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, 2022	Dec. 31, 2021	Sep. 30, 2021
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	$ 2,533	$ 2,585	$ 2,038
2021-2020	1,697	1,027	1,234
2020-2019	712	499	585
2019-2018	336	353	407
2018-2017	238	236	255
Prior	604	579	663
Revolving Loans Amortized Cost Basis	136	110	104
Total	6,256	5,389	5,286
C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	507	537	492
2021-2020	233	183	192
2020-2019	68	57	65
2019-2018	26	32	38
2018-2017	21	17	21
Prior	16	11	12
Revolving Loans Amortized Cost Basis	0
Total	871	837	820
Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	67	39	22
2021-2020	21	15	17
2020-2019	16	6	6
2019-2018	5	22	22
2018-2017	20	17	17
Prior	86	79	82
Revolving Loans Amortized Cost Basis	0
Total	215	178	166
Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	46	73	57
2021-2020	22	9	21
2020-2019	4	6	11
2019-2018	3	6	6
2018-2017	1	3	3
Prior	1	1	1
Revolving Loans Amortized Cost Basis	0
Total	77	98	99
Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	508	561	388
2021-2020	320	136	196
2020-2019	78	24	38
2019-2018	15	14	18
2018-2017	8	7	8
Prior	9	8	8
Revolving Loans Amortized Cost Basis	0
Total	938	750	656
Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	102	118	94
2021-2020	89	43	47
2020-2019	33	15	16
2019-2018	12	11	12
2018-2017	8	7	8
Prior	24	23	26
Revolving Loans Amortized Cost Basis	0
Total	268	217	203
Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	228	214	159
2021-2020	202	130	157
2020-2019	109	92	114
2019-2018	64	57	76
2018-2017	23	41	45
Prior	92	90	124
Revolving Loans Amortized Cost Basis	0
Total	718	624	675
Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	281	260	205
2021-2020	226	147	169
2020-2019	137	118	130
2019-2018	88	103	113
2018-2017	78	65	63
Prior	136	129	144
Revolving Loans Amortized Cost Basis	0
Total	946	822	824
Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	437	483	382
2021-2020	387	237	286
2020-2019	189	118	129
2019-2018	89	87	97
2018-2017	69	71	79
Prior	236	235	258
Revolving Loans Amortized Cost Basis	129	103	98
Total	1,536	1,334	1,329
Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	284	219	174
2021-2020	149	105	122
2020-2019	65	55	65
2019-2018	30	17	21
2018-2017	8	7	9
Prior	3	2	3
Revolving Loans Amortized Cost Basis	0
Total	539	405	394
Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	73	81	65
2021-2020	48	22	27
2020-2019	13	8	11
2019-2018	4	4	4
2018-2017	2	1	2
Prior	1	1	5
Revolving Loans Amortized Cost Basis	7	7	6
Total	148	124	120
Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	2,506	2,548	1,999
2021-2020	1,676	1,005	1,211
2020-2019	698	471	556
2019-2018	311	332	385
2018-2017	222	217	236
Prior	566	548	631
Revolving Loans Amortized Cost Basis	133	107	102
Total	6,112	5,228	5,120
Pass [Member] | C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	495	526	485
2021-2020	226	178	185
2020-2019	65	52	62
2019-2018	24	29	35
2018-2017	18	17	19
Prior	16	11	12
Total	844	813	798
Pass [Member] | Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	67	39	22
2021-2020	21	15	17
2020-2019	16	6	6
2019-2018	5	22	22
2018-2017	20	17	17
Prior	86	79	82
Total	215	178	166
Pass [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	46	69	51
2021-2020	21	8	19
2020-2019	4	6	11
2019-2018	3	6	6
2018-2017	1	3	3
Prior	1	1	1
Total	76	93	91
Pass [Member] | Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	504	557	383
2021-2020	317	134	195
2020-2019	78	24	37
2019-2018	15	14	18
2018-2017	8	7	8
Prior	9	8	8
Total	931	744	649
Pass [Member] | Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	101	117	94
2021-2020	89	42	46
2020-2019	33	15	16
2019-2018	12	10	11
2018-2017	8	7	8
Prior	23	23	25
Total	266	214	200
Pass [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	223	214	158
2021-2020	200	128	156
2020-2019	108	77	98
2019-2018	51	56	75
2018-2017	22	31	35
Prior	79	84	117
Total	683	590	639
Pass [Member] | Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	280	250	193
2021-2020	223	143	166
2020-2019	134	114	125
2019-2018	81	90	99
2018-2017	67	59	59
Prior	124	117	133
Total	909	773	775
Pass [Member] | Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	433	477	374
2021-2020	382	230	279
2020-2019	183	115	126
2019-2018	87	84	94
2018-2017	68	68	76
Prior	224	222	245
Revolving Loans Amortized Cost Basis	126	100	96
Total	1,503	1,296	1,290
Pass [Member] | Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	284	218	174
2021-2020	149	105	121
2020-2019	64	54	64
2019-2018	29	17	21
2018-2017	8	7	9
Prior	3	2	3
Total	537	403	392
Pass [Member] | Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	73	81	65
2021-2020	48	22	27
2020-2019	13	8	11
2019-2018	4	4	4
2018-2017	2	1	2
Prior	1	1	5
Revolving Loans Amortized Cost Basis	7	7	6
Total	148	124	120
Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	1	12	12
2021-2020	6	8	7
2020-2019	5	17	15
2019-2018	16	1	2
2018-2017	0	9	10
Prior	11	7	7
Revolving Loans Amortized Cost Basis		1
Total	39	55	53
Special Mention [Member] | C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	1	4	2
2021-2020		1	1
2020-2019		4	1
2019-2018	1		1
2018-2017			1
Total	2	9	6
Special Mention [Member] | Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020			1
Total			1
Special Mention [Member] | Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021		2	3
2021-2020	2
Total	2	2	3
Special Mention [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020	1	1	1
2020-2019		12	13
2019-2018	11
2018-2017		7	7
Prior	7	3	3
Total	19	23	24
Special Mention [Member] | Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021		2	3
2021-2020	1	2
2020-2019	2	1	1
2019-2018	4
2018-2017	0	1	1
Prior	1	1	1
Total	8	7	6
Special Mention [Member] | Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021		3	4
2021-2020	2	4	4
2020-2019	3
2019-2018		1	1
2018-2017		1	1
Prior	3	3	3
Revolving Loans Amortized Cost Basis		1
Total	8	13	13
Special Mention [Member] | Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021		1
Total	0	1
Special Mention [Member] | Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0
Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	26	25	27
2021-2020	15	14	16
2020-2019	9	11	14
2019-2018	9	20	20
2018-2017	16	10	9
Prior	27	24	25
Revolving Loans Amortized Cost Basis	3	2	2
Total	105	106	113
Substandard [Member] | C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	11	7	5
2021-2020	7	4	6
2020-2019	3	1	2
2019-2018	1	3	2
2018-2017	3		1
Total	25	15	16
Substandard [Member] | Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Prior			0
Total	0		0
Substandard [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021		4	6
2021-2020	1	1	1
2018-2017			0
Total	1	5	7
Substandard [Member] | Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	4	2	2
2021-2020	1	2	1
2020-2019			1
Total	5	4	4
Substandard [Member] | Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	1	1
2021-2020		1	1
2019-2018		1	1
Prior	1		1
Total	2	3	3
Substandard [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	5		1
2021-2020	1	1
2020-2019	1	3	3
2019-2018	2	1	1
2018-2017	1	3	3
Prior	6	3	4
Total	16	11	12
Substandard [Member] | Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	1	8	9
2021-2020	2	2	3
2020-2019	1	3	4
2019-2018	3	13	14
2018-2017	11	5	3
Prior	11	11	10
Total	29	42	43
Substandard [Member] | Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	4	3	4
2021-2020	3	3	3
2020-2019	3	3	3
2019-2018	2	2	2
2018-2017	1	2	2
Prior	9	10	10
Revolving Loans Amortized Cost Basis	3	2	2
Total	25	25	26
Substandard [Member] | Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020			1
2020-2019	1	1	1
2019-2018	1
Total	2	$ 1	$ 2
Substandard [Member] | Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 0