Condensed Financial Statements of Parent Company (Schedule of Condensed Statements of Income) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Total interest income	$ 749,496	$ 701,471	$ 642,500
Interest expense	12,864	14,537	21,945
Salaries and employee benefits	310,504	292,349	273,692
Other	165,561	167,656	152,077
Income before income taxes and equity in undistributed earnings of subsidiaries	319,799	336,024	290,881
Income tax benefit	(40,471)	(58,047)	(53,015)
Net income	279,328	277,977	237,866
Preferred stock dividends	8,063	8,063	6,719
Net income available to common shareholders	271,265	269,914	231,147
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Dividend income paid by Frost Bank	126,375	114,439	144,642
Dividend income paid by non-banks	1,830	4,323	2,819
Interest and other income	82	69	79
Total interest income	128,287	118,831	147,540
Interest expense	3,673	3,381	7,365
Salaries and employee benefits	1,376	1,218	1,175
Other	5,727	8,526	6,735
Total expenses	10,776	13,125	15,275
Income before income taxes and equity in undistributed earnings of subsidiaries	117,511	105,706	132,265
Income tax benefit	6,062	6,702	7,845
Equity in undistributed earnings of subsidiaries	155,755	165,569	97,756
Net income	279,328	277,977	237,866
Preferred stock dividends	8,063	8,063	6,719
Net income available to common shareholders	$ 271,265	$ 269,914	$ 231,147