Segment Reporting - Segment Results and Reconciliations to GAAP (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Interest income:
Cash and investments	$ 1	$ 23	$ 15	$ 75
Total interest income	761	1,166	2,567	3,605
Total interest expense	425	854	1,658	2,714
Net interest income	336	312	909	891
Less: provisions for loan losses	14	64	153	208
Net interest income after provisions for loan losses	322	248	756	683
Other income (loss):
Servicing revenue	54	60	163	182
Asset recovery and business processing revenue	125	123	337	364
Other income (loss)	(2)	13	(238)	16
Gains on sales of loans				16
Gains on debt repurchases				59
Total other income (loss)	177	196	262	637
Expenses:
Direct operating expenses	170	187	506	552
Unallocated shared services expenses	62	64	189	197
Total operating expenses	232	251	695	749
Goodwill and acquired intangible asset impairment and amortization	5	6	16	23
Restructuring/other reorganization expenses	3	2	9	4
Total expenses	240	259	720	776
Income before income tax expense	259	185	298	544
Income tax expense (benefit)	52	40	71	119
Net income	207	145	227	425
Education Loan Portfolio [Member]
Interest income:
Total interest income	760	1,143	2,552	3,528
Other Loans [Member]
Interest income:
Total interest income				2
Operating Segments [Member]
Interest income:
Cash and investments	1	23	15	75
Total interest income	744	1,181	2,562	3,652
Total interest expense	423	872	1,615	2,746
Net interest income	321	309	947	906
Less: provisions for loan losses	14	64	153	208
Net interest income after provisions for loan losses	307	245	794	698
Other income (loss):
Servicing revenue	54	60	163	182
Asset recovery and business processing revenue	125	123	337	364
Other income (loss)		9	17	35
Gains on sales of loans				16
Gains on debt repurchases				47
Total other income (loss)	179	192	517	644
Expenses:
Direct operating expenses	170	187	506	552
Unallocated shared services expenses	62	64	189	197
Total operating expenses	232	251	695	749
Restructuring/other reorganization expenses	3	2	9	4
Total expenses	235	253	704	753
Income before income tax expense	251	184	607	589
Income tax expense (benefit)	59	42	143	135
Net income	192	142	464	454
Operating Segments [Member] | Federal Education Loans [Member]
Interest income:
Cash and investments		11	7	41
Total interest income	393	732	1,437	2,300
Total interest expense	232	588	974	1,866
Net interest income	161	144	463	434
Less: provisions for loan losses	4	8	13	23
Net interest income after provisions for loan losses	157	136	450	411
Other income (loss):
Servicing revenue	53	57	158	174
Asset recovery and business processing revenue	35	57	126	165
Other income (loss)	(1)	6	8	22
Total other income (loss)	87	120	292	361
Expenses:
Direct operating expenses	64	89	217	270
Total operating expenses	64	89	217	270
Total expenses	64	89	217	270
Income before income tax expense	180	167	525	502
Income tax expense (benefit)	43	39	124	115
Net income	137	128	401	387
Operating Segments [Member] | Consumer Lending [Member]
Interest income:
Cash and investments		4	3	13
Total interest income	350	441	1,120	1,331
Total interest expense	161	242	545	758
Net interest income	189	199	575	573
Less: provisions for loan losses	10	56	140	185
Net interest income after provisions for loan losses	179	143	435	388
Other income (loss):
Servicing revenue	1	3	5	8
Other income (loss)				1
Gains on sales of loans				16
Total other income (loss)	1	3	5	25
Expenses:
Direct operating expenses	37	44	109	117
Total operating expenses	37	44	109	117
Total expenses	37	44	109	117
Income before income tax expense	143	102	331	296
Income tax expense (benefit)	33	23	78	68
Net income	110	79	253	228
Operating Segments [Member] | Business Processing [Member]
Other income (loss):
Asset recovery and business processing revenue	90	66	211	199
Total other income (loss)	90	66	211	199
Expenses:
Direct operating expenses	69	54	180	165
Total operating expenses	69	54	180	165
Total expenses	69	54	180	165
Income before income tax expense	21	12	31	34
Income tax expense (benefit)	5	3	7	8
Net income	16	9	24	26
Operating Segments [Member] | Other Segment [Member]
Interest income:
Cash and investments	1	8	5	21
Total interest income	1	8	5	21
Total interest expense	30	42	96	122
Net interest income	(29)	(34)	(91)	(101)
Net interest income after provisions for loan losses	(29)	(34)	(91)	(101)
Other income (loss):
Other income (loss)	1	3	9	12
Gains on debt repurchases				47
Total other income (loss)	1	3	9	59
Expenses:
Unallocated shared services expenses	62	64	189	197
Total operating expenses	62	64	189	197
Restructuring/other reorganization expenses	3	2	9	4
Total expenses	65	66	198	201
Income before income tax expense	(93)	(97)	(280)	(243)
Income tax expense (benefit)	(22)	(23)	(66)	(56)
Net income	(71)	(74)	(214)	(187)
Operating Segments [Member] | Education Loan Portfolio [Member]
Interest income:
Total interest income	743	1,158	2,547	3,575
Operating Segments [Member] | Education Loan Portfolio [Member] | Federal Education Loans [Member]
Interest income:
Total interest income	393	721	1,430	2,258
Operating Segments [Member] | Education Loan Portfolio [Member] | Consumer Lending [Member]
Interest income:
Total interest income	350	437	1,117	1,317
Operating Segments [Member] | Other Loans [Member]
Interest income:
Total interest income				2
Operating Segments [Member] | Other Loans [Member] | Federal Education Loans [Member]
Interest income:
Total interest income				1
Operating Segments [Member] | Other Loans [Member] | Consumer Lending [Member]
Interest income:
Total interest income				1
Adjustments [Member] | Reclassifications [Member]
Interest income:
Total interest income	31	2	47	4
Total interest expense	7	(4)	39	(2)
Net interest income	24	6	8	6
Net interest income after provisions for loan losses	24	6	8	6
Other income (loss):
Other income (loss)	(24)	(6)	(8)	(45)
Gains on debt repurchases				39
Total other income (loss)	(24)	(6)	(8)	(6)
Adjustments [Member] | Additions (Subtractions) [Member]
Interest income:
Total interest income	(14)	(17)	(42)	(51)
Total interest expense	(5)	(14)	4	(30)
Net interest income	(9)	(3)	(46)	(21)
Net interest income after provisions for loan losses	(9)	(3)	(46)	(21)
Other income (loss):
Other income (loss)	22	10	(247)	26
Gains on debt repurchases				(27)
Total other income (loss)	22	10	(247)	(1)
Expenses:
Goodwill and acquired intangible asset impairment and amortization	5	6	16	23
Total expenses	5	6	16	23
Income before income tax expense	8	1	(309)	(45)
Income tax expense (benefit)	(7)	(2)	(72)	(16)
Net income	15	3	(237)	(29)
Adjustments [Member] | Total Adjustments [Member]
Interest income:
Total interest income	17	(15)	5	(47)
Total interest expense	2	(18)	43	(32)
Net interest income	15	3	(38)	(15)
Net interest income after provisions for loan losses	15	3	(38)	(15)
Other income (loss):
Other income (loss)	(2)	4	(255)	(19)
Gains on debt repurchases				12
Total other income (loss)	(2)	4	(255)	(7)
Expenses:
Goodwill and acquired intangible asset impairment and amortization	5	6	16	23
Total expenses	5	6	16	23
Income before income tax expense	8	1	(309)	(45)
Income tax expense (benefit)	(7)	(2)	(72)	(16)
Net income	15	3	(237)	(29)
Adjustments [Member] | Education Loan Portfolio [Member] | Reclassifications [Member]
Interest income:
Total interest income	31	2	47	4
Adjustments [Member] | Education Loan Portfolio [Member] | Additions (Subtractions) [Member]
Interest income:
Total interest income	(14)	(17)	(42)	(51)
Adjustments [Member] | Education Loan Portfolio [Member] | Total Adjustments [Member]
Interest income:
Total interest income	$ 17	$ (15)	$ 5	$ (47)