Segment Reporting - Segment Results and Reconciliations to GAAP (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Interest income:
Cash and investments	$ 25	$ 24	$ 52	$ 41
Total interest income	1,204	1,227	2,439	2,399
Total interest expense	911	929	1,860	1,773
Net interest income	293	298	579	626
Less: provisions for loan losses	68	112	144	199
Net interest income after provisions for loan losses	225	186	435	427
Other income (loss):
Servicing revenue	60	71	122	140
Asset recovery and business processing revenue	123	99	242	207
Other income (loss)	(21)	(27)	2	9
Gains on sales of loans	16		16
Gains (losses) on debt repurchases	44	(7)	59	(8)
Total other income	222	136	441	348
Expenses:
Direct operating expenses	179	129	363	323
Unallocated shared services expenses	62	72	134	153
Total operating expenses	241	201	497	476
Goodwill and acquired intangible asset impairment and amortization	11	6	18	16
Restructuring/other reorganization expenses	1	2	2	9
Total expenses	253	209	517	501
Income before income tax expense	194	113	359	274
Income tax expense (benefit)	41	30	78	64
Net income (loss)	153	83	281	210
Education Loan Portfolio [Member]
Interest income:
Total interest income	1,178	1,202	2,385	2,356
Other Loans [Member]
Interest income:
Total interest income	1	1	2	2
Operating Segments [Member]
Interest income:
Cash and investments	25	24	52	41
Total interest income	1,220	1,242	2,470	2,423
Total interest expense	924	923	1,874	1,774
Net interest income	296	319	596	649
Less: provisions for loan losses	68	112	144	199
Net interest income after provisions for loan losses	228	207	452	450
Other income (loss):
Servicing revenue	60	71	122	140
Asset recovery and business processing revenue	123	99	242	207
Other income (loss)	11	3	25	6
Gains on sales of loans	16		16
Gains (losses) on debt repurchases	32	(7)	47	(8)
Total other income	242	166	452	345
Expenses:
Direct operating expenses	179	129	363	323
Unallocated shared services expenses	62	72	134	153
Total operating expenses	241	201	497	476
Restructuring/other reorganization expenses	1	2	2	9
Total expenses	242	203	499	485
Income before income tax expense	228	170	405	310
Income tax expense (benefit)	53	39	93	72
Net income (loss)	175	131	312	238
Operating Segments [Member] | Federal Education Loans [Member]
Interest income:
Cash and investments	14	12	30	19
Total interest income	772	788	1,568	1,528
Total interest expense	627	622	1,278	1,193
Net interest income	145	166	290	335
Less: provisions for loan losses	8	40	15	50
Net interest income after provisions for loan losses	137	126	275	285
Other income (loss):
Servicing revenue	57	68	117	134
Asset recovery and business processing revenue	58	34	108	70
Other income (loss)	7		15	1
Total other income	122	102	240	205
Expenses:
Direct operating expenses	89	36	180	115
Total operating expenses	89	36	180	115
Total expenses	89	36	180	115
Income before income tax expense	170	192	335	375
Income tax expense (benefit)	39	44	77	86
Net income (loss)	131	148	258	289
Operating Segments [Member] | Consumer Lending [Member]
Interest income:
Cash and investments	4	3	9	6
Total interest income	441	445	889	879
Total interest expense	255	252	516	490
Net interest income	186	193	373	389
Less: provisions for loan losses	60	72	129	149
Net interest income after provisions for loan losses	126	121	244	240
Other income (loss):
Servicing revenue	3	3	5	6
Other income (loss)			1
Gains on sales of loans	16		16
Total other income	19	3	22	6
Expenses:
Direct operating expenses	34	39	72	95
Total operating expenses	34	39	72	95
Total expenses	34	39	72	95
Income before income tax expense	111	85	194	151
Income tax expense (benefit)	26	19	44	35
Net income (loss)	85	66	150	116
Operating Segments [Member] | Business Processing [Member]
Other income (loss):
Asset recovery and business processing revenue	65	65	134	137
Total other income	65	65	134	137
Expenses:
Direct operating expenses	56	54	111	113
Total operating expenses	56	54	111	113
Total expenses	56	54	111	113
Income before income tax expense	9	11	23	24
Income tax expense (benefit)	2	3	6	6
Net income (loss)	7	8	17	18
Operating Segments [Member] | Other Segment [Member]
Interest income:
Cash and investments	7	9	13	16
Total interest income	7	9	13	16
Total interest expense	42	49	80	91
Net interest income	(35)	(40)	(67)	(75)
Net interest income after provisions for loan losses	(35)	(40)	(67)	(75)
Other income (loss):
Other income (loss)	4	3	9	5
Gains (losses) on debt repurchases	32	(7)	47	(8)
Total other income	36	(4)	56	(3)
Expenses:
Unallocated shared services expenses	62	72	134	153
Total operating expenses	62	72	134	153
Restructuring/other reorganization expenses	1	2	2	9
Total expenses	63	74	136	162
Income before income tax expense	(62)	(118)	(147)	(240)
Income tax expense (benefit)	(14)	(27)	(34)	(55)
Net income (loss)	(48)	(91)	(113)	(185)
Operating Segments [Member] | Education Loan Portfolio [Member]
Interest income:
Total interest income	1,194	1,217	2,416	2,380
Operating Segments [Member] | Education Loan Portfolio [Member] | Federal Education Loans [Member]
Interest income:
Total interest income	758	775	1,537	1,507
Operating Segments [Member] | Education Loan Portfolio [Member] | Consumer Lending [Member]
Interest income:
Total interest income	436	442	879	873
Operating Segments [Member] | Other Loans [Member]
Interest income:
Total interest income	1	1	2	2
Operating Segments [Member] | Other Loans [Member] | Federal Education Loans [Member]
Interest income:
Total interest income		1	1	2
Operating Segments [Member] | Other Loans [Member] | Consumer Lending [Member]
Interest income:
Total interest income	1		1
Adjustments [Member] | Reclassifications [Member]
Interest income:
Total interest income	1	3	2	11
Total interest expense	(2)	10	2	4
Net interest income	3	(7)		7
Net interest income after provisions for loan losses	3	(7)		7
Other income (loss):
Other income (loss)	(38)	7	(39)	(7)
Gains (losses) on debt repurchases	35		39
Total other income	(3)	7		(7)
Adjustments [Member] | Additions (Subtractions) [Member]
Interest income:
Total interest income	(17)	(18)	(33)	(35)
Total interest expense	(11)	(4)	(16)	(5)
Net interest income	(6)	(14)	(17)	(30)
Net interest income after provisions for loan losses	(6)	(14)	(17)	(30)
Other income (loss):
Other income (loss)	6	(37)	16	10
Gains (losses) on debt repurchases	(23)		(27)
Total other income	(17)	(37)	(11)	10
Expenses:
Goodwill and acquired intangible asset impairment and amortization	11	6	18	16
Total expenses	11	6	18	16
Income before income tax expense	(34)	(57)	(46)	(36)
Income tax expense (benefit)	(12)	(9)	(15)	(8)
Net income (loss)	(22)	(48)	(31)	(28)
Adjustments [Member] | Total Adjustments [Member]
Interest income:
Total interest income	(16)	(15)	(31)	(24)
Total interest expense	(13)	6	(14)	(1)
Net interest income	(3)	(21)	(17)	(23)
Net interest income after provisions for loan losses	(3)	(21)	(17)	(23)
Other income (loss):
Other income (loss)	(32)	(30)	(23)	3
Gains (losses) on debt repurchases	12		12
Total other income	(20)	(30)	(11)	3
Expenses:
Goodwill and acquired intangible asset impairment and amortization	11	6	18	16
Total expenses	11	6	18	16
Income before income tax expense	(34)	(57)	(46)	(36)
Income tax expense (benefit)	(12)	(9)	(15)	(8)
Net income (loss)	(22)	(48)	(31)	(28)
Adjustments [Member] | Education Loan Portfolio [Member] | Reclassifications [Member]
Interest income:
Total interest income	1	3	2	11
Adjustments [Member] | Education Loan Portfolio [Member] | Additions (Subtractions) [Member]
Interest income:
Total interest income	(17)	(18)	(33)	(35)
Adjustments [Member] | Education Loan Portfolio [Member] | Total Adjustments [Member]
Interest income:
Total interest income	$ (16)	$ (15)	$ (31)	$ (24)