Condensed Parent Company Financial Statements - Summary of Cash Flow of Parent Company (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 income	$ 63,887us-gaap_NetIncomeLoss	$ 54,317us-gaap_NetIncomeLoss	$ 51,900us-gaap_NetIncomeLoss
Adjustments to reconcile to net cash provided by operating activities:
Amortization of stock-based compensation arrangements	1,676us-gaap_ShareBasedCompensation	1,360us-gaap_ShareBasedCompensation	1,530us-gaap_ShareBasedCompensation
Other, net	5,719	5,512	(3,148)
Net cash provided by operating activities	76,922	78,090	60,296
INVESTING ACTIVITIES
Net cash and due from banks provided by acquisitions	174,283
Net cash used in investing activities	(183,376)	(131,078)	(185,890)
FINANCING ACTIVITIES
Issuance of common stock	6,532	5,200	3,540
Common stock acquired		(2,710)	(256)
Cash dividends paid	(19,543)us-gaap_PaymentsOfDividendsCommonStock	(13,583)us-gaap_PaymentsOfDividendsCommonStock	(21,090)us-gaap_PaymentsOfDividendsCommonStock
Redemption of junior subordinated debentures			(9,279)
Net cash provided by (used in) financing activities	162,814	(34,625)	363,009
Net increase /(decrease) in cash, due from banks and interest-bearing deposits	56,360	(87,613)	237,415
Cash, due from banks and interest-bearing deposits at the beginning of the period	1,857,535	1,945,148	1,707,733
Cash, due from banks and interest-bearing deposits at the end of the period	1,913,895	1,857,535	1,945,148
SUPPLEMENTAL DISCLOSURE
Cash paid during the year for interest	13,135us-gaap_InterestPaidNet	14,142us-gaap_InterestPaidNet	18,312us-gaap_InterestPaidNet
Cash received during the period for income taxes, net	26,178us-gaap_IncomeTaxesPaidNet	20,946us-gaap_IncomeTaxesPaidNet	25,975us-gaap_IncomeTaxesPaidNet
Parent Company [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	63,887	54,317	51,900
Adjustments to reconcile to net cash provided by operating activities:
Equity in undistributed earnings of subsidiaries	(42,196)	(34,351)	(34,281)
Amortization of stock-based compensation arrangements	1,076	1,137	1,517
Other, net	(656)	(2,116)	2,944
Net cash provided by operating activities	22,111	18,987	22,080
INVESTING ACTIVITIES
Net cash and due from banks provided by acquisitions			19,866
Sales and maturities of held for investment and available for sale securities	8	63
Net cash used in investing activities	8	63	19,866
FINANCING ACTIVITIES
Issuance of common stock	6,532	5,200	3,540
Common stock acquired		(2,710)	(256)
Cash dividends paid	(19,543)	(13,583)	(21,090)
Redemption of junior subordinated debentures			(9,279)
Net cash provided by (used in) financing activities	(13,011)	(11,093)	(27,085)
Net increase /(decrease) in cash, due from banks and interest-bearing deposits	9,108	7,957	14,861
Cash, due from banks and interest-bearing deposits at the beginning of the period	27,592	19,635	4,774
Cash, due from banks and interest-bearing deposits at the end of the period	36,700	27,592	19,635
SUPPLEMENTAL DISCLOSURE
Cash paid during the year for interest	1,965	1,965	2,128
Cash received during the period for income taxes, net	$ 1,822	$ 4,449	$ 1,884

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