Operating Segments (Summary of Operating Results by Segment) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net interest income (expense)	$ 976,001	$ 1,004,005	$ 957,892
Credit loss expense	241,230	33,759	21,685
Non-interest income	465,454	363,902	351,286
Non-interest expense	848,904	834,679	778,812
Income before income taxes	351,321	499,469	508,681
Income tax expense (benefit)	20,170	55,870	53,763
Net income	331,151	443,599	454,918
Preferred stock dividends	2,016	8,063	8,063
Redemption of preferred stock	5,514	0	0
Net income available to common shareholders	323,621	435,536	446,855
Revenues from (expenses to) external customers	1,441,455	1,367,907	1,309,178
Average assets	37,961,000	32,086,000	31,030,000
Banking [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	981,441	1,010,368	963,757
Credit loss expense	241,230	33,758	21,685
Non-interest income	321,136	218,447	213,763
Non-interest expense	718,519	703,121	657,376
Income before income taxes	342,828	491,936	498,459
Income tax expense (benefit)	20,347	55,520	52,928
Net income	322,481	436,416	445,531
Preferred stock dividends	0	0	0
Redemption of preferred stock	0
Net income available to common shareholders	322,481	436,416	445,531
Revenues from (expenses to) external customers	1,302,577	1,228,815	1,177,520
Average assets	37,892,000	32,019,000	30,964,000
Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	2,776	4,001	4,083
Credit loss expense	0	1	0
Non-interest income	145,268	145,905	138,045
Non-interest expense	123,630	124,622	114,166
Income before income taxes	24,414	25,283	27,962
Income tax expense (benefit)	5,127	5,308	5,872
Net income	19,287	19,975	22,090
Preferred stock dividends	0	0	0
Redemption of preferred stock	0
Net income available to common shareholders	19,287	19,975	22,090
Revenues from (expenses to) external customers	148,044	149,906	142,128
Average assets	59,000	56,000	54,000
Non-Banks [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	(8,216)	(10,364)	(9,948)
Credit loss expense	0	0	0
Non-interest income	(950)	(450)	(522)
Non-interest expense	6,755	6,936	7,270
Income before income taxes	(15,921)	(17,750)	(17,740)
Income tax expense (benefit)	(5,304)	(4,958)	(5,037)
Net income	(10,617)	(12,792)	(12,703)
Preferred stock dividends	2,016	8,063	8,063
Redemption of preferred stock	5,514
Net income available to common shareholders	(18,147)	(20,855)	(20,766)
Revenues from (expenses to) external customers	(9,166)	(10,814)	(10,470)
Average assets	$ 10,000	$ 11,000	$ 12,000