Borrowings - Additional Information (Details) - USD ($)			3 Months Ended	12 Months Ended
	Feb. 19, 2020	Feb. 12, 2020	Mar. 31, 2020	Dec. 31, 2019	Feb. 20, 2019
Line of Credit Facility [Line Items]
Short-term borrowings			$ 0	$ 289,230,000
Unsecured offering issued			$ 636,000,000	$ 1,110,000,000
Estimated weighted average life of student loans			5 years 3 months 18 days	5 years 4 months 24 days
Uncommitted federal funds			$ 125,000,000
Lendable value of collateral			2,400,000,000	$ 3,200,000,000
Unsecured borrowings
Line of Credit Facility [Line Items]
Short-term borrowings			0	0
Private Education Loans 2019 Term A
Line of Credit Facility [Line Items]
Total loan amount securitized at inception		$ 636,000,000
Amount of loan securitization sold to third parties		$ 636,000,000
Ownership interest percentage		100.00%
Proceeds from loan securitization sold to third parties		$ 634,000,000
Loans pledged as collateral			665,000,000
Principal amount of collateral			622,000,000
Capitalized interest			43,000,000
Class A and B Notes
Line of Credit Facility [Line Items]
Estimated weighted average life of student loans		4 years 2 months 4 days
Class A and B Notes | London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Basis spread on variable rate		0.88%
Commercial Paper | ABCP borrowings
Line of Credit Facility [Line Items]
Private asset backed commercial paper education loan funding facility	$ 2,000,000,000				$ 750,000,000
Ownership interest percentage in residual interest in ABCP facility	100.00%
Short-term borrowings			$ 0	$ 289,000,000