Borrowings - Additional Information (Details) - USD ($)						1 Months Ended	9 Months Ended	12 Months Ended
	Aug. 18, 2021	Jul. 30, 2021	May 27, 2021	May 19, 2021	Feb. 09, 2021	Mar. 31, 2020	Sep. 30, 2021	Dec. 31, 2020
Line of Credit Facility [Line Items]
Short-term borrowings							$ 199,583,000	$ 0
Estimated weighted average life of student loans							4 years 8 months 12 days	5 years 4 months 24 days
Vertical risk retention interest						5.00%	5.00%
Uncommitted federal funds							$ 125,000,000
Lendable value of collateral							2,700,000,000	$ 3,800,000,000
2021-B
Line of Credit Facility [Line Items]
Principal Amount Outstanding on Loans Held-for-sale or Securitization or Asset-backed Financing Arrangement				$ 531,000,000
Loans sold to third parties				$ 531,000,000
Ownership interest percentage				100.00%
Proceeds from Issuance of Unsecured Debt				$ 529,000,000
Estimated weighted average life of student loans				4 years 3 months 3 days
Loans pledged as collateral							521,000,000
Principal amount of collateral							484,000,000
Capitalized interest							37,000,000
2021-B | London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Basis spread on variable rate				0.77%
2021-D
Line of Credit Facility [Line Items]
Principal Amount Outstanding on Loans Held-for-sale or Securitization or Asset-backed Financing Arrangement	$ 527,000,000
Loans sold to third parties	$ 527,000,000
Ownership interest percentage	100.00%
Proceeds from Issuance of Unsecured Debt	$ 525,000,000
Estimated weighted average life of student loans	4 years 2 months 19 days
Loans pledged as collateral							539,000,000
Principal amount of collateral							501,000,000
Capitalized interest							38,000,000
2021-D | London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.69%
2021-A
Line of Credit Facility [Line Items]
Vertical risk retention interest					5.00%
2021-A | Student Loan
Line of Credit Facility [Line Items]
Private education loans sold					$ 2,500,000,000
Additional net proceeds from sales of loans held for investment, capitalized interest					130,000,000
Gain on sale					$ 18,000,000
2021-C
Line of Credit Facility [Line Items]
Vertical risk retention interest			5.00%
2021-C | Student Loan
Line of Credit Facility [Line Items]
Private education loans sold			$ 505,000,000
Additional net proceeds from sales of loans held for investment, capitalized interest			27,000,000
Gain on sale			$ 4,000,000
Commercial Paper | ABCP borrowings
Line of Credit Facility [Line Items]
Private asset backed commercial paper education loan funding facility		$ 2,000,000,000
Ownership interest percentage in residual interest in ABCP facility		100.00%
Short-term borrowings							$ 0	$ 0