Segment Reporting - Segment Results and Reconciliations to GAAP (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Interest income:
Cash and investments	$ 41	$ 19	$ 111	$ 25
Total interest income	1,170	881	3,338	2,199
Total interest expense	879	641	2,636	1,301
Net interest income	291	240	702	898
Less: provisions for loan losses	72	28	68	62
Net interest income after provisions for loan losses	219	212	634	836
Other income (loss):
Servicing revenue	15	24	48	60
Asset recovery and business processing revenue	85	80	240	264
Other revenue	31	46	59	183
Total other income	131	150	347	507
Expenses:
Direct operating expenses	134	135	394	408
Unallocated shared services expenses	99	59	207	180
Total operating expenses	233	194	601	588
Goodwill and acquired intangible asset impairment and amortization	3	10	8	17
Restructuring/other reorganization expenses	4	21	23	25
Total expenses	240	225	632	630
Income before income tax expense	110	137	349	713
Income tax expense (benefit)	31	32	93	173
Net income	79	105	256	540
Operating Segments [Member]
Interest income:
Net interest income	280	247	753	794
Less: provisions for loan losses	72	28	68	62
Other income (loss):
Total other income	105	110	303	346
Expenses:
Total operating expenses	233	194	601	588
Goodwill and acquired intangible asset impairment and amortization	0	0	0	0
Restructuring/other reorganization expenses	4	21	23	25
Total expenses	237	215	624	613
Income before income tax expense	76	114	364	465
Income tax expense (benefit)	19	27	86	109
Net income	57	87	278	356
Operating Segments [Member] | Federal Education Loans [Member]
Interest income:
Cash and investments	19	9	56	12
Total interest income	797	564	2,250	1,310
Total interest expense	636	444	1,859	905
Net interest income	161	120	391	405
Less: provisions for loan losses	36	0	51	0
Net interest income after provisions for loan losses	125	120	340	405
Other income (loss):
Servicing revenue	12	21	39	51
Asset recovery and business processing revenue	0	1	0	4
Other revenue	3	6	10	24
Total other income	15	28	49	79
Expenses:
Direct operating expenses	17	25	55	79
Unallocated shared services expenses	0	0	0	0
Total operating expenses	17	25	55	79
Goodwill and acquired intangible asset impairment and amortization	0	0	0	0
Restructuring/other reorganization expenses	0	0	0	0
Total expenses	17	25	55	79
Income before income tax expense	123	123	334	405
Income tax expense (benefit)	29	29	78	95
Net income	94	94	256	310
Operating Segments [Member] | Consumer Lending [Member]
Interest income:
Cash and investments	7	3	20	5
Total interest income	358	312	1,056	867
Total interest expense	208	159	610	421
Net interest income	150	153	446	446
Less: provisions for loan losses	36	28	17	62
Net interest income after provisions for loan losses	114	125	429	384
Other income (loss):
Servicing revenue	3	3	9	9
Asset recovery and business processing revenue	0	0	0	0
Other revenue	1	0	2	1
Total other income	4	3	11	10
Expenses:
Direct operating expenses	44	43	124	113
Unallocated shared services expenses	0	0	0	0
Total operating expenses	44	43	124	113
Goodwill and acquired intangible asset impairment and amortization	0	0	0	0
Restructuring/other reorganization expenses	0	0	0	0
Total expenses	44	43	124	113
Income before income tax expense	74	85	316	281
Income tax expense (benefit)	18	20	75	66
Net income	56	65	241	215
Operating Segments [Member] | Business Processing [Member]
Interest income:
Cash and investments	0	0	0	0
Total interest income	0	0	0	0
Total interest expense	0	0	0	0
Net interest income	0	0	0	0
Less: provisions for loan losses	0	0	0	0
Net interest income after provisions for loan losses	0	0	0	0
Other income (loss):
Servicing revenue	0	0	0	0
Asset recovery and business processing revenue	85	79	240	260
Other revenue	0	0	0	0
Total other income	85	79	240	260
Expenses:
Direct operating expenses	73	67	215	216
Unallocated shared services expenses	0	0	0	0
Total operating expenses	73	67	215	216
Goodwill and acquired intangible asset impairment and amortization	0	0	0	0
Restructuring/other reorganization expenses	0	0	0	0
Total expenses	73	67	215	216
Income before income tax expense	12	12	25	44
Income tax expense (benefit)	3	3	6	11
Net income	9	9	19	33
Operating Segments [Member] | Other Segment [Member]
Interest income:
Cash and investments	15	7	35	8
Total interest income	15	7		8
Total interest expense	46	33	119	65
Net interest income	(31)	(26)	(84)	(57)
Less: provisions for loan losses	0	0	0	0
Net interest income after provisions for loan losses	(31)	(26)	(84)	(57)
Other income (loss):
Servicing revenue	0	0	0	0
Asset recovery and business processing revenue	0	0	0	0
Other revenue	1	0	3	(3)
Total other income	1	0	3	(3)
Expenses:
Direct operating expenses	0	0	0	0
Unallocated shared services expenses	99	59	207	180
Total operating expenses	99	59	207	180
Goodwill and acquired intangible asset impairment and amortization	0	0	0	0
Restructuring/other reorganization expenses	4	21	23	25
Total expenses	103	80	230	205
Income before income tax expense	(133)	(106)	(311)	(265)
Income tax expense (benefit)	(31)	(25)	(73)	(63)
Net income	(102)	(81)	(238)	(202)
Adjustments [Member] | Reclassifications [Member]
Interest income:
Net interest income	7	(1)	24	(20)
Other income (loss):
Total other income	(7)	1	(24)	20
Expenses:
Total operating expenses	0	0	0	0
Goodwill and acquired intangible asset impairment and amortization	0	0	0	0
Restructuring/other reorganization expenses	0	0	0	0
Total expenses	0	0	0	0
Income before income tax expense	0	0	0	0
Income tax expense (benefit)	0	0	0	0
Net income	0	0	0	0
Adjustments [Member] | Additions (Subtractions) [Member]
Interest income:
Net interest income	(18)	8	27	(84)
Other income (loss):
Total other income	(19)	(41)	(20)	(181)
Expenses:
Total operating expenses	0	0	0	0
Goodwill and acquired intangible asset impairment and amortization	(3)	(10)	(8)	(17)
Restructuring/other reorganization expenses	0	0	0	0
Total expenses	(3)	(10)	(8)	(17)
Income before income tax expense	(34)	(23)	15	(248)
Income tax expense (benefit)	(12)	(5)	(7)	(64)
Net income	(22)	(18)	22	(184)
Adjustments [Member] | Total Adjustments [Member]
Interest income:
Net interest income	(11)	7	51	(104)
Net interest income after provisions for loan losses	(11)	7	51	(104)
Other income (loss):
Total other income	(26)	(40)	(44)	(161)
Expenses:
Total operating expenses	0	0	0	0
Goodwill and acquired intangible asset impairment and amortization	(3)	(10)	(8)	(17)
Restructuring/other reorganization expenses	0	0	0	0
Total expenses	(3)	(10)	(8)	(17)
Income before income tax expense	(34)	(23)	15	(248)
Income tax expense (benefit)	(12)	(5)	(7)	(64)
Net income	(22)	(18)	22	(184)
Education Loan Portfolio [Member]
Interest income:
Total interest income	1,129	862	3,227	2,174
Education Loan Portfolio [Member] | Operating Segments [Member] | Federal Education Loans [Member]
Interest income:
Total interest income	778	555	2,194	1,298
Education Loan Portfolio [Member] | Operating Segments [Member] | Consumer Lending [Member]
Interest income:
Total interest income	351	309	1,036	862
Education Loan Portfolio [Member] | Operating Segments [Member] | Business Processing [Member]
Interest income:
Total interest income	0	0	0	0
Education Loan Portfolio [Member] | Operating Segments [Member] | Other Segment [Member]
Interest income:
Total interest income	$ 0	$ 0	$ 0	$ 0