Loans Held-for-Investment - Loan Quality Indicators (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	$ 14,268	$ 16,227
Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	601	662
Year Two	469	901
Year Three	564	396
Year Four	249	301
Year Five	156	255
Prior	465	481
Revolving Loans Amortized Cost Basis	889	1,043
Revolving Loans Converted to Term Loans Amortized Cost Basis	77	70
Total	3,470	4,109	$ 4,895
Total loans	3,486	4,126
Consumer loans | Residential first mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	1,626	2,266
Consumer loans | Residential first mortgage loans | Debt-to-Value Ratio, Greater than 90
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	82	84
Year Two	76	260
Year Three	160	123
Year Four	67	35
Year Five	19	3
Prior	19	19
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total	423	524
Consumer loans | Residential first mortgage loans | Debt-to-Value Ratio, 71-90
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	104	169
Year Two	89	180
Year Three	76	66
Year Four	32	99
Year Five	44	72
Prior	207	238
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total	552	824
Consumer loans | Residential first mortgage loans | Debt-to-Vale Ratio, 55-70
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	56	83
Year Two	32	60
Year Three	33	22
Year Four	14	82
Year Five	48	96
Prior	135	122
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total	318	465
Consumer loans | Residential first mortgage loans | Debt-to-Value Ratio, Less than 55
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	30	26
Year Two	22	48
Year Three	29	26
Year Four	17	76
Year Five	39	81
Prior	83	73
Revolving Loans Amortized Cost Basis	82	93
Revolving Loans Converted to Term Loans Amortized Cost Basis	16	15
Total	318	438
Consumer loans | Residential first mortgage loans | FICO Score, Greater than 750
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	118	195
Year Two	109	272
Year Three	126	118
Year Four	54	193
Year Five	95	181
Prior	238	231
Revolving Loans Amortized Cost Basis	50	55
Revolving Loans Converted to Term Loans Amortized Cost Basis	7	6
Total	797	1,251
Consumer loans | Residential first mortgage loans | FICO Score, 700 to 750
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	97	119
Year Two	60	180
Year Three	91	90
Year Four	50	85
Year Five	45	64
Prior	132	130
Revolving Loans Amortized Cost Basis	22	25
Revolving Loans Converted to Term Loans Amortized Cost Basis	6	7
Total	503	700
Consumer loans | Residential first mortgage loans | FICO Score, Less than 700
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	57	48
Year Two	50	96
Year Three	81	29
Year Four	26	14
Year Five	10	7
Prior	74	91
Revolving Loans Amortized Cost Basis	10	13
Revolving Loans Converted to Term Loans Amortized Cost Basis	3	2
Total	311	300
Consumer loans | Residential first mortgage loans | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	272	362
Year Two	218	544
Year Three	291	231
Year Four	123	289
Year Five	147	252
Prior	419	420
Revolving Loans Amortized Cost Basis	82	92
Revolving Loans Converted to Term Loans Amortized Cost Basis	15	15
Total	1,567	2,205	3,107
Consumer loans | Residential first mortgage loans | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	0
Year Two	0	1
Year Three	0	1
Year Four	0	1
Year Five	1	0
Prior	2	17
Revolving Loans Amortized Cost Basis	0	1
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total	3	21	23
Consumer loans | Residential first mortgage loans | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	0
Year Two	1	3
Year Three	7	5
Year Four	7	2
Year Five	2	0
Prior	23	15
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	1	0
Total	41	25	15
Consumer loans | Home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	657	856
Consumer loans | Home equity | Debt-to-Value Ratio, Greater than 90
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	0
Year Two	0	0
Year Three	0	0
Year Four	0	1
Year Five	1	1
Prior	7	10
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total	8	12
Consumer loans | Home equity | Debt-to-Value Ratio, 71-90
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	2	5
Year Two	3	24
Year Three	14	10
Year Four	5	4
Year Five	2	1
Prior	7	9
Revolving Loans Amortized Cost Basis	395	548
Revolving Loans Converted to Term Loans Amortized Cost Basis	37	33
Total	465	634
Consumer loans | Home equity | Debt-to-Value Ratio, Less than or Equal to 70
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	1	2
Year Two	2	7
Year Three	4	3
Year Four	2	1
Year Five	1	0
Prior	3	4
Revolving Loans Amortized Cost Basis	151	175
Revolving Loans Converted to Term Loans Amortized Cost Basis	18	16
Total	182	208
Consumer loans | Home equity | FICO Score, Greater than 750
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	1	2
Year Two	2	9
Year Three	5	6
Year Four	2	2
Year Five	2	1
Prior	5	7
Revolving Loans Amortized Cost Basis	257	324
Revolving Loans Converted to Term Loans Amortized Cost Basis	15	13
Total	289	364
Consumer loans | Home equity | FICO Score, 700 to 750
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	2	3
Year Two	2	12
Year Three	7	4
Year Four	3	3
Year Five	1	1
Prior	6	8
Revolving Loans Amortized Cost Basis	218	289
Revolving Loans Converted to Term Loans Amortized Cost Basis	22	20
Total	261	340
Consumer loans | Home equity | FICO Score, Less than 700
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	2
Year Two	1	10
Year Three	6	3
Year Four	2	1
Year Five	1	0
Prior	6	8
Revolving Loans Amortized Cost Basis	71	110
Revolving Loans Converted to Term Loans Amortized Cost Basis	18	16
Total	105	150
Consumer loans | Home equity | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	3	7
Year Two	5	31
Year Three	18	13
Year Four	7	6
Year Five	4	2
Prior	15	11
Revolving Loans Amortized Cost Basis	544	720
Revolving Loans Converted to Term Loans Amortized Cost Basis	52	48
Total	648	838	1,002
Consumer loans | Home equity | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	0
Year Two	0	0
Year Three	0	0
Year Four	0	0
Year Five	0	0
Prior	0	11
Revolving Loans Amortized Cost Basis	0	2
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total	0	13	16
Consumer loans | Home equity | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	0
Year Two	0	0
Year Three	0	0
Year Four	0	0
Year Five	0	0
Prior	2	1
Revolving Loans Amortized Cost Basis	2	1
Revolving Loans Converted to Term Loans Amortized Cost Basis	3	1
Total	7	3	3
Consumer loans | Other consumer | FICO Score, Greater than 750
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	326	209
Year Two	245	205
Year Three	248	80
Year Four	98	2
Year Five	2	1
Prior	4	5
Revolving Loans Amortized Cost Basis	250	213
Revolving Loans Converted to Term Loans Amortized Cost Basis	4	6
Total	1,177	721
Consumer loans | Other consumer | FICO Score, 700 to 750
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	79
Year Two	0	107
Year Three	0	55
Year Four	12	1
Year Five	0	0
Prior	0	1
Revolving Loans Amortized Cost Basis	7	9
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total	19	252
Consumer loans | Other consumer | FICO Score, Less than 700
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	5
Year Two	0	10
Year Three	0	11
Year Four	2	0
Year Five	0	0
Prior	0	0
Revolving Loans Amortized Cost Basis	4	5
Revolving Loans Converted to Term Loans Amortized Cost Basis	2	0
Total	8	31
Consumer loans | Other consumer | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	326	292
Year Two	244	321
Year Three	246	145
Year Four	111	3
Year Five	2	1
Prior	4	6
Revolving Loans Amortized Cost Basis	261	227
Revolving Loans Converted to Term Loans Amortized Cost Basis	6	5
Total	1,200	1,000	727
Consumer loans | Other consumer | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	0
Year Two	0	0
Year Three	1	0
Year Four	0	0
Year Five	0	0
Prior	0	0
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	1
Total	1	1	1
Consumer loans | Other consumer | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	1
Year Two	1	1
Year Three	1	1
Year Four	1	0
Year Five	0	0
Prior	0	0
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total	3	3	1
Consumer loans | Residential first mortgage and home equity
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	275	369
Year Two	224	579
Year Three	316	250
Year Four	137	298
Year Five	154	254
Prior	461	475
Revolving Loans Amortized Cost Basis	628	816
Revolving Loans Converted to Term Loans Amortized Cost Basis	71	64
Total	2,266	3,105
Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	6,631	8,376
Year Two	388	1,508
Year Three	836	711
Year Four	459	701
Year Five	446	400
Prior	438	386
Revolving Loans Amortized Cost Basis	1,584	19
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total loans	10,782	12,101
Commercial loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	3,216	3,061
Commercial loans | Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	330	347
Year Two	294	993
Year Three	621	439
Year Four	335	438
Year Five	252	308
Prior	372	280
Revolving Loans Amortized Cost Basis	826	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total loans	3,030	2,805	2,794
Commercial loans | Commercial real estate | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	21
Year Two	5	19
Year Three	8	35
Year Four	23	51
Year Five	72	21
Prior	52	19
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total loans	160	166	24
Commercial loans | Commercial real estate | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	5
Year Two	0	1
Year Three	3	16
Year Four	0	0
Year Five	0	17
Prior	0	14
Revolving Loans Amortized Cost Basis	1	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total loans	4	53	5
Commercial loans | Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	0
Year Two	0	11
Year Three	0	1
Year Four	0	25
Year Five	22	0
Prior	0	0
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total loans	22	37	5
Commercial loans | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans	1,387	1,382
Commercial loans | Commercial and industrial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	122	319
Year Two	87	425
Year Three	176	163
Year Four	77	149
Year Five	96	54
Prior	14	71
Revolving Loans Amortized Cost Basis	682	19
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total loans	1,254	1,200	1,533
Commercial loans | Commercial and industrial | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	3
Year Two	2	48
Year Three	11	28
Year Four	5	25
Year Five	0	0
Prior	0	2
Revolving Loans Amortized Cost Basis	43	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total loans	61	106	72
Commercial loans | Commercial and industrial | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	1
Year Two	0	0
Year Three	0	14
Year Four	0	9
Year Five	0	0
Prior	0	0
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total loans	0	24	24
Commercial loans | Commercial and industrial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	22
Year Two	0	11
Year Three	17	15
Year Four	19	4
Year Five	4	0
Prior	0	0
Revolving Loans Amortized Cost Basis	32	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total loans	72	52	5
Commercial loans | Warehouse loan commitments | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	6,039	7,398
Year Two	0	0
Year Three	0	0
Year Four	0	0
Year Five	0	0
Prior	0	0
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total loans	6,039	7,398	2,556
Commercial loans | Warehouse loan commitments | Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	140	260
Year Two	0	0
Year Three	0	0
Year Four	0	0
Year Five	0	0
Prior	0	0
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total loans	140	260	189
Commercial loans | Warehouse loan commitments | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	0
Year Two	0	0
Year Three	0	0
Year Four	0	0
Year Five	0	0
Prior	0	0
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total loans	0	0	15
Commercial loans | Warehouse loan commitments | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Year One	0	0
Year Two	0	0
Year Three	0	0
Year Four	0	0
Year Five	0	0
Prior	0	0
Revolving Loans Amortized Cost Basis	0	0
Revolving Loans Converted to Term Loans Amortized Cost Basis	0	0
Total loans	$ 0	$ 0	$ 0