Condensed Financial Statements of Parent Company (Schedule of Condensed Statements of Income) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Condensed Financial Statements, Captions [Line Items]
Total interest income	$ 1,008,769	$ 1,021,035	$ 1,133,785
Interest expense	23,902	45,034	129,780
Other	171,457	166,310	180,665
Income before income taxes and equity in undistributed earnings of subsidiaries	489,538	351,321	499,469
Income tax benefit	(46,459)	(20,170)	(55,870)
Net income	443,079	331,151	443,599
Preferred stock dividends	7,157	2,016	8,063
Redemption of preferred stock	0	5,514	0
Net income available to common shareholders	435,922	323,621	435,536
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Dividend income paid by Frost Bank	219,386	298,884	234,531
Dividend income paid by non-banks	473	736	1,822
Interest and other income	101	446	2,868
Total interest income	219,960	300,066	239,221
Interest expense	7,141	8,216	10,363
Salaries and employee benefits	1,499	1,581	1,551
Other	5,867	6,833	7,033
Total expenses	14,507	16,630	18,947
Income before income taxes and equity in undistributed earnings of subsidiaries	205,453	283,436	220,274
Income tax benefit	4,899	5,406	5,135
Equity in undistributed earnings of subsidiaries	232,727	42,309	218,190
Net income	443,079	331,151	443,599
Preferred stock dividends	7,157	2,016	8,063
Redemption of preferred stock	0	5,514	0
Net income available to common shareholders	$ 435,922	$ 323,621	$ 435,536