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	Jun. 30, 2022	Dec. 31, 2021	Jun. 30, 2021
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	$ 1,721	$ 2,585	$ 1,503
2021-2020	1,996	1,027	1,535
2020-2019	760	499	652
2019-2018	372	353	448
2018-2017	256	236	285
Prior	650	579	779
Revolving Loans Amortized Cost Basis	123	110	103
Total	5,878	5,389	5,305
C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	357	537	493
2021-2020	323	183	324
2020-2019	82	57	74
2019-2018	33	32	50
2018-2017	23	17	23
Prior	22	11	19
Revolving Loans Amortized Cost Basis	0
Total	840	837	983
Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	49	39	23
2021-2020	21	15	23
2020-2019	14	6	8
2019-2018	5	22	22
2018-2017	21	17	18
Prior	91	79	85
Revolving Loans Amortized Cost Basis	0
Total	201	178	179
Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	40	73	30
2021-2020	41	9	39
2020-2019	4	6	15
2019-2018	3	6	7
2018-2017	1	3	4
Prior	1	1	1
Revolving Loans Amortized Cost Basis	0
Total	90	98	96
Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	332	561	223
2021-2020	449	136	241
2020-2019	104	24	43
2019-2018	22	14	18
2018-2017	11	7	9
Prior	11	8	17
Revolving Loans Amortized Cost Basis	0
Total	929	750	551
Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	63	118	59
2021-2020	104	43	55
2020-2019	35	15	17
2019-2018	13	11	13
2018-2017	8	7	9
Prior	27	23	32
Revolving Loans Amortized Cost Basis	0
Total	250	217	185
Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	136	214	103
2021-2020	203	130	172
2020-2019	111	92	120
2019-2018	65	57	81
2018-2017	25	41	50
Prior	96	90	148
Revolving Loans Amortized Cost Basis	0
Total	636	624	674
Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	241	260	146
2021-2020	224	147	183
2020-2019	126	118	135
2019-2018	92	103	118
2018-2017	82	65	70
Prior	145	129	168
Revolving Loans Amortized Cost Basis	0
Total	910	822	820
Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	280	483	268
2021-2020	404	237	324
2020-2019	192	118	149
2019-2018	97	87	108
2018-2017	72	71	87
Prior	251	235	295
Revolving Loans Amortized Cost Basis	116	103	97
Total	1,412	1,334	1,328
Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	168	219	118
2021-2020	171	105	141
2020-2019	77	55	77
2019-2018	37	17	26
2018-2017	11	7	13
Prior	4	2	9
Revolving Loans Amortized Cost Basis	0
Total	468	405	384
Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	55	81	40
2021-2020	56	22	33
2020-2019	15	8	14
2019-2018	5	4	5
2018-2017	2	1	2
Prior	2	1	5
Revolving Loans Amortized Cost Basis	7	7	6
Total	142	124	105
Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	1,692	2,548	1,479
2021-2020	1,970	1,005	1,501
2020-2019	747	471	618
2019-2018	346	332	424
2018-2017	239	217	264
Prior	612	548	743
Revolving Loans Amortized Cost Basis	119	107	102
Total	5,725	5,228	5,131
Pass [Member] \| C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	342	526	485
2021-2020	317	178	316
2020-2019	79	52	71
2019-2018	28	29	46
2018-2017	20	17	22
Prior	22	11	19
Total	808	813	959
Pass [Member] \| Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	49	39	23
2021-2020	21	15	23
2020-2019	14	6	8
2019-2018	5	22	22
2018-2017	21	17	18
Prior	91	79	84
Total	201	178	178
Pass [Member] \| Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	39	69	30
2021-2020	41	8	34
2020-2019	4	6	15
2019-2018	3	6	7
2018-2017	1	3	3
Prior	1	1	1
Total	89	93	90
Pass [Member] \| Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	330	557	39
2021-2020	440	134	237
2020-2019	104	24	220
2019-2018	22	14	18
2018-2017	11	7	9
Prior	11	8	17
Total	918	744	540
Pass [Member] \| Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	63	117	59
2021-2020	104	42	53
2020-2019	35	15	17
2019-2018	13	10	12
2018-2017	8	7	9
Prior	26	23	31
Total	249	214	181
Pass [Member] \| Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	130	214	101
2021-2020	203	128	171
2020-2019	109	77	104
2019-2018	52	56	80
2018-2017	24	31	40
Prior	83	84	138
Total	601	590	634
Pass [Member] \| Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	240	250	137
2021-2020	218	143	178
2020-2019	122	114	129
2019-2018	88	90	103
2018-2017	71	59	64
Prior	133	117	157
Total	872	773	768
Pass [Member] \| Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	277	477	266
2021-2020	399	230	316
2020-2019	188	115	145
2019-2018	94	84	105
2018-2017	70	68	84
Prior	239	222	282
Revolving Loans Amortized Cost Basis	112	100	96
Total	1,379	1,296	1,294
Pass [Member] \| Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	168	218	118
2021-2020	171	105	140
2020-2019	77	54	76
2019-2018	36	17	26
2018-2017	11	7	13
Prior	4	2	9
Total	467	403	382
Pass [Member] \| Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	54	81	40
2021-2020	56	22	33
2020-2019	15	8	14
2019-2018	5	4	5
2018-2017	2	1	2
Prior	2	1	5
Revolving Loans Amortized Cost Basis	7	7	6
Total	141	124	105
Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	6	12	6
2021-2020	9	8	10
2020-2019	5	17	16
2019-2018	15	1	4
2018-2017		9	13
Prior	12	7	7
Revolving Loans Amortized Cost Basis	0	1
Total	47	55	56
Special Mention [Member] \| C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	4	4	4
2021-2020	1	1	2
2020-2019		4	1
2019-2018	2		1
2018-2017			1
Total	7	9	9
Special Mention [Member] \| Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total	0
Special Mention [Member] \| Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021		2
2021-2020	4		2
2020-2019			1
Total	4	2	3
Special Mention [Member] \| Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020		1	1
2020-2019	1	12	13
2019-2018	11		1
2018-2017		7	7
Prior	7	3	3
Total	19	23	25
Special Mention [Member] \| Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021		2	1
2021-2020	2	2	2
2020-2019	2	1	1
2019-2018	1		1
2018-2017		1	4
Prior	1	1	1
Total	6	7	10
Special Mention [Member] \| Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	1	3
2021-2020	2	4	3
2020-2019	2		1
2019-2018	1	1	1
2018-2017		1	1
Prior	4	3	3
Revolving Loans Amortized Cost Basis		1
Total	10	13	9
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]
2022-2021	1
Total	1
Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	23	25	18
2021-2020	17	14	24
2020-2019	8	11	18
2019-2018	11	20	20
2018-2017	17	10	8
Prior	26	24	29
Revolving Loans Amortized Cost Basis	4	2	1
Total	106	106	118
Substandard [Member] \| C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	11	7	4
2021-2020	5	4	6
2020-2019	3	1	2
2019-2018	3	3	3
2018-2017	3
Total	25	15	15
Substandard [Member] \| Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Prior			1
Total	0		1
Substandard [Member] \| Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	1	4
2021-2020		1	5
2018-2017			1
Total	1	5	6
Substandard [Member] \| Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	2	2	4
2021-2020	5	2	2
2020-2019			2
Total	7	4	8
Substandard [Member] \| Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021		1
2021-2020		1	2
2019-2018		1	1
Prior	1		1
Total	1	3	4
Substandard [Member] \| Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	6		2
2021-2020		1
2020-2019	1	3	3
2019-2018	2	1
2018-2017	1	3	3
Prior	6	3	7
Total	16	11	15
Substandard [Member] \| Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	1	8	8
2021-2020	4	2	3
2020-2019	2	3	5
2019-2018	3	13	14
2018-2017	11	5	2
Prior	11	11	10
Total	32	42	42
Substandard [Member] \| Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022-2021	2	3	2
2021-2020	3	3	5
2020-2019	2	3	3
2019-2018	2	2	2
2018-2017	2	2	2
Prior	8	10	10
Revolving Loans Amortized Cost Basis	4	2	1
Total	23	25	25
Substandard [Member] \| Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021-2020			1
2020-2019		1	1
2019-2018	1
Total	1	$ 1	$ 2
Substandard [Member] \| Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 0

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