Condensed Financial Statements of Parent Company (Schedule of Condensed Statements of Income) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Total interest income	$ 1,021,035	$ 1,133,785	$ 1,051,198
Interest expense	45,034	129,780	93,306
Other	166,310	180,665	173,466
Income before income taxes and equity in undistributed earnings of subsidiaries	351,321	499,469	508,681
Income tax benefit	(20,170)	(55,870)	(53,763)
Net income	331,151	443,599	454,918
Preferred stock dividends	2,016	8,063	8,063
Redemption of preferred stock	5,514	0	0
Net income available to common shareholders	323,621	435,536	446,855
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Dividend income paid by Frost Bank	298,884	234,531	223,371
Dividend income paid by non-banks	736	1,822	953
Interest and other income	446	2,868	1,828
Total interest income	300,066	239,221	226,152
Interest expense	8,216	10,363	9,948
Salaries and employee benefits	1,581	1,551	1,973
Other	6,833	7,033	7,016
Total expenses	16,630	18,947	18,937
Income before income taxes and equity in undistributed earnings of subsidiaries	283,436	220,274	207,215
Income tax benefit	5,406	5,135	5,218
Equity in undistributed earnings of subsidiaries	42,309	218,190	242,485
Net income	331,151	443,599	454,918
Preferred stock dividends	2,016	8,063	8,063
Redemption of preferred stock	5,514	0	0
Net income available to common shareholders	$ 323,621	$ 435,536	$ 446,855