Condensed Financial Statements of Parent Company (Schedule of Condensed Statements of Income) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Total interest income	$ 788,412	$ 749,496	$ 701,471
Interest expense	12,076	12,864	14,537
Salaries and employee benefits	318,665	310,504	292,349
Other	178,988	165,561	167,656
Income before income taxes and equity in undistributed earnings of subsidiaries	341,411	319,799	336,024
Income tax benefit	(37,150)	(40,471)	(58,047)
Net income	304,261	279,328	277,977
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	296,198	271,265	269,914
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Dividend income paid by Frost Bank	141,377	126,375	114,439
Dividend income paid by non-banks	895	1,830	4,323
Interest and other income	33	82	69
Total interest income	142,305	128,287	118,831
Interest expense	4,624	3,673	3,381
Salaries and employee benefits	1,828	1,376	1,218
Other	5,933	5,727	8,526
Total expenses	12,385	10,776	13,125
Income before income taxes and equity in undistributed earnings of subsidiaries	129,920	117,511	105,706
Income tax benefit	7,015	6,062	6,702
Equity in undistributed earnings of subsidiaries	167,326	155,755	165,569
Net income	304,261	279,328	277,977
Preferred stock dividends	8,063	8,063	8,063
Net income available to common shareholders	$ 296,198	$ 271,265	$ 269,914