Condensed Financial Statements of Parent Company (Schedule of Condensed Statements of Income) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Total interest income	$ 1,133,785	$ 1,051,198	$ 892,947
Interest expense	129,780	93,306	26,525
Salaries and employee benefits	375,029	350,312	337,068
Other	180,665	173,538	175,289
Income before income taxes and equity in undistributed earnings of subsidiaries	499,469	508,681	408,363
Income tax benefit	(55,870)	(53,763)	(44,214)
Net income	443,599	454,918	364,149
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	435,536	446,855	356,086
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Dividend income paid by Frost Bank	234,531	223,371	149,671
Dividend income paid by non-banks	1,822	953	915
Interest and other income	2,868	1,828	421
Total interest income	239,221	226,152	151,007
Interest expense	10,363	9,948	7,815
Salaries and employee benefits	1,551	1,973	1,202
Other	7,033	7,016	6,373
Total expenses	18,947	18,937	15,390
Income before income taxes and equity in undistributed earnings of subsidiaries	220,274	207,215	135,617
Income tax benefit	5,135	5,218	7,092
Equity in undistributed earnings of subsidiaries	218,190	242,485	221,440
Net income	443,599	454,918	364,149
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	$ 435,536	$ 446,855	$ 356,086