Operating Segments (Summary of Operating Results by Segment) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net interest income (expense)	$ 957,892	$ 866,422	$ 776,336
Provision for loan losses	21,613	35,460	51,673
Non-interest income	351,286	336,470	349,708
Non-interest expense	778,884	759,069	732,960
Income before income taxes	508,681	408,363	341,411
Income tax expense (benefit)	53,763	44,214	37,150
Net income	454,918	364,149	304,261
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	446,855	356,086	296,198
Revenues from (expenses to) external customers	1,309,178	1,202,892	1,126,044
Average assets	31,030,000	30,450,000	28,832,000
Banking [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	963,757	856,593	769,625
Provision for loan losses	21,613	35,460	51,672
Non-interest income	213,763	207,810	229,791
Non-interest expense	657,448	644,072	624,396
Income before income taxes	498,459	384,871	323,348
Income tax expense (benefit)	52,928	37,837	33,683
Net income	445,531	347,034	289,665
Preferred stock dividends	0	0	0
Net income available to common shareholders	445,531	347,034	289,665
Revenues from (expenses to) external customers	1,177,520	1,064,403	999,416
Average assets	30,964,000	30,391,000	28,795,000
Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	4,083	17,644	11,335
Provision for loan losses	0	0	1
Non-interest income	138,045	128,819	120,102
Non-interest expense	114,166	108,931	102,062
Income before income taxes	27,962	37,532	29,374
Income tax expense (benefit)	5,872	13,137	10,281
Net income	22,090	24,395	19,093
Preferred stock dividends	0	0	0
Net income available to common shareholders	22,090	24,395	19,093
Revenues from (expenses to) external customers	142,128	146,463	131,437
Average assets	54,000	43,000	34,000
Non-Banks [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	(9,948)	(7,815)	(4,624)
Provision for loan losses	0	0	0
Non-interest income	(522)	(159)	(185)
Non-interest expense	7,270	6,066	6,502
Income before income taxes	(17,740)	(14,040)	(11,311)
Income tax expense (benefit)	(5,037)	(6,760)	(6,814)
Net income	(12,703)	(7,280)	(4,497)
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	(20,766)	(15,343)	(12,560)
Revenues from (expenses to) external customers	(10,470)	(7,974)	(4,809)
Average assets	$ 12,000	$ 16,000	$ 3,000