Operating Segments - Summary of Operating Results by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Segment Reporting Information [Line Items]
Interest income	$ 631,778	$ 620,105	$ 1,804,508	$ 1,807,180
Interest expense	163,911	178,487	450,391	519,738
Net interest income (expense)	467,867	441,618	1,354,117	1,287,442
Credit loss expense	12,090	6,779	28,602	32,978
Net interest income after credit loss expense	455,777	434,839	1,325,515	1,254,464
Trust and investment management fees	49,472	44,846	145,072	131,446
Service charges on deposit accounts	36,304	31,440	102,638	89,212
Insurance commissions and fees	15,838	15,424	52,079	50,322
Interchange and card transaction fees	6,843	5,547	19,921	16,568
Other charges, commissions, and fees	13,911	14,730	40,966	42,283
Net gain (loss) on securities transactions	(7,512)	0	(7,512)	(14)
Other	21,106	13,660	47,394	37,114
Total non-interest income	135,962	125,647	400,558	366,931
Salaries and wages	182,183	169,155	521,328	492,161
Employee benefits	33,649	34,465	113,461	109,448
Net occupancy	35,848	34,682	105,824	102,599
Technology, furniture, and equipment	46,145	43,479	130,383	124,169
Deposit insurance	6,941	6,328	20,449	20,102
Other	72,434	64,369	213,141	199,193
Total non-interest expense	377,200	352,478	1,104,586	1,047,672
Income (loss) before income taxes	214,539	208,008	621,487	573,723
Income tax expense (benefit)	34,330	33,628	98,232	91,418
Net income (loss)	180,209	174,380	523,255	482,305
Preferred stock dividends	1,668	1,668	5,006	5,006
Net income (loss) available to common shareholders	178,541	172,712	518,249	477,299
Revenues from (expenses to) external customers	603,829	567,265	1,754,675	1,654,373
Average assets (in millions)	54,115,000	51,911,000	52,974,000	51,344,000
Bank
Segment Reporting Information [Line Items]
Interest income	629,805	618,033	1,799,074	1,801,149
Interest expense	160,910	175,279	441,434	510,129
Net interest income (expense)	468,895	442,754	1,357,640	1,291,020
Credit loss expense	12,090	6,779	28,602	32,978
Net interest income after credit loss expense	456,805	435,975	1,329,038	1,258,042
Trust and investment management fees	0	0	0	0
Service charges on deposit accounts	36,299	31,436	102,623	89,203
Insurance commissions and fees	15,838	15,424	52,079	50,322
Interchange and card transaction fees	6,843	5,547	19,921	16,568
Other charges, commissions, and fees	7,890	8,356	23,073	23,793
Net gain (loss) on securities transactions	(7,512)	0	(7,512)	(14)
Other	19,958	11,941	43,547	32,328
Total non-interest income	79,316	72,704	233,731	212,200
Salaries and wages	161,634	147,908	458,478	431,089
Employee benefits	30,344	31,151	101,059	97,733
Net occupancy	32,591	31,197	94,653	91,826
Technology, furniture, and equipment	44,637	41,611	125,637	119,346
Deposit insurance	6,929	6,315	20,417	20,065
Other	55,397	50,120	164,975	155,360
Total non-interest expense	331,532	308,302	965,219	915,419
Income (loss) before income taxes	204,589	200,377	597,550	554,823
Income tax expense (benefit)	32,719	32,591	94,729	89,008
Net income (loss)	171,870	167,786	502,821	465,815
Preferred stock dividends	0	0	0	0
Net income (loss) available to common shareholders	171,870	167,786	502,821	465,815
Revenues from (expenses to) external customers	548,211	515,458	1,591,371	1,503,220
Average assets (in millions)	54,024,000	51,828,000	52,885,000	51,265,000
Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Interest income	1,973	2,072	5,434	6,031
Interest expense	77	104	243	292
Net interest income (expense)	1,896	1,968	5,191	5,739
Credit loss expense	0	0	0	0
Net interest income after credit loss expense	1,896	1,968	5,191	5,739
Trust and investment management fees	49,509	45,021	145,322	132,409
Service charges on deposit accounts	5	4	15	9
Insurance commissions and fees	0	0	0	0
Interchange and card transaction fees	0	0	0	0
Other charges, commissions, and fees	6,021	6,374	17,893	18,490
Net gain (loss) on securities transactions	0	0	0	0
Other	1,078	1,662	3,685	4,613
Total non-interest income	56,613	53,061	166,915	155,521
Salaries and wages	20,127	20,852	61,585	59,885
Employee benefits	3,279	3,289	12,324	11,640
Net occupancy	3,258	3,485	11,172	10,773
Technology, furniture, and equipment	1,453	1,820	4,589	4,672
Deposit insurance	12	13	32	37
Other	16,200	13,775	44,196	39,829
Total non-interest expense	44,329	43,234	133,898	126,836
Income (loss) before income taxes	14,180	11,795	38,208	34,424
Income tax expense (benefit)	2,978	2,477	8,024	7,229
Net income (loss)	11,202	9,318	30,184	27,195
Preferred stock dividends	0	0	0	0
Net income (loss) available to common shareholders	11,202	9,318	30,184	27,195
Revenues from (expenses to) external customers	58,509	55,029	172,106	161,260
Average assets (in millions)	80,000	74,000	80,000	70,000
Non-Banks [Member]
Segment Reporting Information [Line Items]
Interest income	0	0	0	0
Interest expense	2,924	3,104	8,714	9,317
Net interest income (expense)	(2,924)	(3,104)	(8,714)	(9,317)
Credit loss expense	0	0	0	0
Net interest income after credit loss expense	(2,924)	(3,104)	(8,714)	(9,317)
Trust and investment management fees	(37)	(175)	(250)	(963)
Service charges on deposit accounts	0	0	0	0
Insurance commissions and fees	0	0	0	0
Interchange and card transaction fees	0	0	0	0
Other charges, commissions, and fees	0	0	0	0
Net gain (loss) on securities transactions	0	0	0	0
Other	70	57	162	173
Total non-interest income	33	(118)	(88)	(790)
Salaries and wages	422	395	1,265	1,187
Employee benefits	26	25	78	75
Net occupancy	(1)	0	(1)	0
Technology, furniture, and equipment	55	48	157	151
Deposit insurance	0	0	0	0
Other	837	474	3,970	4,004
Total non-interest expense	1,339	942	5,469	5,417
Income (loss) before income taxes	(4,230)	(4,164)	(14,271)	(15,524)
Income tax expense (benefit)	(1,367)	(1,440)	(4,521)	(4,819)
Net income (loss)	(2,863)	(2,724)	(9,750)	(10,705)
Preferred stock dividends	1,668	1,668	5,006	5,006
Net income (loss) available to common shareholders	(4,531)	(4,392)	(14,756)	(15,711)
Revenues from (expenses to) external customers	(2,891)	(3,222)	(8,802)	(10,107)
Average assets (in millions)	$ 11,000	$ 9,000	$ 9,000	$ 9,000