Operating Segments (Summary of Operating Results by Segment) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net interest income (expense)	$ 1,004,005	$ 957,892	$ 866,422
Provision for loan losses	33,759	21,613	35,460
Non-interest income	363,902	351,286	336,470
Non-interest expense	834,679	778,884	759,069
Income before income taxes	499,469	508,681	408,363
Income tax expense (benefit)	55,870	53,763	44,214
Net income	443,599	454,918	364,149
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	435,536	446,855	356,086
Revenues from (expenses to) external customers	1,367,907	1,309,178	1,202,892
Average assets	32,086,000	31,030,000	30,450,000
Banking [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	1,010,368	963,757	856,593
Provision for loan losses	33,758	21,613	35,460
Non-interest income	218,447	213,763	207,810
Non-interest expense	703,121	657,448	644,072
Income before income taxes	491,936	498,459	384,871
Income tax expense (benefit)	55,520	52,928	37,837
Net income	436,416	445,531	347,034
Preferred stock dividends	0	0	0
Net income available to common shareholders	436,416	445,531	347,034
Revenues from (expenses to) external customers	1,228,815	1,177,520	1,064,403
Average assets	32,019,000	30,964,000	30,391,000
Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	4,001	4,083	17,644
Provision for loan losses	1	0	0
Non-interest income	145,905	138,045	128,819
Non-interest expense	124,622	114,166	108,931
Income before income taxes	25,283	27,962	37,532
Income tax expense (benefit)	5,308	5,872	13,137
Net income	19,975	22,090	24,395
Preferred stock dividends	0	0	0
Net income available to common shareholders	19,975	22,090	24,395
Revenues from (expenses to) external customers	149,906	142,128	146,463
Average assets	56,000	54,000	43,000
Non-Banks [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	(10,364)	(9,948)	(7,815)
Provision for loan losses	0	0	0
Non-interest income	(450)	(522)	(159)
Non-interest expense	6,936	7,270	6,066
Income before income taxes	(17,750)	(17,740)	(14,040)
Income tax expense (benefit)	(4,958)	(5,037)	(6,760)
Net income	(12,792)	(12,703)	(7,280)
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	(20,855)	(20,766)	(15,343)
Revenues from (expenses to) external customers	(10,814)	(10,470)	(7,974)
Average assets	$ 11,000	$ 12,000	$ 16,000