Condensed Financial Statements of Parent Company (Schedule of Condensed Statements of Income) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Total interest income	$ 701,471	$ 642,500	$ 631,612
Interest expense	14,537	21,945	26,751
Salaries and employee benefits	292,349	273,692	258,752
Other	167,656	152,077	139,469
Income before income taxes and equity in undistributed earnings of subsidiaries	336,024	290,881	308,475
Income tax benefit	(58,047)	(53,015)	(70,523)
Net income	277,977	237,866	237,952
Less: Preferred stock dividends	8,063	6,719	0
Net income available to common shareholders	269,914	231,147	237,952
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Dividend income paid by Frost Bank	114,439	144,642	143,623
Dividend income paid by non-banks	4,323	2,819	3,077
Interest and other income	69	79	308
Total interest income	118,831	147,540	147,008
Interest expense	3,381	7,365	8,512
Salaries and employee benefits	1,218	1,175	1,167
Other	8,526	6,735	6,727
Total expenses	13,125	15,275	16,406
Income before income taxes and equity in undistributed earnings of subsidiaries	105,706	132,265	130,602
Income tax benefit	6,702	7,845	7,463
Equity in undistributed earnings of subsidiaries	165,569	97,756	99,887
Net income	277,977	237,866	237,952
Less: Preferred stock dividends	8,063	6,719	0
Net income available to common shareholders	$ 269,914	$ 231,147	$ 237,952