Operating Segments (Summary of Operating Results by Segment) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Net interest income (expense)	$ 776,336	$ 736,632	$ 686,934
Provision for loan losses	51,673	51,845	16,314
Non-interest income	349,708	328,730	320,144
Non-interest expense	732,960	693,718	654,740
Income before income taxes	341,411	319,799	336,024
Income tax expense (benefit)	37,150	40,471	58,047
Net income	304,261	279,328	277,977
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	296,198	271,265	269,914
Revenues from (expenses to) external customers	1,126,044	1,065,362	1,007,078
Average assets	28,832,000	28,061,000	25,766,000
Banking [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	769,625	732,671	683,579
Provision for loan losses	51,672	51,848	16,312
Non-interest income	229,791	205,606	193,883
Non-interest expense	624,396	589,394	549,812
Income before income taxes	323,348	297,035	311,338
Income tax expense (benefit)	33,683	34,997	51,881
Net income	289,665	262,038	259,457
Preferred stock dividends	0	0	0
Net income available to common shareholders	289,665	262,038	259,457
Revenues from (expenses to) external customers	999,416	938,277	877,462
Average assets	28,795,000	28,023,000	25,732,000
Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	11,335	7,634	6,734
Provision for loan losses	1	(3)	2
Non-interest income	120,102	121,489	122,261
Non-interest expense	102,062	98,405	96,330
Income before income taxes	29,374	30,721	32,663
Income tax expense (benefit)	10,281	10,753	11,431
Net income	19,093	19,968	21,232
Preferred stock dividends	0	0	0
Net income available to common shareholders	19,093	19,968	21,232
Revenues from (expenses to) external customers	131,437	129,123	128,995
Average assets	34,000	36,000	32,000
Non-Banks [Member]
Segment Reporting Information [Line Items]
Net interest income (expense)	(4,624)	(3,673)	(3,379)
Provision for loan losses	0	0	0
Non-interest income	(185)	1,635	4,000
Non-interest expense	6,502	5,919	8,598
Income before income taxes	(11,311)	(7,957)	(7,977)
Income tax expense (benefit)	(6,814)	(5,279)	(5,265)
Net income	(4,497)	(2,678)	(2,712)
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	(12,560)	(10,741)	(10,775)
Revenues from (expenses to) external customers	(4,809)	(2,038)	621
Average assets	$ 3,000	$ 2,000	$ 2,000