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Borrowings - Narrative (Details) - USD ($)			3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 13, 2025	Feb. 18, 2025	Jun. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Jan. 31, 2025	Aug. 31, 2024	Jun. 14, 2024	May 31, 2024
Line of Credit Facility [Line Items]
Short-term borrowings			$ 0	$ 0	$ 0
Estimated weighted average life of student loans			5 years 7 months 6 days		5 years 7 months 6 days
Vertical risk retention interest				5.00%
Uncommitted federal funds			$ 125,000,000	$ 125,000,000
Lendable value of collateral			2,500,000,000	2,500,000,000	$ 2,200,000,000
Unsecured borrowings
Line of Credit Facility [Line Items]
Short-term borrowings			0	0	0
Secured Borrowings | Line of Credit
Line of Credit Facility [Line Items]
Secured borrowing facility maximum borrowing capacity	$ 2,500,000,000							$ 2,000,000,000
Line of credit facility, interest holding percentage	100.00%
Short-term borrowings			0	0	$ 0
2024-C ABS Transaction
Line of Credit Facility [Line Items]
Notes Issued									$ 668,000,000
2024-E ABS Transaction
Line of Credit Facility [Line Items]
Notes Issued							$ 868,000,000
ABCP borrowings | Secured Borrowings
Line of Credit Facility [Line Items]
Line of credit facility, expiration period	2 years
Senior Unsecured Notes Due January 31, 2030 | Unsecured borrowings
Line of Credit Facility [Line Items]
Notes Issued						$ 500,000,000
Debt instrument, interest rate, stated percentage						6.50%
Unsecured debt			$ 493,000,000	$ 493,000,000
Senior Unsecured Notes Due October 29, 2025 | Unsecured borrowings
Line of Credit Facility [Line Items]
Debt instrument, interest rate, stated percentage		4.20%
Repayments of long-term debt		$ 500,000,000
Debt instrument, redemption price, percentage		100.00%
Gain (loss) on extinguishment of debt		$ (1,000,000)

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Contractual interest rate for funds borrowed, under the debt agreement.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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