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	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024
Financing Receivable, Credit Quality Indicator [Line Items]
2025	$ 730	$ 3,112	$ 746
2024	2,701	1,205	2,152
2023	1,093	1,571	1,876
2022	1,471	836	1,015
2021	803	355	493
Prior	930	637	796
Revolving Loans Amortized Cost Basis	218	198	152
Total	7,946	7,914	7,230
Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		3	0
2023		2
2022			0
Total	0	5	0
C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	175	696	244
2024	560	229	578
2023	197	151	228
2022	125	46	71
2021	43	24	34
Prior	44	31	37
Total	1,144	1,177	1,192
C&I [Member] \| Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		1
Total		1	0
Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	10	175	5
2024	140	26	20
2023	26	77	81
2022	75	12	16
2021	12	8	10
Prior	76	71	79
Total	339	369	211
Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	24	79	21
2024	56	8	56
2023	4	5	6
2022	3	2	3
2021	2	1	1
Prior	1		1
Total	90	95	88
Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	172	675	125
2024	571	134	451
2023	117	167	244
2022	163	56	69
2021	54	14	19
Prior	20	8	14
Revolving Loans Amortized Cost Basis	1	1
Total	1,098	1,055	922
Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	21	115	23
2024	101	43	60
2023	40	76	101
2022	67	60	71
2021	58	22	27
Prior	44	24	29
Total	331	340	311
Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	24	180	59
2024	155	90	135
2023	81	246	245
2022	226	140	178
2021	135	65	112
Prior	133	85	125
Total	754	806	854
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	39	173	28
2024	217	142	159
2023	136	305	312
2022	305	194	215
2021	192	100	111
Prior	254	169	208
Total	1,143	1,083	1,033
Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	130	612	132
2024	536	375	458
2023	356	391	429
2022	376	273	307
2021	263	109	154
Prior	347	246	293
Revolving Loans Amortized Cost Basis	210	190	145
Total	2,218	2,196	1,918
Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	117	339	91
2024	308	123	175
2023	106	127	189
2022	108	39	65
2021	32	10	22
Prior	8	2	8
Total	679	640	550
Auto [Member] \| Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		1
2023		1	0
Total		2	0
Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	18	68	18
2024	57	35	60
2023	30	26	41
2022	23	14	20
2021	12	2	3
Prior	3	1	2
Revolving Loans Amortized Cost Basis	7	7	7
Total	150	153	151
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	716	3,057	725
2024	2,653	1,173	2,093
2023	1,066	1,506	1,856
2022	1,410	802	984
2021	765	338	475
Prior	875	608	747
Revolving Loans Amortized Cost Basis	215	196	148
Total	7,700	7,680	7,028
Pass [Member] \| C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	167	677	236
2024	550	226	551
2023	193	146	226
2022	121	45	70
2021	42	23	32
Prior	42	30	36
Total	1,115	1,147	1,151
Pass [Member] \| Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	10	175	5
2024	140	26	20
2023	26	77	81
2022	74	12	15
2021	12	8	10
Prior	67	71	79
Total	329	369	210
Pass [Member] \| Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	24	76	18
2024	53	8	54
2023	4	4	6
2022	3	2	3
2021	2	1	1
Prior	1
Total	87	91	82
Pass [Member] \| Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	169	667	117
2024	567	133	438
2023	116	166	240
2022	162	55	63
2021	53	14	19
Prior	20	8	14
Revolving Loans Amortized Cost Basis	1	1
Total	1,088	1,044	891
Pass [Member] \| Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	21	114	23
2024	100	42	58
2023	40	72	101
2022	63	60	71
2021	57	18	23
Prior	39	23	29
Total	320	329	305
Pass [Member] \| Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	24	166	59
2024	143	86	133
2023	77	242	244
2022	223	133	174
2021	125	58	104
Prior	122	81	108
Total	714	766	822
Pass [Member] \| Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	38	170	28
2024	208	128	155
2023	128	270	305
2022	272	177	202
2021	174	99	110
Prior	244	160	190
Total	1,064	1,004	990
Pass [Member] \| Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	128	606	130
2024	528	367	450
2023	347	378	424
2022	363	266	302
2021	257	105	151
Prior	329	232	281
Revolving Loans Amortized Cost Basis	207	188	141
Total	2,159	2,142	1,879
Pass [Member] \| Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	117	338	91
2024	307	122	175
2023	105	126	188
2022	107	38	64
2021	31	10	22
Prior	8	2	8
Total	675	636	548
Pass [Member] \| Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	18	68	18
2024	57	35	59
2023	30	25	41
2022	22	14	20
2021	12	2	3
Prior	3	1	2
Revolving Loans Amortized Cost Basis	7	7	7
Total	149	152	150
Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	7	15	13
2024	6	3	28
2023	4	8	6
2022	4	11	10
2021	11	1	2
Prior	14	4	3
Revolving Loans Amortized Cost Basis	0	0	1
Total	46	42	63
Special Mention [Member] \| C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	7	7	6
2024	1	0	17
2023	1	1	1
2022	1
2021		1
Total	10	9	24
Special Mention [Member] \| Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025			0
2022	1
Prior	9
Total	10		0
Special Mention [Member] \| Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025			0
2024			1
Total			1
Special Mention [Member] \| Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		1	7
2024			6
2023		0	3
Total		1	16
Special Mention [Member] \| Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024			1
2021			0
Total			1
Special Mention [Member] \| Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		4	0
2024	2	2	0
2023	2	1	0
2022	1	1	1
2021	1
Prior	1	1	1
Total	7	9	3
Special Mention [Member] \| Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		2
2024	2
2023		5
2022		7	8
2021	8
Prior	2	1	0
Total	12	15	11
Special Mention [Member] \| Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		1
2024	1	1	1
2023	1	1	1
2022	1	3	1
2021	2		1
Prior	2	2	2
Revolving Loans Amortized Cost Basis		0	1
Total	7	8	7
Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	7	40	8
2024	42	29	31
2023	23	57	14
2022	57	23	21
2021	27	16	16
Prior	41	25	46
Revolving Loans Amortized Cost Basis	3	2	3
Total	200	192	139
Substandard [Member] \| C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	1	12	2
2024	9	3	10
2023	3	4	1
2022	3	1	1
2021	1	0	2
Prior	2	1	1
Total	19	21	17
Substandard [Member] \| Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		0
2022			1
Total		0	1
Substandard [Member] \| Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		3	3
2024	3		1
2023		1
Prior			1
Total	3	4	5
Substandard [Member] \| Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	3	7	1
2024	4	1	7
2023	1	1	1
2022	1	1	6
2021	1
Total	10	10	15
Substandard [Member] \| Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		1
2024	1	1	1
2023		4	0
2022	4
2021	1	4	4
Prior	5	1	0
Total	11	11	5
Substandard [Member] \| Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		10	0
2024	10	2	2
2023	2	3	1
2022	2	6	3
2021	9	7	7
Prior	10	3	16
Total	33	31	29
Substandard [Member] \| Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	1	1	0
2024	7	14	2
2023	8	30	6
2022	33	10	5
2021	10	1	1
Prior	8	8	18
Total	67	64	32
Substandard [Member] \| Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025	2	5	2
2024	7	7	7
2023	8	12	4
2022	12	4	4
2021	4	4	2
Prior	16	12	10
Revolving Loans Amortized Cost Basis	3	2	3
Total	52	46	32
Substandard [Member] \| Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2025		1
2024	1	1
2023	1	1	1
2022	1	1	1
2021	1
Total	4	4	2
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