Operating Segments (Summary of Operating Results by Segment) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Interest income	$ 2,420,747	$ 2,390,637	$ 2,217,716
Interest expense	684,598	786,025	659,052
Net interest income (expense)	1,736,149	1,604,612	1,558,664
Credit loss expense	44,202	64,985	46,171
Net interest income after credit loss expense	1,691,947	1,539,627	1,512,493
Trust and investment management fees	177,097	165,270	153,315
Service charges on deposit accounts	121,572	106,230	93,504
Insurance commissions and fees	65,502	61,269	58,271
Interchange and card transaction fees	22,858	21,017	19,419
Other charges, commissions and fees	57,511	53,348	49,026
Net gain (loss) on securities transactions	(850)	(96)	66
Other	55,405	52,060	54,941
Total non-interest income	499,095	459,098	428,542
Salaries and wages	674,647	621,394	547,718
Employee benefits	146,101	122,446	115,306
Net occupancy	136,940	128,751	124,396
Technology, furniture and equipment	165,744	148,487	135,286
Deposit insurance	18,752	37,269	76,589
Other	277,156	244,411	229,367
Total non-interest expense	1,419,340	1,302,758	1,228,662
Income (loss) before income taxes	771,702	695,967	712,373
Income tax expense (benefit)	123,145	113,425	114,400
Net income (loss)	648,557	582,542	597,973
Preferred stock dividends	6,675	6,675	6,675
Net income (loss) available to common shareholders	641,882	575,867	591,298
Revenues from (expenses to) external customers	2,235,244	2,063,710	1,987,206
Average assets (in millions)	51,889,000	49,694,000	49,604,000
Banking [Member]
Segment Reporting Information [Line Items]
Interest income	2,412,807	2,382,675	2,209,659
Interest expense	671,866	772,075	647,218
Net interest income (expense)	1,740,941	1,610,600	1,562,441
Credit loss expense	44,202	64,985	46,171
Net interest income after credit loss expense	1,696,739	1,545,615	1,516,270
Trust and investment management fees	0	0	0
Service charges on deposit accounts	121,558	106,217	93,478
Insurance commissions and fees	65,502	61,269	58,271
Interchange and card transaction fees	22,858	21,017	19,419
Other charges, commissions and fees	32,037	30,626	30,217
Net gain (loss) on securities transactions	(850)	(96)	66
Other	49,110	45,790	49,868
Total non-interest income	290,215	264,823	251,319
Salaries and wages	593,755	544,843	480,083
Employee benefits	131,159	109,431	103,427
Net occupancy	122,458	115,136	109,953
Technology, furniture and equipment	159,341	142,517	129,042
Deposit insurance	18,720	37,203	76,535
Other	215,405	189,700	178,784
Total non-interest expense	1,240,838	1,138,830	1,077,824
Income (loss) before income taxes	746,116	671,608	689,765
Income tax expense (benefit)	119,877	110,524	111,145
Net income (loss)	626,239	561,084	578,620
Preferred stock dividends	0	0	0
Net income (loss) available to common shareholders	626,239	561,084	578,620
Revenues from (expenses to) external customers	2,031,156	1,875,423	1,813,760
Average assets (in millions)	51,807,000	49,621,000	49,536,000
Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Interest income	7,940	7,962	8,057
Interest expense	386	421	437
Net interest income (expense)	7,554	7,541	7,620
Credit loss expense	0	0	0
Net interest income after credit loss expense	7,554	7,541	7,620
Trust and investment management fees	178,234	167,448	155,278
Service charges on deposit accounts	14	13	26
Insurance commissions and fees	0	0	0
Interchange and card transaction fees	0	0	0
Other charges, commissions and fees	25,474	22,717	18,809
Net gain (loss) on securities transactions	0	0	0
Other	6,071	6,011	4,793
Total non-interest income	209,793	196,189	178,906
Salaries and wages	79,309	74,928	66,070
Employee benefits	14,842	12,911	11,776
Net occupancy	14,482	13,615	14,443
Technology, furniture and equipment	6,203	5,764	6,054
Deposit insurance	32	66	54
Other	56,999	49,790	46,081
Total non-interest expense	171,867	157,074	144,478
Income (loss) before income taxes	45,480	46,656	42,048
Income tax expense (benefit)	9,551	9,798	8,830
Net income (loss)	35,929	36,858	33,218
Preferred stock dividends	0	0	0
Net income (loss) available to common shareholders	35,929	36,858	33,218
Revenues from (expenses to) external customers	217,347	203,730	186,526
Average assets (in millions)	73,000	64,000	59,000
Non-Banks [Member]
Segment Reporting Information [Line Items]
Interest income	0	0	0
Interest expense	12,346	13,529	11,397
Net interest income (expense)	(12,346)	(13,529)	(11,397)
Credit loss expense	0	0	0
Net interest income after credit loss expense	(12,346)	(13,529)	(11,397)
Trust and investment management fees	(1,137)	(2,178)	(1,963)
Service charges on deposit accounts	0	0	0
Insurance commissions and fees	0	0	0
Interchange and card transaction fees	0	0	0
Other charges, commissions and fees	0	5	0
Net gain (loss) on securities transactions	0	0	0
Other	224	259	280
Total non-interest income	(913)	(1,914)	(1,683)
Salaries and wages	1,583	1,623	1,565
Employee benefits	100	104	103
Net occupancy	0	0	0
Technology, furniture and equipment	200	206	190
Deposit insurance	0	0	0
Other	4,752	4,921	4,502
Total non-interest expense	6,635	6,854	6,360
Income (loss) before income taxes	(19,894)	(22,297)	(19,440)
Income tax expense (benefit)	(6,283)	(6,897)	(5,575)
Net income (loss)	(13,611)	(15,400)	(13,865)
Preferred stock dividends	6,675	6,675	6,675
Net income (loss) available to common shareholders	(20,286)	(22,075)	(20,540)
Revenues from (expenses to) external customers	(13,259)	(15,443)	(13,080)
Average assets (in millions)	$ 9,000	$ 9,000	$ 9,000