Segment Information (Details) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2020 USD ($) segments	Jun. 30, 2019 USD ($)
Segment Reporting [Abstract]
Number of operating segments | segments					3
Summary of Operations
Net interest income	$ 168		$ 138		$ 316	$ 264
Provision (benefit) for credit losses	102		17		116	17
Net interest income after provision for credit losses	66		121		200	247
Net gain on loan sales	303		75		393	124
Loan fees and charges	41		24		67	41
Loan administration (expense) income	21		6		33	17
Net return on mortgage servicing rights	(8)		5		(2)	11
Other noninterest income	21		58		44	84
Total noninterest income	378		168		535	277
Compensation and benefits	116		90		218	177
Commissions	61		25		90	38
Loan processing expense	25		21		45	38
Other noninterest expense	94		78		179	152
Total noninterest expense	296		214		532	405
Income (loss) before indirect overhead allocations and income taxes	148		75		203	119
Indirect overhead allocation income (expense)	0		0		0	0
Provision (benefit) for income taxes	32		14		42	22
Net income	116	$ 46	61	$ 36	161	97
Average balances
Loans held-for-sale	5,645		3,539		5,447	3,403
Loans with government guarantees	858		502		834	478
Loans held-for-investment	13,596		10,613		12,709	9,893
Total assets	26,261		19,966		24,837	19,206
Deposits	17,715		14,159		16,755	13,536
Intersegment Eliminations
Summary of Operations
Intersegment (expense) revenue	0		0		0	0
Community Banking
Summary of Operations
Net interest income	133		98		237	191
Provision (benefit) for credit losses	(3)		15		5	16
Net interest income after provision for credit losses	136		83		232	175
Net gain on loan sales	0		(2)		0	(6)
Loan fees and charges	0		0		0	1
Loan administration (expense) income	(1)		0		(2)	(2)
Net return on mortgage servicing rights	0		0		0	0
Other noninterest income	12		15		28	28
Total noninterest income	11		13		26	21
Compensation and benefits	24		25		51	50
Commissions	0		0		1	0
Loan processing expense	1		2		3	4
Other noninterest expense	124		44		168	83
Total noninterest expense	149		71		223	137
Income (loss) before indirect overhead allocations and income taxes	(2)		25		35	59
Indirect overhead allocation income (expense)	(11)		(10)		(20)	(20)
Provision (benefit) for income taxes	(3)		3		3	8
Net income	(10)		12		12	31
Average balances
Loans held-for-sale	0		19		0	45
Loans with government guarantees	0		0		0	0
Loans held-for-investment	10,878		10,563		9,888	9,847
Total assets	11,292		11,061		10,340	10,323
Deposits	10,747		10,238		10,590	10,111
Community Banking | Intersegment Eliminations
Summary of Operations
Intersegment (expense) revenue	(69)		(8)		(75)	(8)
Mortgage Originations
Summary of Operations
Net interest income	56		33		98	66
Provision (benefit) for credit losses	(2)		1		(5)	1
Net interest income after provision for credit losses	58		32		103	65
Net gain on loan sales	303		78		393	129
Loan fees and charges	22		17		39	27
Loan administration (expense) income	(8)		(5)		(15)	(10)
Net return on mortgage servicing rights	(8)		4		(2)	11
Other noninterest income	1		3		2	7
Total noninterest income	310		97		417	164
Compensation and benefits	38		27		69	50
Commissions	61		25		89	38
Loan processing expense	14		8		24	13
Other noninterest expense	56		20		82	38
Total noninterest expense	169		80		264	139
Income (loss) before indirect overhead allocations and income taxes	199		49		256	90
Indirect overhead allocation income (expense)	(15)		(10)		(27)	(20)
Provision (benefit) for income taxes	39		8		48	15
Net income	145		31		181	55
Average balances
Loans held-for-sale	5,645		3,520		5,447	3,358
Loans with government guarantees	858		502		834	478
Loans held-for-investment	2,688		21		2,792	16
Total assets	10,598		5,045		10,207	4,836
Deposits	0		0		0	0
Mortgage Originations | Intersegment Eliminations
Summary of Operations
Intersegment (expense) revenue	(34)		9		(34)	12
Mortgage Servicing
Summary of Operations
Net interest income	4		3		8	6
Provision (benefit) for credit losses	0		0		0	0
Net interest income after provision for credit losses	4		3		8	6
Net gain on loan sales	0		0		0	0
Loan fees and charges	19		8		28	14
Loan administration (expense) income	36		30		72	59
Net return on mortgage servicing rights	0		0		0	0
Other noninterest income	0		0		0	0
Total noninterest income	55		38		100	73
Compensation and benefits	11		6		21	12
Commissions	0		0		0	0
Loan processing expense	9		10		16	20
Other noninterest expense	17		14		36	29
Total noninterest expense	37		30		73	61
Income (loss) before indirect overhead allocations and income taxes	22		11		35	18
Indirect overhead allocation income (expense)	(6)		(4)		(11)	(9)
Provision (benefit) for income taxes	3		2		5	2
Net income	13		5		19	7
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	69		48		59	52
Deposits	6,222		3,501		5,499	3,017
Mortgage Servicing | Intersegment Eliminations
Summary of Operations
Intersegment (expense) revenue	9		6		17	12
Other
Summary of Operations
Net interest income	(25)		4		(27)	1
Provision (benefit) for credit losses	107		1		116	0
Net interest income after provision for credit losses	(132)		3		(143)	1
Net gain on loan sales	0		(1)		0	1
Loan fees and charges	0		(1)		0	(1)
Loan administration (expense) income	(6)		(19)		(22)	(30)
Net return on mortgage servicing rights	0		1		0	0
Other noninterest income	8		40		14	49
Total noninterest income	2		20		(8)	19
Compensation and benefits	43		32		77	65
Commissions	0		0		0	0
Loan processing expense	1		1		2	1
Other noninterest expense	(103)		0		(107)	2
Total noninterest expense	(59)		33		(28)	68
Income (loss) before indirect overhead allocations and income taxes	(71)		(10)		(123)	(48)
Indirect overhead allocation income (expense)	32		24		58	49
Provision (benefit) for income taxes	(7)		1		(14)	(3)
Net income	(32)		13		(51)	4
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	30
Total assets	4,302		3,812		4,231	3,995
Deposits	746		420		666	408
Other | Intersegment Eliminations
Summary of Operations
Intersegment (expense) revenue	$ 94		$ (7)		$ 92	$ (16)