Segment Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segments	Sep. 30, 2019 USD ($)
Segment Reporting [Abstract]
Number of operating segments | segments			3
Summary of Operations
Net interest income	$ 180	$ 146	$ 496	$ 410
Provision (benefit) for credit losses	32	1	148	18
Net interest income after provision for credit losses	148	145	348	392
Net gain on loan sales	346	110	739	234
Loan fees and charges	45	29	112	70
Net return on mortgage servicing rights	12	(2)	10	9
Loan administrative (expense) income	26	5	59	22
Other noninterest income	23	29	68	113
Total noninterest income	452	171	988	448
Compensation and benefits	123	98	341	275
Commissions	72	38	162	76
Loan processing expense	24	22	69	60
Other noninterest expense	86	80	265	232
Total noninterest expense	305	238	837	643
Income (loss) before indirect overhead allocations and income taxes	295	78	499	197
Indirect overhead allocation income (expense)	0	0	0	0
Provision (benefit) for income taxes	73	15	115	37
Net income	222	63	384	160
Average balances
Loans held-for-sale	5,602	3,786	5,499	3,532
Loans with government guarantees	2,122	574	1,267	511
Loans held-for-investment	14,839	11,743	13,424	10,516
Total assets	28,277	21,197	25,992	19,877
Deposits	19,561	15,817	17,698	14,305
Other
Summary of Operations
Net interest income	(32)	(1)	(60)	0
Provision (benefit) for credit losses	37	3	153	4
Net interest income after provision for credit losses	(69)	(4)	(213)	(4)
Net gain on loan sales	0	0	0	1
Loan fees and charges	0	1	0	0
Net return on mortgage servicing rights	0	0	0	0
Loan administrative (expense) income	(3)	(19)	(26)	(50)
Other noninterest income	5	9	20	58
Total noninterest income	2	(9)	(6)	9
Compensation and benefits	41	35	118	100
Commissions	0	1	0	0
Loan processing expense	0	1	2	2
Other noninterest expense	(30)	(1)	(137)	1
Total noninterest expense	11	36	(17)	103
Income (loss) before indirect overhead allocations and income taxes	(78)	(49)	(202)	(98)
Indirect overhead allocation income (expense)	33	24	91	73
Provision (benefit) for income taxes	2	(6)	(13)	(9)
Net income	(47)	(19)	(98)	(16)
Average balances
Loans held-for-sale	0	0	0	0
Loans with government guarantees	0	0	0	0
Loans held-for-investment	30	29	29	29
Total assets	4,401	3,597	4,288	3,862
Deposits	967	748	767	522
Intersegment Eliminations
Summary of Operations
Intersegment (expense) revenue	24	(9)	116	(24)
Community Banking | Operating Segments
Summary of Operations
Net interest income	158	105	395	296
Provision (benefit) for credit losses	(2)	0	3	17
Net interest income after provision for credit losses	160	105	392	279
Net gain on loan sales	2	(4)	2	(10)
Loan fees and charges	0	0	1	1
Net return on mortgage servicing rights	0	0	0	0
Loan administrative (expense) income	(1)	(1)	(2)	(3)
Other noninterest income	15	18	43	46
Total noninterest income	16	13	44	34
Compensation and benefits	28	27	79	77
Commissions	1	0	2	1
Loan processing expense	1	1	4	5
Other noninterest expense	63	42	231	125
Total noninterest expense	93	70	316	208
Income (loss) before indirect overhead allocations and income taxes	83	48	120	105
Indirect overhead allocation income (expense)	(11)	(10)	(31)	(30)
Provision (benefit) for income taxes	15	8	19	16
Net income	57	30	70	59
Average balances
Loans held-for-sale	0	20	0	37
Loans with government guarantees	0	0	0	0
Loans held-for-investment	12,311	11,681	10,702	10,465
Total assets	12,603	12,184	11,100	10,950
Deposits	11,265	10,513	10,817	10,247
Community Banking | Intersegment Eliminations
Summary of Operations
Intersegment (expense) revenue	(22)	(6)	(97)	(14)
Mortgage Originations | Operating Segments
Summary of Operations
Net interest income	49	37	147	103
Provision (benefit) for credit losses	(3)	(2)	(8)	(3)
Net interest income after provision for credit losses	52	39	155	106
Net gain on loan sales	344	114	737	243
Loan fees and charges	29	20	68	47
Net return on mortgage servicing rights	12	(2)	10	9
Loan administrative (expense) income	(10)	(7)	(25)	(16)
Other noninterest income	3	2	5	9
Total noninterest income	378	127	795	292
Compensation and benefits	42	29	111	79
Commissions	71	37	160	75
Loan processing expense	15	10	39	23
Other noninterest expense	32	25	114	63
Total noninterest expense	160	101	424	240
Income (loss) before indirect overhead allocations and income taxes	270	65	526	158
Indirect overhead allocation income (expense)	(18)	(10)	(45)	(30)
Provision (benefit) for income taxes	53	11	101	26
Net income	199	44	380	102
Average balances
Loans held-for-sale	5,602	3,766	5,499	3,495
Loans with government guarantees	2,122	574	1,267	511
Loans held-for-investment	2,498	33	2,693	22
Total assets	11,195	5,378	10,539	5,018
Deposits	0	0	0	0
Mortgage Originations | Intersegment Eliminations
Summary of Operations
Intersegment (expense) revenue	(12)	8	(46)	19
Mortgage Servicing | Operating Segments
Summary of Operations
Net interest income	5	5	14	11
Provision (benefit) for credit losses	0	0	0	0
Net interest income after provision for credit losses	5	5	14	11
Net gain on loan sales	0	0	0	0
Loan fees and charges	16	8	43	22
Net return on mortgage servicing rights	0	0	0	0
Loan administrative (expense) income	40	32	112	91
Other noninterest income	0	0	0	0
Total noninterest income	56	40	155	113
Compensation and benefits	12	7	33	19
Commissions	0	0	0	0
Loan processing expense	8	10	24	30
Other noninterest expense	21	14	57	43
Total noninterest expense	41	31	114	92
Income (loss) before indirect overhead allocations and income taxes	20	14	55	32
Indirect overhead allocation income (expense)	(4)	(4)	(15)	(13)
Provision (benefit) for income taxes	3	2	8	4
Net income	13	8	32	15
Average balances
Loans held-for-sale	0	0	0	0
Loans with government guarantees	0	0	0	0
Loans held-for-investment	0	0	0	0
Total assets	78	38	65	47
Deposits	7,329	4,556	6,114	3,536
Mortgage Servicing | Intersegment Eliminations
Summary of Operations
Intersegment (expense) revenue	$ 10	$ 7	$ 27	$ 19