Operating Segments (Summary of Operating Results by Segment) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net interest income (expense)	$ 866,422	$ 776,336	$ 736,632
Provision for loan losses	35,460	51,673	51,845
Non-interest income	336,470	349,708	328,730
Non-interest expense	759,069	732,960	693,718
Income before income taxes	408,363	341,411	319,799
Income tax expense (benefit)	44,214	37,150	40,471
Net income	364,149	304,261	279,328
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	356,086	296,198	271,265
Revenues from (expenses to) external customers	1,202,892	1,126,044	1,065,362
Average assets	30,450,000	28,832,000	28,061,000
Banking [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	856,593	769,625	732,671
Provision for loan losses	35,460	51,672	51,848
Non-interest income	207,810	229,791	205,606
Non-interest expense	644,072	624,396	589,394
Income before income taxes	384,871	323,348	297,035
Income tax expense (benefit)	37,837	33,683	34,997
Net income	347,034	289,665	262,038
Preferred stock dividends	0	0	0
Net income available to common shareholders	347,034	289,665	262,038
Revenues from (expenses to) external customers	1,064,403	999,416	938,277
Average assets	30,391,000	28,795,000	28,023,000
Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	17,644	11,335	7,634
Provision for loan losses	0	1	(3)
Non-interest income	128,819	120,102	121,489
Non-interest expense	108,931	102,062	98,405
Income before income taxes	37,532	29,374	30,721
Income tax expense (benefit)	13,137	10,281	10,753
Net income	24,395	19,093	19,968
Preferred stock dividends	0	0	0
Net income available to common shareholders	24,395	19,093	19,968
Revenues from (expenses to) external customers	146,463	131,437	129,123
Average assets	43,000	34,000	36,000
Non-Banks [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	(7,815)	(4,624)	(3,673)
Provision for loan losses	0	0	0
Non-interest income	(159)	(185)	1,635
Non-interest expense	6,066	6,502	5,919
Income before income taxes	(14,040)	(11,311)	(7,957)
Income tax expense (benefit)	(6,760)	(6,814)	(5,279)
Net income	(7,280)	(4,497)	(2,678)
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	(15,343)	(12,560)	(10,741)
Revenues from (expenses to) external customers	(7,974)	(4,809)	(2,038)
Average assets	$ 16,000	$ 3,000	$ 2,000