Condensed Financial Statements of Parent Company (Schedule of Condensed Statements of Income) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Condensed Financial Statements, Captions [Line Items]
Interest income	$ 2,420,747	$ 2,390,637	$ 2,217,716
Other	277,156	244,411	229,367
Income before income taxes and equity in undistributed earnings of subsidiaries	771,702	695,967	712,373
Income tax benefit	(123,145)	(113,425)	(114,400)
Net income (loss)	648,557	582,542	597,973
Preferred stock dividends	6,675	6,675	6,675
Net income (loss) available to common shareholders	641,882	575,867	591,298
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Dividend income paid by Frost Bank	352,292	262,214	279,679
Dividend income paid by non-banks	231	268	255
Interest and other income	0	0	1,929
Interest income	352,523	262,482	281,863
Interest expense	12,346	13,529	13,304
Salaries and employee benefits	1,682	1,727	1,668
Other	6,042	7,304	6,653
Total expenses	20,070	22,560	21,625
Income before income taxes and equity in undistributed earnings of subsidiaries	332,453	239,922	260,238
Income tax benefit	6,281	6,896	5,974
Equity in undistributed earnings of subsidiaries	309,823	335,724	331,761
Net income (loss)	648,557	582,542	597,973
Preferred stock dividends	6,675	6,675	6,675
Net income (loss) available to common shareholders	$ 641,882	$ 575,867	$ 591,298