Loans Held-for-Investment and Allowance for Credit 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, 2025	Dec. 31, 2024	Jun. 30, 2024
Financing Receivable, Credit Quality Indicator [Line Items]
2025	$ 1,681	$ 3,112	$ 1,669
2024	755	1,205	1,768
2023	996	1,571	1,770
2022	1,389	836	968
2021	2,174	355	435
Prior	850	637	743
Revolving Loans Amortized Cost Basis	230	198	167
Total	8,075	7,914	7,520
Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		3
2023	1	2	1
2022	1
Prior			1
Total	2	5	2
C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	432	696	373
2024	413	229	434
2023	165	151	203
2022	111	46	67
2021	40	24	30
Prior	41	31	35
Revolving Loans Amortized Cost Basis	0	0	0
Total	1,202	1,177	1,142
C&I [Member] \| Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		1
Prior			1
Total		1	1
Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	28	175	148
2024	96	26	31
2023	25	77	80
2022	74	12	15
2021	11	8	9
Prior	72	71	76
Revolving Loans Amortized Cost Basis	0	0	0
Total	306	369	359
Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	42	79	41
2024	35	8	36
2023	4	5	5
2022	4	2	3
2021	1	1	1
Prior	0	0	0
Revolving Loans Amortized Cost Basis	0	0	0
Total	86	95	86
Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	389	675	317
2024	470	134	355
2023	111	167	218
2022	133	56	66
2021	52	14	17
Prior	17	8	13
Revolving Loans Amortized Cost Basis	1	1	0
Total	1,173	1,055	986
Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	35	115	63
2024	71	43	52
2023	37	76	82
2022	63	60	68
2021	57	22	27
Prior	40	24	27
Revolving Loans Amortized Cost Basis	0		0
Total	303	340	319
Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	90	180	119
2024	112	90	89
2023	75	246	242
2022	219	140	169
2021	132	65	77
Prior	118	85	120
Revolving Loans Amortized Cost Basis	0	0	0
Total	746	806	816
Non-Owner Occupied CRE [Member] \| Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		1
Total		1
Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	117	173	65
2024	168	142	151
2023	127	305	325
2022	295	194	212
2021	183	100	107
Prior	235	169	190
Revolving Loans Amortized Cost Basis	0	0	0
Total	1,125	1,083	1,050
Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	298	612	287
2024	488	375	414
2023	336	391	413
2022	378	273	296
2021	246	109	147
Prior	319	246	275
Revolving Loans Amortized Cost Basis	221	190	159
Total	2,286	2,196	1,991
Residential [Member] \| Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023	1
Total	1
Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	210	339	215
2024	274	123	155
2023	92	127	167
2022	92	39	55
2021	25	10	17
Prior	6	2	6
Revolving Loans Amortized Cost Basis	0	0	0
Total	699	640	615
Auto [Member] \| Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		1
2023		1	1
2022	1
Total	1	2	1
Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	40	68	41
2024	47	35	51
2023	24	26	35
2022	20	14	17
2021	8	2	3
Prior	2	1	1
Revolving Loans Amortized Cost Basis	8	7	8
Total	149	153	156
Non-Auto [Member] \| Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023		1
Total		1
Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	1,644	3,057	1,615
2024	705	1,173	1,749
2023	970	1,506	1,721
2022	1,326	802	937
2021	2,140	338	418
Prior	806	608	697
Revolving Loans Amortized Cost Basis	227	196	163
Total	7,818	7,680	7,300
Pass [Member] \| C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	417	677	340
2024	409	226	431
2023	161	146	201
2022	107	45	65
2021	27	23	29
Prior	40	30	33
Total	1,161	1,147	1,099
Pass [Member] \| Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	28	175	148
2024	96	26	31
2023	25	77	80
2022	73	12	14
2021	11	8	9
Prior	63	71	76
Total	296	369	358
Pass [Member] \| Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	42	76	38
2024	33	8	36
2023	4	4	5
2022	3	2	3
2021	1	1	1
Total	83	91	83
Pass [Member] \| Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	383	667	304
2024	462	133	350
2023	110	166	215
2022	132	55	60
2021	51	14	17
Prior	17	8	13
Revolving Loans Amortized Cost Basis	1	1
Total	1,156	1,044	959
Pass [Member] \| Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	35	114	63
2024	70	42	51
2023	37	72	78
2022	59	60	68
2021	56	18	22
Prior	36	23	27
Total	293	329	309
Pass [Member] \| Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	77	166	118
2024	111	86	88
2023	72	242	241
2022	213	133	165
2021	120	58	72
Prior	113	81	102
Total	706	766	786
Pass [Member] \| Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	116	170	62
2024	161	128	148
2023	119	270	293
2022	264	177	200
2021	166	99	105
Prior	226	160	175
Total	1,052	1,004	983
Pass [Member] \| Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	296	606	286
2024	478	367	409
2023	327	378	407
2022	365	266	292
2021	240	105	143
Prior	303	232	264
Revolving Loans Amortized Cost Basis	218	188	155
Total	2,227	2,142	1,956
Pass [Member] \| Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	210	338	215
2024	273	122	155
2023	91	126	166
2022	91	38	53
2021	25	10	17
Prior	6	2	6
Total	696	636	612
Pass [Member] \| Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	40	68	41
2024	47	35	50
2023	24	25	35
2022	19	14	17
2021	8	2	3
Prior	2	1	1
Revolving Loans Amortized Cost Basis	8	7	8
Total	148	152	155
Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	7	15	30
2024	25	3	4
2023	4	8	4
2022	6	11	11
2021	6	1	1
Prior	14	4	7
Revolving Loans Amortized Cost Basis	0		1
Total	62	42	58
Special Mention [Member] \| C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	7	7	22
2024	2
2023	1	1
2022	1
2021	13	1
Prior	1
Total	25	9	22
Special Mention [Member] \| Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022	1
Prior	9
Total	10		0
Special Mention [Member] \| Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	2
2022	1
Total	3
Special Mention [Member] \| Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	6	1	6
2024	8		2
2023			2
Total		1	10
Special Mention [Member] \| Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		4
2024		2
2023	2	1
2022	3	1	1
2021	3
Prior	1	1	3
Total	9	9	4
Special Mention [Member] \| Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		2	2
2024	2		1
2023		5
2022		7	8
2021	7
Prior	1	1	2
Total	10	15	13
Special Mention [Member] \| Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		1
2024	2	1	1
2023	1	1	2
2022	1	3	2
2021	2		1
Prior	2	2	2
Revolving Loans Amortized Cost Basis		0	1
Total	8	8	9
Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	30	40	24
2024	25	29	15
2023	22	57	45
2022	57	23	20
2021	28	16	16
Prior	30	25	39
Revolving Loans Amortized Cost Basis	3	2	3
Total	195	192	162
Substandard [Member] \| C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	8	12	11
2024	2	3	3
2023	3	4	2
2022	3	1	2
2021			1
Prior		1	2
Total	16	21	21
Substandard [Member] \| Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022			1
Total			1
Substandard [Member] \| Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		3	3
2023		1
Total		4	3
Substandard [Member] \| Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		7	7
2024		1	3
2023	1	1	1
2022	1	1	6
2021	1
Total	17	10	17
Substandard [Member] \| Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		1
2024	1	1	1
2023		4	4
2022	4
2021	1	4	5
Prior	4	1
Total	10	11	10
Substandard [Member] \| Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	13	10	1
2024	1	2	1
2023	1	3	1
2022	3	6	3
2021	9	7	5
Prior	4	3	15
Total	31	31	26
Substandard [Member] \| Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	1	1	1
2024	5	14	2
2023	8	30	32
2022	31	10	4
2021	10	1	2
Prior	8	8	13
Total	63	64	54
Substandard [Member] \| Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	2	5	1
2024	8	7	4
2023	8	12	4
2022	12	4	2
2021	4	4	3
Prior	14	12	9
Revolving Loans Amortized Cost Basis	3	2	3
Total	51	46	26
Substandard [Member] \| Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		1
2024	1	1
2023	1	1	1
2022	1	1	2
Total	3	4	3
Substandard [Member] \| Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024			1
2023		1
2022	1
Total	$ 1	$ 1	$ 1

Loans Held-for-Investment and Allowance for Credit 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