Condensed Financial Statements of Parent Company (Schedule of Condensed Statements of Cash Flows) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net income	$ 364,149	$ 304,261	$ 279,328
Stock-based compensation	13,013	11,799	12,737
Net tax benefit from stock-based compensation	9,062	5,063	1,434
Net cash from operating activities	538,079	437,842	394,860
Net cash received in acquisitions	0	(492)	0
Net cash from investing activities	(670,595)	(1,373,981)	(1,238,920)
Proceeds from issuance of subordinated notes	98,434	0	0
Principal payments on subordinated notes	(100,000)	0	0
Proceeds from stock option exercises	67,746	78,866	14,853
Purchase of treasury stock	(101,473)	(1,290)	(101,237)
Cash dividends paid on preferred stock	(8,063)	(8,063)	(8,063)
Cash dividends paid on common stock	(144,172)	(134,902)	(132,161)
Net cash from financing activities	1,044,118	1,486,061	71,460
Net change in cash and cash equivalents	911,602	549,922	(772,600)
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	364,149	304,261	279,328
Equity in undistributed earnings of subsidiaries	(221,440)	(167,326)	(155,755)
Stock-based compensation	519	520	480
Net tax benefit from stock-based compensation	318	185	161
Net change in other assets and other liabilities	7,665	(940)	2,460
Net cash from operating activities	151,211	136,700	126,674
Redemption of investment in Frost Securities, Inc.	0	0	216
Net cash from investing activities	0	0	216
Proceeds from issuance of subordinated notes	98,434	0	0
Principal payments on subordinated notes	(100,000)	0	0
Proceeds from stock option exercises	67,746	78,866	14,853
Proceeds from stock-based compensation activities of subsidiaries	12,494	11,279	12,257
Purchase of treasury stock	(101,473)	(1,290)	(101,237)
Cash dividends paid on preferred stock	(8,063)	(8,063)	(8,063)
Cash dividends paid on common stock	(144,172)	(134,902)	(132,161)
Net cash from financing activities	(175,034)	(54,110)	(214,351)
Net change in cash and cash equivalents	(23,823)	82,590	(87,461)
Cash and cash equivalents at beginning of year	289,124	206,534	293,995
Cash and cash equivalents at end of year	$ 265,301	$ 289,124	$ 206,534