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	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023
Financing Receivable, Credit Quality Indicator [Line Items]
2024	$ 2,397	$ 2,531	$ 1,888
2023	1,478	2,036	2,344
2022	1,671	1,062	1,155
2021	905	518	558
2020	405	240	256
Prior	689	613	645
Revolving Loans Amortized Cost Basis	179	149	149
Total	7,724	7,149	6,995
Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023	1	1	1
2022	2	1	1
Total	3	2	2
C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	547	733	533
2023	335	279	392
2022	178	75	97
2021	58	37	42
2020	26	16	18
Prior	32	25	26
Total	1,176	1,165	1,108
C&I [Member] | Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023	1
2022		1	1
Total	1	1	1
Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	131	25	20
2023	29	83	83
2022	79	16	17
2021	13	10	14
2020	9	1	2
Prior	73	80	82
Total	334	215	218
Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	56	64	43
2023	19	15	31
2022	5	4	5
2021	2	1	2
2020	1	1	1
Total	83	85	82
Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	520	532	388
2023	229	317	408
2022	181	79	95
2021	57	20	22
2020	17	9	10
Prior	9	6	7
Revolving Loans Amortized Cost Basis	1
Total	1,014	963	930
Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	76	101	83
2023	45	111	117
2022	80	73	76
2021	65	29	30
2020	24	8	9
Prior	26	23	26
Total	316	345	341
Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	151	169	135
2023	94	238	245
2022	245	175	184
2021	163	113	123
2020	66	54	58
Prior	107	79	84
Total	826	828	829
Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	132	158	85
2023	147	312	326
2022	321	225	237
2021	203	114	118
2020	106	70	72
Prior	178	158	165
Total	1,087	1,037	1,003
Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	454	482	372
2023	400	421	450
2022	405	318	331
2021	283	163	170
2020	140	70	72
Prior	259	239	252
Revolving Loans Amortized Cost Basis	171	142	142
Total	2,112	1,835	1,789
Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	271	195	168
2023	138	213	238
2022	145	75	87
2021	46	27	32
2020	14	10	13
Prior	4	2	2
Total	618	522	540
Auto [Member] | Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023			1
2022	1	1
Total	1	1	1
Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	59	72	61
2023	42	47	54
2022	32	22	26
2021	15	4	5
2020	2	1	1
Prior	1	1	1
Revolving Loans Amortized Cost Basis	7	7	7
Total	158	154	155
Non-Auto [Member] | Charge-Off [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022	1
Total	1
Pass [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	2,359	2,488	1,853
2023	1,450	2,007	2,310
2022	1,605	1,041	1,131
2021	872	499	539
2020	388	214	231
Prior	645	578	606
Revolving Loans Amortized Cost Basis	175	145	146
Total	7,494	6,972	6,816
Pass [Member] | C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	533	720	524
2023	331	276	386
2022	175	73	94
2021	57	36	40
2020	25	14	16
Prior	30	23	24
Total	1,151	1,142	1,084
Pass [Member] | Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	131	25	20
2023	29	83	83
2022	79	15	16
2021	13	10	14
2020	9	1	2
Prior	73	80	82
Total	334	214	217
Pass [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	53	63	42
2023	19	15	30
2022	5	4	5
2021	2	1	2
2020	1	1	1
Total	80	84	80
Pass [Member] | Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	513	515	373
2023	227	311	401
2022	180	78	94
2021	57	20	22
2020	17	9	10
Prior	9	6	7
Revolving Loans Amortized Cost Basis	1
Total	1,004	939	907
Pass [Member] | Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	76	101	83
2023	44	111	116
2022	76	73	75
2021	65	24	25
2020	20	8	9
Prior	25	22	25
Total	306	339	333
Pass [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	147	167	134
2023	91	232	240
2022	244	172	182
2021	155	106	116
2020	60	41	45
Prior	91	73	75
Total	788	791	792
Pass [Member] | Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	128	154	81
2023	138	305	320
2022	279	217	228
2021	185	114	118
2020	104	62	64
Prior	166	142	149
Total	1,000	994	960
Pass [Member] | Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	448	477	368
2023	392	415	443
2022	392	313	325
2021	278	158	165
2020	136	67	70
Prior	246	229	241
Revolving Loans Amortized Cost Basis	167	138	139
Total	2,059	1,797	1,751
Pass [Member] | Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	271	195	168
2023	137	212	237
2022	144	74	86
2021	45	26	32
2020	14	10	13
Prior	4	2	2
Total	615	519	538
Pass [Member] | Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	59	71	60
2023	42	47	54
2022	31	22	26
2021	15	4	5
2020	2	1	1
Prior	1	1	1
Revolving Loans Amortized Cost Basis	7	7	7
Total	157	153	154
Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	16	11	15
2023	7	11	11
2022	1	7	8
2021	8	2	2
2020	1	2	15
Prior	4	6	5
Revolving Loans Amortized Cost Basis	0	1	1
Total	37	40	57
Special Mention [Member] | C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	9	1	7
2023	1	1	1
2022	0	0
2020		1	1
Total	10	3	9
Special Mention [Member] | Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020			0
Total			0
Special Mention [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024			0
2023			0
Total	0		0
Special Mention [Member] | Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	2	8	7
2023		2	4
Total	2	10	11
Special Mention [Member] | Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2021			0
Total			0
Special Mention [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	1		0
2023		5	4
2022		1	1
2021	1
2020			11
Prior	1	2	2
Total	3	8	18
Special Mention [Member] | Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	2	1
2023	5	1
2022		4	5
2021	5	0
2020	1	1	3
Prior	1	1	0
Total	14	8	9
Special Mention [Member] | Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	2	1
2023	1	2	2
2022	1	2	2
2021	2	2	2
Prior	2	3	3
Revolving Loans Amortized Cost Basis		1	1
Total	8	11	10
Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	22	32	20
2023	21	18	23
2022	65	14	16
2021	25	17	17
2020	16	24	10
Prior	40	29	34
Revolving Loans Amortized Cost Basis	4	3	2
Total	193	137	122
Substandard [Member] | C&I [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	5	12	2
2023	3	2	5
2022	3	2	3
2021	1	1	2
2020	1	1	1
Prior	2	2	2
Total	15	20	15
Substandard [Member] | Municipal [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2022		1	1
Total		1	1
Substandard [Member] | Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	3	1	1
2023			1
Total	3	1	2
Substandard [Member] | Construction & Development [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	5	9	8
2023	2	4	3
2022	1	1	1
Total	8	14	12
Substandard [Member] | Farm [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023	1		1
2022	4		1
2021		5	5
2020	4
Prior	1	1	1
Total	10	6	8
Substandard [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	3	2	1
2023	3	1	1
2022	1	2	1
2021	7	7	7
2020	6	13	2
Prior	15	4	7
Total	35	29	19
Substandard [Member] | Owner Occupied CRE [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	2	3	3
2023	4	6	6
2022	42	4	4
2021	13	0	0
2020	1	7	5
Prior	11	15	16
Total	73	35	34
Substandard [Member] | Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024	4	4	4
2023	7	4	5
2022	12	3	4
2021	3	3	3
2020	4	3	2
Prior	11	7	8
Revolving Loans Amortized Cost Basis	4	3	2
Total	45	27	28
Substandard [Member] | Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2023	1	1	1
2022	1	1	1
2021	1	1
Total	3	3	2
Substandard [Member] | Non-Auto [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2024		1	1
2022	1
Total	$ 1	$ 1	$ 1