Operating Segments - Summary of Operating Results by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	$ 336,069	$ 322,336	$ 679,323	$ 643,529
Net income	111,586	111,341	228,082	217,821
Operating Segments [Member] | Banking [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	302,747	290,433	611,360	578,994
Net income	110,893	109,276	223,810	212,917
Operating Segments [Member] | Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	36,164	34,526	73,515	69,607
Net income	4,897	5,901	11,297	11,535
Operating Segments [Member] | Non-Banks [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	(2,842)	(2,623)	(5,552)	(5,072)
Net income	$ (4,204)	$ (3,836)	$ (7,025)	$ (6,631)