Condensed Financial Statements of Parent Company (Schedule of Condensed Statements of Income) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Total interest income	$ 1,051,198	$ 892,947	$ 788,412
Interest expense	93,306	26,525	12,076
Salaries and employee benefits	350,312	337,068	318,665
Other	173,538	175,289	178,988
Income before income taxes and equity in undistributed earnings of subsidiaries	508,681	408,363	341,411
Income tax benefit	(53,763)	(44,214)	(37,150)
Net income	454,918	364,149	304,261
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	446,855	356,086	296,198
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Dividend income paid by Frost Bank	223,371	149,671	141,377
Dividend income paid by non-banks	953	915	895
Interest and other income	1,828	421	33
Total interest income	226,152	151,007	142,305
Interest expense	9,948	7,815	4,624
Salaries and employee benefits	1,973	1,202	1,828
Other	7,016	6,373	5,933
Total expenses	18,937	15,390	12,385
Income before income taxes and equity in undistributed earnings of subsidiaries	207,215	135,617	129,920
Income tax benefit	5,218	7,092	7,015
Equity in undistributed earnings of subsidiaries	242,485	221,440	167,326
Net income	454,918	364,149	304,261
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	$ 446,855	$ 356,086	$ 296,198