Operating Segments - Summary of Operating Results by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	$ 342,231	$ 329,322	$ 1,021,554	$ 972,851
Net income	111,836	117,844	339,918	335,665
Operating Segments [Member] | Banking [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	307,312	295,062	918,672	874,056
Net income	110,583	114,214	334,393	327,131
Operating Segments [Member] | Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	37,549	36,868	111,064	106,475
Net income	4,106	6,442	15,403	17,977
Operating Segments [Member] | Non-Banks [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	(2,630)	(2,608)	(8,182)	(7,680)
Net income	$ (2,853)	$ (2,812)	$ (9,878)	$ (9,443)