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	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024
Financing Receivable, Credit Quality Indicator [Line Items]
2025	$ 2,552
2024	1,749	$ 3,112	$ 2,397
2023	910	1,205	1,478
2022	1,317	1,571	1,671
2021	710	836	905
2020		355	405
Prior	768	637	689
Revolving Loans Amortized Cost Basis	238	198	179
Total	8,244	7,914	7,724
Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023	23	3	1
2022	1	2	2
Prior			0
Total	24	5	3
C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	526
2024	330	696	547
2023	139	229	335
2022	105	151	178
2021	37	46	58
2020		24	26
Prior	38	31	32
Revolving Loans Amortized Cost Basis	0	0	0
Total	1,175	1,177	1,176
C&I [Member] \| Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023	17	1	1
Total	17	1	1
Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	124
2024	50	175	131
2023	24	26	29
2022	72	77	79
2021	11	12	13
2020		8	9
Prior	67	71	73
Revolving Loans Amortized Cost Basis	0	0	0
Total	348	369	334
Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	63
2024	18	79	56
2023	3	8	19
2022	4	5	5
2021	1	2	2
2020		1	1
Prior	0	0	0
Revolving Loans Amortized Cost Basis	0	0	0
Total	89	95	83
Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	648
2024	333	675	520
2023	100	134	229
2022	72	167	181
2021	47	56	57
2020		14	17
Prior	14	8	9
Revolving Loans Amortized Cost Basis	1	1	1
Total	1,215	1,055	1,014
Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	71
2024	61	115	76
2023	36	43	45
2022	62	76	80
2021	56	60	65
2020		22	24
Prior	37	24	26
Revolving Loans Amortized Cost Basis	0		0
Total	323	340	316
Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	145
2024	104	180	151
2023	66	90	94
2022	256	246	245
2021	127	140	163
2020		65	66
Prior	104	85	107
Revolving Loans Amortized Cost Basis	0	0	0
Total	802	806	826
Non-Owner Occupied CRE [Member] \| Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023	1	1
Total	1	1
Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	183
2024	149	173	132
2023	121	142	147
2022	284	305	321
2021	175	194	203
2020		100	106
Prior	207	169	178
Revolving Loans Amortized Cost Basis	0	0	0
Total	1,119	1,083	1,087
Owner Occupied CRE [Member] \| Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023	4
Total	4
Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	438
2024	423	612	454
2023	322	375	400
2022	368	391	405
2021	231	273	283
2020		109	140
Prior	297	246	259
Revolving Loans Amortized Cost Basis	230	190	171
Total	2,309	2,196	2,112
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	298
2024	243	339	271
2023	79	123	138
2022	76	127	145
2021	19	39	46
2020		10	14
Prior	3	2	4
Revolving Loans Amortized Cost Basis	0	0	0
Total	718	640	618
Auto [Member] \| Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023		1
2022	1	1	1
Total	1	2	1
Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	56
2024	38	68	59
2023	20	35	42
2022	18	26	32
2021	6	14	15
2020		2	2
Prior	1	1	1
Revolving Loans Amortized Cost Basis	7	7	7
Total	146	153	158
Non-Auto [Member] \| Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022		1	1
Total		1	1
Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	2,520
2024	1,713	3,057	2,359
2023	882	1,173	1,450
2022	1,254	1,506	1,605
2021	662	802	872
2020		338	388
Prior	725	608	645
Revolving Loans Amortized Cost Basis	235	196	175
Total	7,991	7,680	7,494
Pass [Member] \| C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	514
2024	324	677	533
2023	136	226	331
2022	102	146	175
2021	22	45	57
2020		23	25
Prior	37	30	30
Total	1,135	1,147	1,151
Pass [Member] \| Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	124
2024	50	175	131
2023	24	26	29
2022	71	77	79
2021		12	13
2020		8	9
Prior	59	71	73
Total	339	369	334
Pass [Member] \| Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	62
2024	17	76	53
2023	3	8	19
2022	3	4	5
2021		2	2
2020		1	1
Total	86	91	80
Pass [Member] \| Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	641
2024	327	667	513
2023	99	133	227
2022	71	166	180
2021	46	55	57
2020		14	17
Prior	14	8	9
Revolving Loans Amortized Cost Basis	1	1	1
Total	1,199	1,044	1,004
Pass [Member] \| Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	70
2024	61	114	76
2023	36	42	44
2022	58	72	76
2021	55	60	65
2020		18	20
Prior	33	23	25
Total	313	329	306
Pass [Member] \| Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	138
2024	104	166	147
2023	61	86	91
2022	248	242	244
2021	115	133	155
2020		58	60
Prior	99	81	91
Total	765	766	788
Pass [Member] \| Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	182
2024	140	170	128
2023	112	128	138
2022	254	270	279
2021	161	177	185
2020		99	104
Prior	198	160	166
Total	1,047	1,004	1,000
Pass [Member] \| Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	435
2024	410	606	448
2023	313	367	392
2022	355	378	392
2021	226	266	278
2020		105	136
Prior	281	232	246
Revolving Loans Amortized Cost Basis	227	188	167
Total	2,247	2,142	2,059
Pass [Member] \| Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	298
2024	242	338	271
2023	78	122	137
2022	75	126	144
2021	19	38	45
2020		10	14
Prior	3	2	4
Revolving Loans Amortized Cost Basis	0
Total	715	636	615
Pass [Member] \| Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	56
2024	38	68	59
2023	20	35	42
2022	17	25	31
2021	6	14	15
2020		2	2
Prior	1	1	1
Revolving Loans Amortized Cost Basis	7	7	7
Total	145	152	157
Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	6
2024	16	15	16
2023	5	3	7
2022	10	8	1
2021	27	11	8
2020		1	1
Prior	13	4	4
Revolving Loans Amortized Cost Basis	0		0
Total	77	42	37
Special Mention [Member] \| C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	5
2024	3	7	9
2023	1		1
2022	1	1
2021	15
2020		1
Prior	1
Total	26	9	10
Special Mention [Member] \| Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022	1
Prior	8
Total	9		0
Special Mention [Member] \| Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total	3
Special Mention [Member] \| Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	1
2024		1	2
2023			0
2021			0
Total	1	1	2
Special Mention [Member] \| Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		4
2023	2	2
2022	6	1
2021	3	1	1
Prior	1	1	1
Total	12	9	3
Special Mention [Member] \| Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	9	2	2
2023	1		5
2022	1	5
2021	7	7	5
2020			1
Prior	1	1	1
Total	19	15	14
Special Mention [Member] \| Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	4	1	2
2023	1	1	1
2022	1	1	1
2021	2	3	2
Prior	2	2	2
Revolving Loans Amortized Cost Basis		0	0
Total	10	8	8
Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	26
2024	20	40	22
2023	23	29	21
2022	53	57	65
2021	21	23	25
2020		16	16
Prior	30	25	40
Revolving Loans Amortized Cost Basis	3	2	4
Total	176	192	193
Substandard [Member] \| C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	7
2024	3	12	5
2023	2	3	3
2022	2	4	3
2021		1	1
2020			1
Prior	0	1	2
Total	14	21	15
Substandard [Member] \| Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021			0
Total			0
Substandard [Member] \| Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	1
2024	1	3	3
2022	1	1
Total		4	3
Substandard [Member] \| Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	6
2024	6	7	5
2023	1	1	2
2022	1	1	1
2021	1	1	0
Total	15	10	8
Substandard [Member] \| Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	1
2024		1
2023		1	1
2022	4	4	4
2021	1
2020		4	4
Prior	4	1	1
Total	10	11	10
Substandard [Member] \| Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	7
2024		10	3
2023	3	2	3
2022	2	3	1
2021	9	6	7
2020		7	6
Prior	4	3	15
Total	25	31	35
Substandard [Member] \| Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	1
2024		1	2
2023	8	14	4
2022	29	30	42
2021	7	10	13
2020		1	1
Prior	8	8	11
Total	53	64	73
Substandard [Member] \| Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	3
2024	9	5	4
2023	8	7	7
2022	12	12	12
2021	3	4	3
2020		4	4
Prior	14	12	11
Revolving Loans Amortized Cost Basis	3	2	4
Total	52	46	45
Substandard [Member] \| Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	1	1
2023	1	1	1
2022	1	1	1
2021		1	1
Total	3	4	3
Substandard [Member] \| Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022	1	1	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