
Total Private Education Loans in forbearance, Year 2	28,747		64,360
Total Private Education Loans in forbearance, Year 3	29,129		38,613
Total Private Education Loans in forbearance, Year 4	21,854		37,802
Total Private Education Loans in forbearance, Year 5	24,555		30,583
Total Private Education Loans in forbearance, After Year 5	104,974		91,681
Total Private Education Loans in forbearance	$ 213,843		$ 279,085