Condensed Financial Statements of Parent Company (Schedule of Condensed Statements of Income) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Total interest income	$ 892,947	$ 788,412	$ 749,496
Interest expense	26,525	12,076	12,864
Salaries and employee benefits	337,068	318,665	310,504
Other	175,289	178,988	165,561
Income before income taxes and equity in undistributed earnings of subsidiaries	408,363	341,411	319,799
Income tax benefit	(44,214)	(37,150)	(40,471)
Net income	364,149	304,261	279,328
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	356,086	296,198	271,265
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Dividend income paid by Frost Bank	149,671	141,377	126,375
Dividend income paid by non-banks	915	895	1,830
Interest and other income	421	33	82
Total interest income	151,007	142,305	128,287
Interest expense	7,815	4,624	3,673
Salaries and employee benefits	1,202	1,828	1,376
Other	6,373	5,933	5,727
Total expenses	15,390	12,385	10,776
Income before income taxes and equity in undistributed earnings of subsidiaries	135,617	129,920	117,511
Income tax benefit	7,092	7,015	6,062
Equity in undistributed earnings of subsidiaries	221,440	167,326	155,755
Net income	364,149	304,261	279,328
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	$ 356,086	$ 296,198	$ 271,265