Condensed Financial Information - Parent Company - Condensed Statements of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
CASH FLOWS FROM OPERATING ACTIVITIES:
NET EARNINGS	$ 89,559	$ 78,868	$ 74,225
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization, net	9,262	8,638	7,920
Decrease (increase) in other assets	9,852	(6,254)	(5,170)
Increase (decrease) in other liabilities	3,486	(250)	(1,216)
Net cash provided by operating activities	131,402	109,858	93,062
CASH FLOWS FROM INVESTING ACTIVITIES:
Acquisition of bank		(25,706)
Maturities of available-for-sale securities	2,917,407	2,181,684	1,909,635
Purchases of bank premises and equipment	(17,412)	(11,324)	(16,180)
Net cash used in investing activities	(575,205)	(556,770)	(294,991)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds of stock issuances	1,437	1,958	824
Cash dividends paid	(34,578)	(24,505)	(38,719)
Net cash provided by financing activities	485,262	298,185	311,832
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	41,459	(148,727)	109,903
CASH AND CASH EQUIVALENTS, beginning of year	212,012	360,739	250,836
CASH AND CASH EQUIVALENTS, end of year	253,471	212,012	360,739
Parent Company [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
NET EARNINGS	89,559	78,868	74,225
Adjustments to reconcile net earnings to net cash provided by operating activities:
Excess of earnings over dividends of subsidiary bank	(58,539)	(17,192)	(17,547)
Depreciation and amortization, net	200	135	142
Decrease (increase) in other assets	(150)	(3)	34
Increase (decrease) in other liabilities	(314)	890	(918)
Net cash provided by operating activities	30,756	62,698	55,936
CASH FLOWS FROM INVESTING ACTIVITIES:
Acquisition of bank		(39,200)
Net decrease (increase) in interest-bearing time deposits in unaffiliated banks	480	2,598	(960)
Maturities of available-for-sale securities			5,430
Purchases of bank premises and equipment	(780)	(224)	(86)
Repayment from (of advances to) investment in and advances to subsidiaries, net	(3,300)	700	(400)
Net cash used in investing activities	(3,600)	(36,126)	3,984
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds of stock issuances	1,437	1,958	824
Cash dividends paid	(34,578)	(24,505)	(38,719)
Net cash provided by financing activities	(33,141)	(22,547)	(37,895)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(5,985)	4,025	22,025
CASH AND CASH EQUIVALENTS, beginning of year	76,070	72,045	50,020
CASH AND CASH EQUIVALENTS, end of year	$ 70,085	$ 76,070	$ 72,045