Segment Reporting - Segment Results and Reconciliations to GAAP (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Interest income:
Cash and investments	$ 153	$ 62	$ 3
Total interest income	4,419	3,223	2,648
Total interest expense	3,557	2,102	1,316
Net interest income	862	1,121	1,332
Less: provisions for loan losses	123	79	(61)
Net interest income after provisions for loan losses	739	1,042	1,393
Other income (loss):
Servicing revenue	64	77	168
Asset recovery and business processing revenue	321	336	539
Other revenue	32	203	94
Gains on sales of loans	0	0	78
Losses on debt repurchases	(8)	0	(73)
Total other income	409	616	806
Expenses:
Direct operating expenses	508	534	745
Unallocated shared services expenses	292	242	462
Total operating expenses	800	776	1,207
Goodwill and acquired intangible asset impairment and amortization	10	19	30
Restructuring/other reorganization expenses	25	36	26
Total expenses	835	831	1,263
Income before income tax expense	313	827	936
Income tax expense (benefit)	85	182	219
Net income	228	645	717
Operating Segments [Member]
Interest income:
Net interest income	946	1,026	1,148
Less: provisions for loan losses	123	79	(61)
Other income (loss):
Losses on debt repurchases	0
Total other income	398	445	755
Expenses:
Total operating expenses	800	776	1,207
Goodwill and acquired intangible asset impairment and amortization	0	0	0
Restructuring/other reorganization expenses	25	36	26
Total expenses	825	812	1,233
Income before income tax expense	396	580	731
Income tax expense (benefit)	93	122	180
Net income	303	458	551
Operating Segments [Member] | Federal Education Loans [Member]
Interest income:
Cash and investments	76	32	0
Total interest income	2,977	1,987	1,405
Total interest expense	2,497	1,468	830
Net interest income	480	519	575
Less: provisions for loan losses	56	0	0
Net interest income after provisions for loan losses	424	519	575
Other income (loss):
Servicing revenue	52	65	162
Asset recovery and business processing revenue	0	6	51
Other revenue	14	31	25
Gains on sales of loans			0
Losses on debt repurchases	0		0
Total other income	66	102	238
Expenses:
Direct operating expenses	72	106	223
Unallocated shared services expenses	0	0	0
Total operating expenses	72	106	223
Goodwill and acquired intangible asset impairment and amortization	0	0	0
Restructuring/other reorganization expenses	0	0	0
Total expenses	72	106	223
Income before income tax expense	418	515	590
Income tax expense (benefit)	99	108	136
Net income	319	407	454
Operating Segments [Member] | Consumer Lending [Member]
Interest income:
Cash and investments	27	10	2
Total interest income	1,396	1,205	1,183
Total interest expense	816	611	541
Net interest income	580	594	642
Less: provisions for loan losses	67	79	(61)
Net interest income after provisions for loan losses	513	515	703
Other income (loss):
Servicing revenue	12	12	6
Asset recovery and business processing revenue	0	0	0
Other revenue	2	1	0
Gains on sales of loans			91
Losses on debt repurchases	0		0
Total other income	14	13	97
Expenses:
Direct operating expenses	151	148	162
Unallocated shared services expenses	0	0	0
Total operating expenses	151	148	162
Goodwill and acquired intangible asset impairment and amortization	0	0	0
Restructuring/other reorganization expenses	0	0	0
Total expenses	151	148	162
Income before income tax expense	376	380	638
Income tax expense (benefit)	89	80	146
Net income	287	300	492
Operating Segments [Member] | Business Processing [Member]
Interest income:
Cash and investments	0	0	0
Total interest income	0	0	0
Total interest expense	0	0	0
Net interest income	0	0	0
Less: provisions for loan losses	0	0	0
Net interest income after provisions for loan losses	0	0	0
Other income (loss):
Servicing revenue	0	0	0
Asset recovery and business processing revenue	321	330	488
Other revenue	0	0	0
Gains on sales of loans			0
Losses on debt repurchases	0		0
Total other income	321	330	488
Expenses:
Direct operating expenses	285	280	360
Unallocated shared services expenses	0	0	0
Total operating expenses	285	280	360
Goodwill and acquired intangible asset impairment and amortization	0	0	0
Restructuring/other reorganization expenses	0	0	0
Total expenses	285	280	360
Income before income tax expense	36	50	128
Income tax expense (benefit)	8	10	29
Net income	28	40	99
Operating Segments [Member] | Other Segment [Member]
Interest income:
Cash and investments	50	20	1
Total interest income	50	20	1
Total interest expense	164	107	70
Net interest income	(114)	(87)	(69)
Less: provisions for loan losses	0	0	0
Net interest income after provisions for loan losses	(114)	(87)	(69)
Other income (loss):
Servicing revenue	0	0	0
Asset recovery and business processing revenue	0	0	0
Other revenue	5	0	5
Gains on sales of loans			0
Losses on debt repurchases	(8)		(73)
Total other income	(3)	0	(68)
Expenses:
Direct operating expenses	0	0	0
Unallocated shared services expenses	292	242	462
Total operating expenses	292	242	462
Goodwill and acquired intangible asset impairment and amortization	0	0	0
Restructuring/other reorganization expenses	25	36	26
Total expenses	317	278	488
Income before income tax expense	(434)	(365)	(625)
Income tax expense (benefit)	(103)	(76)	(131)
Net income	(331)	(289)	(494)
Adjustments [Member] | Reclassifications [Member]
Interest income:
Net interest income	32	(15)	(106)
Other income (loss):
Losses on debt repurchases	0
Total other income	(32)	15	106
Expenses:
Total operating expenses	0	0	0
Goodwill and acquired intangible asset impairment and amortization	0	0	0
Restructuring/other reorganization expenses	0	0	0
Total expenses	0	0	0
Income before income tax expense	0	0	0
Income tax expense (benefit)	0	0	0
Net income	0	0	0
Adjustments [Member] | Additions (Subtractions) [Member]
Interest income:
Net interest income	52	(80)	(78)
Other income (loss):
Losses on debt repurchases	0
Total other income	21	(186)	(157)
Expenses:
Total operating expenses	0	0	0
Goodwill and acquired intangible asset impairment and amortization	(10)	(19)	(30)
Restructuring/other reorganization expenses	0	0	0
Total expenses	(10)	(19)	(30)
Income before income tax expense	83	(247)	(205)
Income tax expense (benefit)	8	(60)	(39)
Net income	75	(187)	(166)
Adjustments [Member] | Total Adjustments [Member]
Interest income:
Net interest income	84	(95)	(184)
Net interest income after provisions for loan losses	84	(95)	(184)
Other income (loss):
Losses on debt repurchases	0
Total other income	(11)	(171)	(51)
Expenses:
Total operating expenses	0	0	0
Goodwill and acquired intangible asset impairment and amortization	(10)	(19)	(30)
Restructuring/other reorganization expenses	0	0	0
Total expenses	(10)	(19)	(30)
Income before income tax expense	83	(247)	(205)
Income tax expense (benefit)	8	(60)	(39)
Net income	75	(187)	(166)
Education Loan Portfolio [Member]
Interest income:
Total interest income	4,266	3,161	2,645
Education Loan Portfolio [Member] | Operating Segments [Member] | Federal Education Loans [Member]
Interest income:
Total interest income	2,901	1,955	1,405
Education Loan Portfolio [Member] | Operating Segments [Member] | Consumer Lending [Member]
Interest income:
Total interest income	1,369	1,195	1,181
Education Loan Portfolio [Member] | Operating Segments [Member] | Business Processing [Member]
Interest income:
Total interest income	0	0	0
Education Loan Portfolio [Member] | Operating Segments [Member] | Other Segment [Member]
Interest income:
Total interest income	$ 0	$ 0	$ 0