Operating Segments - Summary of Operating Results by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Segment Reporting Information [Line Items]
Interest income	$ 577,967	$ 585,117
Interest expense	139,445	168,897
Net interest income (expense)	438,522	416,220
Credit loss expense	6,745	13,070
Net interest income after credit loss expense	431,777	403,150
Trust and investment management fees	47,957	42,931
Service charges on deposit accounts	32,157	28,621
Insurance commissions and fees	22,075	21,019
Interchange and card transaction fees	6,532	5,402
Other charges, commissions, and fees	13,268	13,586
Net gain (loss) on securities transactions	0	(14)
Other	14,326	12,466
Total non-interest income	136,315	124,011
Salaries and wages	166,190	160,857
Employee benefits	44,656	42,157
Net occupancy	34,753	33,277
Technology, furniture, and equipment	41,674	40,118
Deposit insurance	7,203	7,184
Other	71,210	64,473
Total non-interest expense	365,686	348,066
Income (loss) before income taxes	202,406	179,095
Income tax expense (benefit)	31,419	28,173
Net income (loss)	170,987	150,922
Preferred stock dividends	1,669	1,669
Net income (loss) available to common shareholders	169,318	149,253
Revenues from (expenses to) external customers	574,837	540,231
Average assets (in millions)	52,122,000	50,925,000
Bank
Segment Reporting Information [Line Items]
Interest income	576,332	583,241
Interest expense	136,462	165,700
Net interest income (expense)	439,870	417,541
Credit loss expense	6,745	13,070
Net interest income after credit loss expense	433,125	404,471
Trust and investment management fees	0	0
Service charges on deposit accounts	32,152	28,618
Insurance commissions and fees	22,075	21,019
Interchange and card transaction fees	6,532	5,402
Other charges, commissions, and fees	7,239	7,406
Net gain (loss) on securities transactions	0	(14)
Other	13,012	11,385
Total non-interest income	81,010	73,816
Salaries and wages	145,830	141,092
Employee benefits	39,692	37,338
Net occupancy	30,782	29,736
Technology, furniture, and equipment	40,092	38,557
Deposit insurance	7,192	7,171
Other	56,526	50,828
Total non-interest expense	320,114	304,722
Income (loss) before income taxes	194,021	173,565
Income tax expense (benefit)	30,123	27,526
Net income (loss)	163,898	146,039
Preferred stock dividends	0	0
Net income (loss) available to common shareholders	163,898	146,039
Revenues from (expenses to) external customers	520,880	491,357
Average assets (in millions)	52,035,000	50,846,000
Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Interest income	1,635	1,876
Interest expense	84	89
Net interest income (expense)	1,551	1,787
Credit loss expense	0	0
Net interest income after credit loss expense	1,551	1,787
Trust and investment management fees	48,132	43,544
Service charges on deposit accounts	5	3
Insurance commissions and fees	0	0
Interchange and card transaction fees	0	0
Other charges, commissions, and fees	6,029	6,180
Net gain (loss) on securities transactions	0	0
Other	1,273	1,023
Total non-interest income	55,439	50,750
Salaries and wages	19,938	19,369
Employee benefits	4,938	4,794
Net occupancy	3,971	3,541
Technology, furniture, and equipment	1,532	1,508
Deposit insurance	11	13
Other	13,814	12,640
Total non-interest expense	44,204	41,865
Income (loss) before income taxes	12,786	10,672
Income tax expense (benefit)	2,685	2,241
Net income (loss)	10,101	8,431
Preferred stock dividends	0	0
Net income (loss) available to common shareholders	10,101	8,431
Revenues from (expenses to) external customers	56,990	52,537
Average assets (in millions)	78,000	70,000
Non-Banks [Member]
Segment Reporting Information [Line Items]
Interest income	0	0
Interest expense	2,899	3,108
Net interest income (expense)	(2,899)	(3,108)
Credit loss expense	0	0
Net interest income after credit loss expense	(2,899)	(3,108)
Trust and investment management fees	(175)	(613)
Service charges on deposit accounts	0	0
Insurance commissions and fees	0	0
Interchange and card transaction fees	0	0
Other charges, commissions, and fees	0	0
Net gain (loss) on securities transactions	0	0
Other	41	58
Total non-interest income	(134)	(555)
Salaries and wages	422	396
Employee benefits	26	25
Net occupancy	0	0
Technology, furniture, and equipment	50	53
Deposit insurance	0	0
Other	870	1,005
Total non-interest expense	1,368	1,479
Income (loss) before income taxes	(4,401)	(5,142)
Income tax expense (benefit)	(1,389)	(1,594)
Net income (loss)	(3,012)	(3,548)
Preferred stock dividends	1,669	1,669
Net income (loss) available to common shareholders	(4,681)	(5,217)
Revenues from (expenses to) external customers	(3,033)	(3,663)
Average assets (in millions)	$ 9,000	$ 9,000