Segment Information - Financial Information by Business Segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Summary of Operations
Net interest income	$ 148	$ 126
Provision for credit losses	14	0
Net interest income after provision for credit losses	134	126
Net gain on loan sales	90	49
Loan fees and charges	26	17
Loan administration (expense) income	12	11
Net return on mortgage servicing rights	6	6
Other noninterest income	23	26
Total noninterest income	157	109
Compensation and benefits	102	87
Commissions	29	13
Loan processing expense	20	17
Other noninterest expense	84	74
Total noninterest expense	235	191
Income before indirect overhead allocations and income taxes	56	44
Indirect overhead allocation Income (expense)	0	0
Provision for income taxes	10	8
Net income	46	36
Intersegment (expense) revenue	0	0
Average balances
Loans held-for-sale	5,248	3,266
Loans with government guarantees	811	455
Loans held-for-investment	11,823	9,164
Total assets	23,413	18,438
Deposits	15,795	12,906
Community Banking
Summary of Operations
Net interest income	104	93
Provision for credit losses	8	1
Net interest income after provision for credit losses	96	92
Net gain on loan sales	0	(3)
Loan fees and charges	0	0
Loan administration (expense) income	(1)	(1)
Net return on mortgage servicing rights	0	0
Other noninterest income	16	12
Total noninterest income	15	8
Compensation and benefits	27	24
Commissions	1	0
Loan processing expense	2	1
Other noninterest expense	44	40
Total noninterest expense	74	66
Income before indirect overhead allocations and income taxes	37	34
Indirect overhead allocation Income (expense)	(9)	(10)
Provision for income taxes	6	5
Net income	22	19
Intersegment (expense) revenue	(6)
Average balances
Loans held-for-sale	0	0
Loans with government guarantees	0	0
Loans held-for-investment	8,898	6,234
Total assets	9,387	6,589
Deposits	10,434	9,983
Community Banking | Intersegment Eliminations
Summary of Operations
Intersegment (expense) revenue		0
Mortgage Originations
Summary of Operations
Net interest income	42	33
Provision for credit losses	(3)	0
Net interest income after provision for credit losses	45	33
Net gain on loan sales	90	52
Loan fees and charges	17	10
Loan administration (expense) income	(7)	(4)
Net return on mortgage servicing rights	6	6
Other noninterest income	1	4
Total noninterest income	107	68
Compensation and benefits	31	24
Commissions	28	12
Loan processing expense	10	5
Other noninterest expense	26	19
Total noninterest expense	95	60
Income before indirect overhead allocations and income taxes	57	41
Indirect overhead allocation Income (expense)	(12)	(10)
Provision for income taxes	9	7
Net income	36	24
Intersegment (expense) revenue	1
Average balances
Loans held-for-sale	5,248	3,266
Loans with government guarantees	811	455
Loans held-for-investment	2,895	2,901
Total assets	9,817	7,612
Deposits	0	0
Mortgage Originations | Intersegment Eliminations
Summary of Operations
Intersegment (expense) revenue		3
Mortgage Servicing
Summary of Operations
Net interest income	4	3
Provision for credit losses	0	0
Net interest income after provision for credit losses	4	3
Net gain on loan sales	0	0
Loan fees and charges	9	7
Loan administration (expense) income	36	29
Net return on mortgage servicing rights	0	0
Other noninterest income	0	0
Total noninterest income	45	36
Compensation and benefits	10	6
Commissions	0	0
Loan processing expense	7	10
Other noninterest expense	19	15
Total noninterest expense	36	31
Income before indirect overhead allocations and income taxes	13	8
Indirect overhead allocation Income (expense)	(5)	(5)
Provision for income taxes	2	0
Net income	6	3
Intersegment (expense) revenue	8
Average balances
Loans held-for-sale	0	0
Loans with government guarantees	0	0
Loans held-for-investment	0	0
Total assets	48	56
Deposits	4,777	2,528
Mortgage Servicing | Intersegment Eliminations
Summary of Operations
Intersegment (expense) revenue		6
Other
Summary of Operations
Net interest income	(2)	(3)
Provision for credit losses	9	(1)
Net interest income after provision for credit losses	(11)	(2)
Net gain on loan sales	0	0
Loan fees and charges	0	0
Loan administration (expense) income	(16)	(13)
Net return on mortgage servicing rights	0	0
Other noninterest income	6	10
Total noninterest income	(10)	(3)
Compensation and benefits	34	33
Commissions	0	1
Loan processing expense	1	1
Other noninterest expense	(5)	0
Total noninterest expense	30	35
Income before indirect overhead allocations and income taxes	(51)	(40)
Indirect overhead allocation Income (expense)	26	25
Provision for income taxes	(7)	(4)
Net income	(18)	(11)
Intersegment (expense) revenue	(3)
Average balances
Loans held-for-sale	0	0
Loans with government guarantees	0	0
Loans held-for-investment	30	29
Total assets	4,161	4,181
Deposits	$ 584	395
Other | Intersegment Eliminations
Summary of Operations
Intersegment (expense) revenue		$ (9)