Operating Segments - Summary of Operating Results by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	$ 323,412	$ 336,069	$ 780,848	$ 679,323
Net income	93,072	111,586	147,825	228,082
Operating Segments [Member] | Banking [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	290,185	302,747	709,614	611,360
Net income	92,492	110,893	144,516	223,810
Operating Segments [Member] | Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	35,604	36,164	76,211	73,515
Net income	3,925	4,897	9,512	11,297
Operating Segments [Member] | Non-Banks [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	(2,377)	(2,842)	(4,977)	(5,552)
Net income	$ (3,345)	$ (4,204)	$ (6,203)	$ (7,025)