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	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Financing Receivable, Credit Quality Indicator [Line Items]
2024	$ 746	$ 2,531	$ 582
2023	2,152	2,036	2,858
2022	1,876	1,062	1,324
2021	1,015	518	620
2020	493	240	293
Prior	796	613	751
Revolving Loans Amortized Cost Basis	152	149	148
Total	7,230	7,149	6,576
Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023		1
2022		1
2021	0
2020	0
Total	0	2
C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	244	733	157
2023	578	279	557
2022	228	75	137
2021	71	37	52
2020	34	16	21
Prior	37	25	31
Total	1,192	1,165	955
C&I [Member] \| Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022		1
Total	0	1
Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	5	25	3
2023	20	83	82
2022	81	16	19
2021	16	10	15
2020	10	1	3
Prior	79	80	99
Total	211	215	221
Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	21	64	16
2023	56	15	52
2022	6	4	5
2021	3	1	2
2020	1	1	1
Prior	1		1
Total	88	85	77
Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	125	532	116
2023	451	317	599
2022	244	79	125
2021	69	20	56
2020	19	9	12
Prior	14	6	13
Total	922	963	921
Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	23	101	12
2023	60	111	144
2022	101	73	82
2021	71	29	31
2020	27	8	11
Prior	29	23	28
Total	311	345	308
Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	59	169	36
2023	135	238	246
2022	245	175	195
2021	178	113	97
2020	112	54	60
Prior	125	79	103
Total	854	828	737
Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	28	158	74
2023	159	312	322
2022	312	225	250
2021	215	114	127
2020	111	70	83
Prior	208	158	187
Total	1,033	1,037	1,043
Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	132	482	96
2023	458	421	485
2022	429	318	360
2021	307	163	186
2020	154	70	80
Prior	293	239	281
Revolving Loans Amortized Cost Basis	145	142	141
Total	1,918	1,835	1,629
Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	91	195	51
2023	175	213	297
2022	189	75	116
2021	65	27	47
2020	22	10	20
Prior	8	2	6
Total	550	522	537
Auto [Member] \| Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022		1
Total	0	1
Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	18		21
2023	60	72	74
2022	41	47	35
2021	20	22	7
2020	3	4	2
Prior	2	1	2
Revolving Loans Amortized Cost Basis	7	7	7
Total	151	154	148
Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	725	2,488	573
2023	2,093	2,007	2,818
2022	1,856	1,041	1,306
2021	984	499	606
2020	475	214	266
Prior	747	578	706
Revolving Loans Amortized Cost Basis	148	145	144
Total	7,028	6,972	6,419
Pass [Member] \| C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	236	720	155
2023	551	276	548
2022	226	73	132
2021	70	36	48
2020	32	14	19
Prior	36	23	28
Total	1,151	1,142	930
Pass [Member] \| Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	5	25	3
2023	20	83	82
2022	81	15	19
2021	15	10	15
2020	10	1	2
Prior	79	80	99
Total	210	214	220
Pass [Member] \| Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	18	63	16
2023	54	15	51
2022	6	4	5
2021	3	1	2
2020	1	1	1
Prior			1
Total	82	84	76
Pass [Member] \| Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	117	515	111
2023	438	311	586
2022	240	78	124
2021	63	20	56
2020	19	9	12
Prior	14	6	13
Total	891	939	902
Pass [Member] \| Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	23	101	12
2023	58	111	142
2022	101	73	82
2021	71	24	31
2020	23	8	11
Prior	29	22	28
Total	305	339	306
Pass [Member] \| Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	59	167	36
2023	133	232	244
2022	244	172	192
2021	174	106	96
2020	104	41	47
Prior	108	73	91
Total	822	791	706
Pass [Member] \| Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	28	154	73
2023	155	305	316
2022	305	217	247
2021	202	114	124
2020	110	62	74
Prior	190	142	170
Total	990	994	1,004
Pass [Member] \| Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	130	477	95
2023	450	415	479
2022	424	313	355
2021	302	158	181
2020	151	67	78
Prior	281	229	268
Revolving Loans Amortized Cost Basis	141	138	137
Total	1,879	1,797	1,593
Pass [Member] \| Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	91	195	51
2023	175	212	296
2022	188	74	116
2021	64	26	46
2020	22	10	20
Prior	8	2	6
Total	548	519	535
Pass [Member] \| Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	18		21
2023	59	71	74
2022	41	47	34
2021	20	22	7
2020	3	4	2
Prior	2	1	2
Revolving Loans Amortized Cost Basis	7	7	7
Total	150	153	147
Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	13	11	4
2023	28	11	17
2022	6	7	6
2021	10	2	6
2020	2	2	19
Prior	3	6	11
Revolving Loans Amortized Cost Basis	1	1	1
Total	63	40	64
Special Mention [Member] \| C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	6	1	1
2023	17	1	3
2022	1	0	1
2021			2
2020	0	1	1
Total	24	3	8
Special Mention [Member] \| Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020			1
Total			1
Special Mention [Member] \| Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023	1		1
Total	1		1
Special Mention [Member] \| Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	7	8	3
2023	6	2	9
2022	3
Total	16	10	12
Special Mention [Member] \| Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023	1
Total	1
Special Mention [Member] \| Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0
2023		5
2022	0	1	1
2021	1
2020	1		11
Prior	1	2	6
Total	3	8	18
Special Mention [Member] \| Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	1
2023	2	1	3
2022	1	4	1
2021	8	0	2
2020		1	6
Prior		1	1
Total	11	8	13
Special Mention [Member] \| Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		1
2023	1	2	1
2022	1	2	3
2021	1	2	2
2020	1
Prior	2	3	4
Revolving Loans Amortized Cost Basis	1	1	1
Total	7	11	11
Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	8	32	5
2023	31	18	23
2022	14	14	12
2021	21	17	8
2020	16	24	8
Prior	46	29	34
Revolving Loans Amortized Cost Basis	3	3	3
Total	139	137	93
Substandard [Member] \| C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	2	12	1
2023	10	2	6
2022	1	2	4
2021	1	1	2
2020	2	1	1
Prior	1	2	3
Total	17	20	17
Substandard [Member] \| Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022		1
2021	1
Total	1	1
Substandard [Member] \| Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	3	1
2023	1
Prior	1
Total	5	1
Substandard [Member] \| Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	1	9	2
2023	7	4	4
2022	1	1	1
2021	6
Total	15	14	7
Substandard [Member] \| Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0
2023	1		2
2022	0
2021	0	5
2020	4
Prior	0	1
Total	5	6	2
Substandard [Member] \| Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	2
2023	2	1	2
2022	1	2	2
2021	3	7	1
2020	7	13	2
Prior	16	4	6
Total	29	29	13
Substandard [Member] \| Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	3	1
2023	2	6	3
2022	6	4	2
2021	5	0	1
2020	1	7	3
Prior	18	15	16
Total	32	35	26
Substandard [Member] \| Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	2	4	1
2023	7	4	5
2022	4	3	2
2021	4	3	3
2020	2	3	2
Prior	10	7	9
Revolving Loans Amortized Cost Basis	3	3	3
Total	32	27	25
Substandard [Member] \| Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023		1	1
2022	1	1
2021	1	1	1
Total	2	3	2
Substandard [Member] \| Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023	1	1
2022			1
Total	$ 1	$ 1	$ 1

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