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Subsequent Events (Details) - USD ($)						1 Months Ended	3 Months Ended	12 Months Ended
	Feb. 09, 2021	Feb. 02, 2021	Jan. 28, 2021	Jan. 08, 2021	Mar. 10, 2020	Mar. 31, 2020	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Feb. 17, 2021	Feb. 19, 2020	Jan. 22, 2020	Feb. 20, 2019
Subsequent Event [Line Items]
Net proceeds from sales of loans held for investment, principal							$ 2,900,000,000
Net proceeds from sales of loans held for investment, capitalized interest							199,000,000
Net proceeds from sales of loans held for investment, accrued interest							$ 12,000,000
Vertical risk retention interest						500.00%		5.00%
Common stock repurchased (in shares)					44,900,000			47,736,847	16,962,199	0
Repurchase amount (in dollars per share)								$ 75,000,000
Common stock, par value (in dollars per share)								$ 0.20	$ 0.20
Commercial Paper | ABCP borrowings
Subsequent Event [Line Items]
Private education loan funding												$ 2,000,000,000		$ 750,000,000
Common Stock
Subsequent Event [Line Items]
Common stock, par value (in dollars per share)								$ 0.20
2020 Share Repurchase Program
Subsequent Event [Line Items]
Aggregate purchase price (not to exceed)													$ 600,000,000
2020 Share Repurchase Program | Common Stock
Subsequent Event [Line Items]
Common stock repurchased (in shares)					44,900,000
Student Loan
Subsequent Event [Line Items]
Private education loans sold						$ 1,700,000,000
Subsequent Event
Subsequent Event [Line Items]
Net proceeds from sales of loans held for investment, principal				$ 2,800,000,000
Net proceeds from sales of loans held for investment, capitalized interest				185,000,000
Net proceeds from sales of loans held for investment, accrued interest				15,000,000
Vertical risk retention interest	5.00%
Subsequent Event | Commercial Paper | ABCP borrowings
Subsequent Event [Line Items]
Private education loan funding											$ 2,000,000,000
Subsequent Event | 2020 Share Repurchase Program | Common Stock
Subsequent Event [Line Items]
Additional shares repurchased			13,000,000
Common stock repurchased (in shares)			58,000,000
Repurchase amount (in dollars per share)			$ 9.01
Subsequent Event | Tender Offer | Common Stock
Subsequent Event [Line Items]
Aggregate purchase price (not to exceed)		$ 1,000,000,000
Common stock, par value (in dollars per share)		$ 0.20
Subsequent Event | Tender Offer | Common Stock | Maximum
Subsequent Event [Line Items]
Repurchase amount (in dollars per share)		$ 15.00
Subsequent Event | Tender Offer | Common Stock | Minimum
Subsequent Event [Line Items]
Repurchase amount (in dollars per share)		$ 13.10
Subsequent Event | Student Loan
Subsequent Event [Line Items]
Private education loans sold	$ 2,500,000,000			$ 3,000,000,000
Additional net proceeds from sales of loans held for investment, capitalized interest	$ 130,000,000

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