Operating Segments - Summary of Operating Results by Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	$ 322,336	$ 295,868	$ 643,529	$ 588,077
Net income	111,341	85,553	217,821	170,494
Operating Segments [Member] | Banking [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	290,433	261,250	578,994	520,161
Net income	109,276	81,529	212,917	162,398
Operating Segments [Member] | Frost Wealth Advisors [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	34,526	36,712	69,607	71,300
Net income	5,901	6,279	11,535	11,573
Operating Segments [Member] | Non-Banks [Member]
Segment Reporting Information [Line Items]
Revenues from (expenses to) external customers	(2,623)	(2,094)	(5,072)	(3,384)
Net income	$ (3,836)	$ (2,255)	$ (6,631)	$ (3,477)