Segment Reporting - Segment Results and Reconciliations to GAAP - Reportable Segment Significant Operating Expenses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting, Revenue Reconciling Item [Line Items]
Servicing expenses	$ 32	$ 23	$ 67	$ 48
Information technology expenses	28	34	59	71
Corporate expenses	20	24	45	55
Other/remaining expenses	20	85	56	176
Total operating expenses	100	166	227	350
Federal Education Loans [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Servicing expenses	16	10	34	22
Information technology expenses	0	3	0	6
Corporate expenses	0	1	1	2
Other/remaining expenses	1	2	2	3
Total operating expenses	17	16	37	33
Consumer Lending Segment [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Servicing expenses	14	13	28	26
Information technology expenses	8	7	17	14
Corporate expenses	1	1	1	2
Other/remaining expenses	13	13	24	25
Total operating expenses	36	34	70	67
Business Processing [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Servicing expenses	0	0	0	0
Information technology expenses	0	4	1	9
Corporate expenses	0	1	0	2
Other/remaining expenses	0	57	19	120
Total operating expenses	0	62	20	131
Other Segment [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Servicing expenses	2	0	5	0
Information technology expenses	20	20	41	42
Corporate expenses	19	21	43	49
Other/remaining expenses	6	13	11	28
Total operating expenses	$ 47	$ 54	$ 100	$ 119