Operating Segments - Summary of Operating Results by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	$ 329,322	$ 300,826	$ 972,851	$ 888,903
Net income	117,844	93,131	335,665	263,625
Operating Segments [Member] | Banking [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	295,062	266,582	874,056	786,743
Net income	114,214	88,368	327,131	250,766
Operating Segments [Member] | Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	36,868	36,529	106,475	107,829
Net income	6,442	6,417	17,977	17,990
Operating Segments [Member] | Non-Banks [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	(2,608)	(2,285)	(7,680)	(5,669)
Net income	$ (2,812)	$ (1,654)	$ (9,443)	$ (5,131)