Loans Held-for-Investment and Allowance for Loan Losses - Schedule of Company's Internal Ratings of its Loans Held-for-Investment, Including The Year of Origination, by Portfolio Segments (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	$ 2,038	$ 2,418
2020-2019	1,234	832
2019-2018	585	571
2018-2017	407	350
2017-2016	255	279
Prior	663	619
Revolving Loans Amortized Cost Basis	104	102
Total	5,286	5,171
C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	492	895
2020-2019	192	107
2019-2018	65	74
2018-2017	38	29
2017-2016	21	10
Prior	12	16
Total	820	1,131
Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	22	28
2020-2019	17	19
2019-2018	6	29
2018-2017	22	19
2017-2016	17	14
Prior	82	72
Total	166	181
Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	57	63
2020-2019	21	19
2019-2018	11	9
2018-2017	6	3
2017-2016	3	1
Prior	1
Total	99	95
Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	388	377
2020-2019	196	102
2019-2018	38	36
2018-2017	18	22
2017-2016	8	7
Prior	8	10
Total	656	554
Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	94	64
2020-2019	47	23
2019-2018	16	18
2018-2017	12	12
2017-2016	8	11
Prior	26	24
Total	203	152
Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	159	198
2020-2019	157	119
2019-2018	114	94
2018-2017	76	52
2017-2016	45	56
Prior	124	99
Total	675	618
Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	205	186
2020-2019	169	141
2019-2018	130	127
2018-2017	113	83
2017-2016	63	67
Prior	144	143
Total	824	747
Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	382	381
2020-2019	286	176
2019-2018	129	138
2018-2017	97	105
2017-2016	79	103
Prior	258	250
Revolving Loans Amortized Cost Basis	98	95
Total	1,329	1,248
Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	174	178
2020-2019	122	105
2019-2018	65	39
2018-2017	21	21
2017-2016	9	9
Prior	3	2
Total	394	354
Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	65	48
2020-2019	27	21
2019-2018	11	7
2018-2017	4	4
2017-2016	2	1
Prior	5	3
Revolving Loans Amortized Cost Basis	6	7
Total	120	91
Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	1,999	2,356
2020-2019	1,211	804
2019-2018	556	540
2018-2017	385	320
2017-2016	236	273
Prior	631	580
Revolving Loans Amortized Cost Basis	102	100
Total	5,120	4,973
Pass [Member] | C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	485	874
2020-2019	185	101
2019-2018	62	70
2018-2017	35	28
2017-2016	19	10
Prior	12	16
Total	798	1,099
Pass [Member] | Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	22	26
2020-2019	17	19
2019-2018	6	29
2018-2017	22	14
2017-2016	17	13
Prior	82	71
Total	166	172
Pass [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	51	57
2020-2019	19	19
2019-2018	11	9
2018-2017	6	3
2017-2016	3	1
Prior	1
Total	91	89
Pass [Member] | Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	383	371
2020-2019	195	97
2019-2018	37	36
2018-2017	18	19
2017-2016	8	7
Prior	8	9
Total	649	539
Pass [Member] | Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	94	57
2020-2019	46	22
2019-2018	16	18
2018-2017	11	11
2017-2016	8	11
Prior	25	23
Total	200	142
Pass [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	158	197
2020-2019	156	117
2019-2018	98	93
2018-2017	75	44
2017-2016	35	55
Prior	117	88
Total	639	594
Pass [Member] | Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	193	176
2020-2019	166	132
2019-2018	125	105
2018-2017	99	75
2017-2016	59	65
Prior	133	132
Total	775	685
Pass [Member] | Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	374	373
2020-2019	279	172
2019-2018	126	134
2018-2017	94	101
2017-2016	76	101
Prior	245	237
Revolving Loans Amortized Cost Basis	96	93
Total	1,290	1,211
Pass [Member] | Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	174	177
2020-2019	121	104
2019-2018	64	39
2018-2017	21	21
2017-2016	9	9
Prior	3	2
Total	392	352
Pass [Member] | Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	65	48
2020-2019	27	21
2019-2018	11	7
2018-2017	4	4
2017-2016	2	1
Prior	5	2
Revolving Loans Amortized Cost Basis	6	7
Total	120	90
Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	12	20
2020-2019	7	12
2019-2018	15	4
2018-2017	2	14
2017-2016	10	3
Prior	7	4
Total	53	57
Special Mention [Member] | C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	2	9
2020-2019	1	2
2019-2018	1
2018-2017	1	1
2017-2016	1
Total	6	12
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020-2019	1
Total	1
Special Mention [Member] | Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	3	2
2020-2019		4
Total	3	6
Special Mention [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020		1
2020-2019	1
2019-2018	13	1
2018-2017		8
2017-2016	7	1
Prior	3
Total	24	11
Special Mention [Member] | Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	3	5
2020-2019	0	5
2019-2018	1	2
2018-2017	0	4
2017-2016	1	1
Prior	1	1
Total	6	18
Special Mention [Member] | Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	4	3
2020-2019	4	1
2019-2018		1
2018-2017	1	1
2017-2016	1	1
Prior	3	3
Total	13	10
Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	27	42
2020-2019	16	16
2019-2018	14	27
2018-2017	20	16
2017-2016	9	3
Prior	25	35
Revolving Loans Amortized Cost Basis	2	2
Total	113	141
Substandard [Member] | C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	5	12
2020-2019	6	4
2019-2018	2	4
2018-2017	2
2017-2016	1
Total	16	20
Substandard [Member] | Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020		2
2018-2017		5
2017-2016		1
Prior		1
Total		9
Substandard [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	6	6
2020-2019	1
Total	7	6
Substandard [Member] | Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	2	4
2020-2019	1	1
2019-2018	1
2018-2017		3
Prior		1
Total	4	9
Substandard [Member] | Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020		7
2020-2019	1	1
2018-2017	1	1
Prior	1	1
Total	3	10
Substandard [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	1
2020-2019		2
2019-2018	3
2018-2017	1
2017-2016	3
Prior	4	11
Total	12	13
Substandard [Member] | Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	9	5
2020-2019	3	4
2019-2018	4	20
2018-2017	14	4
2017-2016	3	1
Prior	10	10
Total	43	44
Substandard [Member] | Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	4	5
2020-2019	3	3
2019-2018	3	3
2018-2017	2	3
2017-2016	2	1
Prior	10	10
Revolving Loans Amortized Cost Basis	2	2
Total	26	27
Substandard [Member] | Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020		1
2020-2019	1	1
2019-2018	1
Total	$ 2	2
Substandard [Member] | Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Prior		1
Total		$ 1