Condensed Financial Statements of Parent Company (Schedule of Condensed Statements of Cash Flows) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Net income	$ 454,918	$ 364,149	$ 304,261
Stock-based compensation	13,943	13,013	11,799
Net tax benefit from stock-based compensation	3,865	9,062	5,063
Net cash from operating activities	562,388	538,079	437,842
Net cash received in acquisitions	0	0	(492)
Net cash from investing activities	(1,913,320)	(670,595)	(1,373,981)
Proceeds from issuance of subordinated notes	0	98,434	0
Principal payments on subordinated notes	0	(100,000)	0
Proceeds from stock option exercises	31,647	67,746	78,866
Purchase of treasury stock	(101,010)	(101,473)	(1,290)
Cash dividends paid on preferred stock	(8,063)	(8,063)	(8,063)
Cash dividends paid on common stock	(165,449)	(144,172)	(134,902)
Net cash from financing activities	253,664	1,044,118	1,486,061
Net change in cash and cash equivalents	(1,097,268)	911,602	549,922
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	454,918	364,149	304,261
Equity in undistributed earnings of subsidiaries	(242,485)	(221,440)	(167,326)
Stock-based compensation	721	519	520
Net tax benefit from stock-based compensation	304	318	185
Net change in other assets and other liabilities	(12,709)	7,665	(940)
Net cash from operating activities	200,749	151,211	136,700
Net cash from investing activities	0	0	0
Proceeds from issuance of subordinated notes	0	98,434	0
Principal payments on subordinated notes	0	100,000	0
Proceeds from stock option exercises	31,647	67,746	78,866
Proceeds from stock-based compensation activities of subsidiaries	13,222	12,494	11,279
Purchase of treasury stock	(101,010)	(101,473)	(1,290)
Cash dividends paid on preferred stock	(8,063)	(8,063)	(8,063)
Cash dividends paid on common stock	(165,449)	(144,172)	(134,902)
Net cash from financing activities	(229,653)	(175,034)	(54,110)
Net change in cash and cash equivalents	(28,904)	(23,823)	82,590
Cash and cash equivalents at beginning of year	265,301	289,124	206,534
Cash and cash equivalents at end of year	$ 236,397	$ 265,301	$ 289,124