Condensed Financial Information - Parent Company - Condensed Statements of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows from operating activities:
Net earnings	$ 74,225	$ 68,369	$ 59,659
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization, net	7,920	7,352	7,102
Decrease in other assets	(5,170)	(1,075)	4,573
Increase (decrease) in liabilities	1,818	1,132	(2,415)
Net cash provided by operating activities	93,062	95,111	71,828
CASH FLOWS FROM INVESTING ACTIVITIES:
Acquisition of bank			(2,463)
Purchase of available for sale securities	(2,049,275)	(2,171,404)	(2,068,639)
Maturities of available for securities	1,909,635	1,886,632	1,812,241
Purchases of bank premises and equipment	(16,180)	(14,777)	(11,240)
Net cash used in (provided by) investing activities	(294,991)	(331,030)	(355,808)
Cash flows from financing activities:
Proceeds of stock issuances	824	950	789
Cash dividends paid	(38,719)	(29,359)	(28,346)
Net cash provided by (used in) financing activities	311,832	222,488	283,329
Net increase (decrease) in cash and cash equivalents	109,903	(13,431)	(651)
CASH AND CASH EQUIVALENTS, beginning of year	250,836	264,267	264,918
Cash and cash equivalents, end of year	360,739	250,836	264,267
Parent company [Member]
Cash flows from operating activities:
Net earnings	74,225	68,369	59,659
Adjustments to reconcile net earnings to net cash provided by operating activities:
Excess of earnings over dividends of subsidiary bank	(17,547)	(22,722)	(20,149)
Depreciation and amortization, net	142	187	171
Decrease in other assets	34	396	10
Increase (decrease) in liabilities	(918)	774	(29)
Net cash provided by operating activities	55,936	47,004	39,662
CASH FLOWS FROM INVESTING ACTIVITIES:
Acquisition of bank			(18,200)
Net decrease (increase) in interest-bearing time deposits in unaffiliated banks	(960)	2,160	(198)
Purchase of available for sale securities		(5,756)	(5,649)
Maturities of available for securities	5,430	2,500	2,500
Purchases of bank premises and equipment	(86)	(204)	(59)
Repayment from (of advances related to) investment in and advances to subsidiaries, net	(400)	820	(200)
Net cash used in (provided by) investing activities	3,984	(480)	(21,806)
Cash flows from financing activities:
Proceeds of stock issuances	824	950	790
Cash dividends paid	(38,719)	(29,359)	(28,346)
Net cash provided by (used in) financing activities	(37,895)	(28,409)	(27,556)
Net increase (decrease) in cash and cash equivalents	22,025	18,115	(9,700)
CASH AND CASH EQUIVALENTS, beginning of year	50,020	31,905	41,605
Cash and cash equivalents, end of year	$ 72,045	$ 50,020	$ 31,905