Operating Segments - Summary of Operating Results by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	$ 327,024	$ 342,231	$ 1,107,872	$ 1,021,554
Net income	95,056	111,836	242,881	339,918
Operating Segments [Member] | Banking [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	293,855	307,312	1,003,469	918,672
Net income	93,507	110,583	238,023	334,393
Operating Segments [Member] | Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	35,282	37,549	111,493	111,064
Net income	4,130	4,106	13,642	15,403
Operating Segments [Member] | Non-Banks [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	(2,113)	(2,630)	(7,090)	(8,182)
Net income	$ (2,581)	$ (2,853)	$ (8,784)	$ (9,878)