Condensed Parent Company Financial Statements (Detail) - Statements of Cash Flows (Parent Company Only) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating Activities
Net income	$ 82,064	$ 69,306	$ 79,306
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation of premises, furniture, and equipment	13,367	12,224	11,038
Net securities gains	(2,373)	(8,097)	(34,164)
Net losses on early extinguishment of debt	0	2,059	1,034
Share-based compensation expense	7,242	5,926	5,903
Tax expense related to share-based compensation	(1,200)	(106)	(10)
Net decrease in other assets	10,023	(18,015)	30,696
Net (decrease) increase in other liabilities	(1,042)	22,367	(21,859)
Investing Activities
Purchases of securities available-for-sale	(509,481)	(25,856)	(335,442)
Purchases of premises, furniture, and equipment	(11,269)	(14,085)	(11,030)
Net cash paid from acquisitions	(16,047)	200,645	0
Net cash (used in) provided by investing activities	(498,390)	108,357	(372,280)
Financing Activities
Payments for retirement of subordinated debt	0	0	(24,094)
Treasury stock activity	120	0	0
Cash dividends paid	(27,036)	(22,568)	(7,508)
Restricted stock activity	(2,890)	(2,781)	(1,607)
Excess tax benefit related to share-based compensation	794	912	79
Net cash provided by (used in) financing activities	114,817	(215,578)	106,903
Net increase (decrease) in cash and cash equivalents	(225,060)	19,021	(129,025)
Cash and cash equivalents at beginning of year	606,262	587,241	716,266
Cash and cash equivalents at end of year	381,202	606,262	587,241
Supplemental Disclosures of Cash Flow Information:
Common stock issued for acquisitions, net of issuance costs	0	38,300	0
Parent Company
Operating Activities
Net income	82,064	69,306	79,306
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed loss (income) of subsidiaries	26,462	(26,280)	(23,147)
Depreciation of premises, furniture, and equipment	7	6	7
Net securities gains	(1)	(5,702)	(34,119)
Net losses on early extinguishment of debt	0	0	1,034
Share-based compensation expense	7,242	5,926	5,903
Tax expense related to share-based compensation	(1,200)	(106)	(10)
Net decrease in other assets	23,699	4,599	1,084
Net (decrease) increase in other liabilities	(17,132)	14,063	(1,624)
Net cash provided by operating activities	121,141	61,812	28,434
Investing Activities
Purchases of securities available-for-sale	0	0	(46,532)
Proceeds from sales and maturities of securities available-for-sale	310	8,540	43,329
Purchases of premises, furniture, and equipment	(5)	0	0
Net cash paid from acquisitions	(16,047)	(15,809)	0
Net cash (used in) provided by investing activities	(15,742)	(7,269)	(3,203)
Financing Activities
Payments for retirement of subordinated debt	0	0	(24,094)
Treasury stock activity	120	(369)	0
Cash dividends paid	(27,036)	(22,568)	(7,508)
Restricted stock activity	(2,890)	(2,781)	(1,607)
Excess tax benefit related to share-based compensation	794	912	79
Net cash provided by (used in) financing activities	(29,252)	(24,068)	(33,130)
Net increase (decrease) in cash and cash equivalents	76,147	30,475	(7,899)
Cash and cash equivalents at beginning of year	43,546	13,071	20,970
Cash and cash equivalents at end of year	119,693	43,546	13,071
Supplemental Disclosures of Cash Flow Information:
Common stock issued for acquisitions, net of issuance costs	$ 0	$ 38,300	$ 0