Operating Segments - Summary of Operating Results by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Segment Reporting Information [Line Items]
Interest income	$ 594,763	$ 601,958	$ 1,172,730	$ 1,187,075
Interest expense	147,035	172,354	286,480	341,251
Net interest income (expense)	447,728	429,604	886,250	845,824
Credit loss expense	9,767	13,129	16,512	26,199
Net interest income after credit loss expense	437,961	416,475	869,738	819,625
Trust and investment management fees	47,643	43,669	95,600	86,600
Service charges on deposit accounts	34,177	29,151	66,334	57,772
Insurance commissions and fees	14,166	13,879	36,241	34,898
Interchange and card transaction fees	6,546	5,619	13,078	11,021
Other charges, commissions, and fees	13,787	13,967	27,055	27,553
Net gain (loss) on securities transactions	0	0	0	(14)
Other	11,962	10,988	26,288	23,454
Total non-interest income	128,281	117,273	264,596	241,284
Salaries and wages	172,955	162,149	339,145	323,006
Employee benefits	35,156	32,826	79,812	74,983
Net occupancy	35,223	34,640	69,976	67,917
Technology, furniture, and equipment	42,564	40,572	84,238	80,690
Deposit insurance	6,305	6,590	13,508	13,774
Other	69,497	70,351	140,707	134,824
Total non-interest expense	361,700	347,128	727,386	695,194
Income (loss) before income taxes	204,542	186,620	406,948	365,715
Income tax expense (benefit)	32,483	29,617	63,902	57,790
Net income (loss)	172,059	157,003	343,046	307,925
Preferred stock dividends	1,669	1,669	3,338	3,338
Net income (loss) available to common shareholders	170,390	155,334	339,708	304,587
Revenues from (expenses to) external customers	576,009	546,877	1,150,846	1,087,108
Average assets (in millions)	52,626,000	51,191,000	52,373,000	51,064,000
Bank
Segment Reporting Information [Line Items]
Interest income	592,937	599,875	1,169,269	1,183,116
Interest expense	144,062	169,150	280,524	334,850
Net interest income (expense)	448,875	430,725	888,745	848,266
Credit loss expense	9,767	13,129	16,512	26,199
Net interest income after credit loss expense	439,108	417,596	872,233	822,067
Trust and investment management fees	0	0	0	0
Service charges on deposit accounts	34,172	29,149	66,324	57,767
Insurance commissions and fees	14,166	13,879	36,241	34,898
Interchange and card transaction fees	6,546	5,619	13,078	11,021
Other charges, commissions, and fees	7,944	8,031	15,183	15,437
Net gain (loss) on securities transactions	0	0	0	(14)
Other	10,577	9,002	23,589	20,387
Total non-interest income	73,405	65,680	154,415	139,496
Salaries and wages	151,014	142,089	296,844	283,181
Employee benefits	31,023	29,244	70,715	66,582
Net occupancy	31,280	30,893	62,062	60,629
Technology, furniture, and equipment	40,908	39,178	81,000	77,735
Deposit insurance	6,296	6,579	13,488	13,750
Other	53,052	54,412	109,578	105,240
Total non-interest expense	313,573	302,395	633,687	607,117
Income (loss) before income taxes	198,940	180,881	392,961	354,446
Income tax expense (benefit)	31,887	28,891	62,010	56,417
Net income (loss)	167,053	151,990	330,951	298,029
Preferred stock dividends	0	0	0	0
Net income (loss) available to common shareholders	167,053	151,990	330,951	298,029
Revenues from (expenses to) external customers	522,280	496,405	1,043,160	987,762
Average assets (in millions)	52,541,000	51,117,000	52,287,000	50,987,000
Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Interest income	1,826	2,083	3,461	3,959
Interest expense	82	99	166	188
Net interest income (expense)	1,744	1,984	3,295	3,771
Credit loss expense	0	0	0	0
Net interest income after credit loss expense	1,744	1,984	3,295	3,771
Trust and investment management fees	47,681	43,844	95,813	87,388
Service charges on deposit accounts	5	2	10	5
Insurance commissions and fees	0	0	0	0
Interchange and card transaction fees	0	0	0	0
Other charges, commissions, and fees	5,843	5,936	11,872	12,116
Net gain (loss) on securities transactions	0	0	0	0
Other	1,334	1,928	2,607	2,951
Total non-interest income	54,863	51,710	110,302	102,460
Salaries and wages	21,520	19,664	41,458	39,033
Employee benefits	4,107	3,557	9,045	8,351
Net occupancy	3,943	3,747	7,914	7,288
Technology, furniture, and equipment	1,604	1,344	3,136	2,852
Deposit insurance	9	11	20	24
Other	14,182	13,414	27,996	26,054
Total non-interest expense	45,365	41,737	89,569	83,602
Income (loss) before income taxes	11,242	11,957	24,028	22,629
Income tax expense (benefit)	2,361	2,511	5,046	4,752
Net income (loss)	8,881	9,446	18,982	17,877
Preferred stock dividends	0	0	0	0
Net income (loss) available to common shareholders	8,881	9,446	18,982	17,877
Revenues from (expenses to) external customers	56,607	53,694	113,597	106,231
Average assets (in millions)	76,000	65,000	77,000	68,000
Non-Banks [Member]
Segment Reporting Information [Line Items]
Interest income	0	0	0	0
Interest expense	2,891	3,105	5,790	6,213
Net interest income (expense)	(2,891)	(3,105)	(5,790)	(6,213)
Credit loss expense	0	0	0	0
Net interest income after credit loss expense	(2,891)	(3,105)	(5,790)	(6,213)
Trust and investment management fees	(38)	(175)	(213)	(788)
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	51	58	92	116
Total non-interest income	13	(117)	(121)	(672)
Salaries and wages	421	396	843	792
Employee benefits	26	25	52	50
Net occupancy	0	0	0	0
Technology, furniture, and equipment	52	50	102	103
Deposit insurance	0	0	0	0
Other	2,263	2,525	3,133	3,530
Total non-interest expense	2,762	2,996	4,130	4,475
Income (loss) before income taxes	(5,640)	(6,218)	(10,041)	(11,360)
Income tax expense (benefit)	(1,765)	(1,785)	(3,154)	(3,379)
Net income (loss)	(3,875)	(4,433)	(6,887)	(7,981)
Preferred stock dividends	1,669	1,669	3,338	3,338
Net income (loss) available to common shareholders	(5,544)	(6,102)	(10,225)	(11,319)
Revenues from (expenses to) external customers	(2,878)	(3,222)	(5,911)	(6,885)
Average assets (in millions)	$ 9,000	$ 9,000	$ 9,000	$ 9,000