PARENT COMPANY FINANCIAL INFORMATION (Statements of Cash Flows) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows from Operating Activities:
Net income	$ 8,267	$ 15,089	$ 14,741	$ 15,112	$ 13,316	$ 13,474	$ 10,647	$ 10,813	$ 53,209	$ 48,250	$ 45,355
Adjustments to reconcile net income to net cash provided by operating activities:
Share based compensation expense									2,164	2,139	1,979
Net change in other liabilities									(1,007)	(2,932)	(6,267)
Other-net									5,174	(1,873)	4,628
Net cash provided by operating activities									69,417	90,500	51,180
Cash Flows from Investing Activities:
Purchase of investment available-for-sale									(125,028)	(287,211)	(46,190)
Acquistion of business activity, net of cash acquired									0	(1,347)	0
Net cash (used) in investing activities									(392,347)	(422,993)	(291,915)
Cash Flows from Financing Activities:
Retirement of subordinated debt									(30,000)	(5,000)	0
Redemption of stock warrant									0	(13,273)	(22,624)
Proceeds from issuance of common stock									1,200	1,580	1,174
Stock tendered for payment of withholding taxes									(952)	(683)	(687)
Repurchase of common stock									0	(13,273)	(22,624)
Dividends paid									(25,134)	(23,676)	(22,397)
Net cash provided by financing activities									301,305	393,736	217,400
Net increase (decrease) in cash and cash equivalents									(21,625)	61,243	(23,335)
Cash and cash equivalents at beginning of period				134,125				72,882	134,125	72,882	96,217
Cash and cash equivalents at end of period	112,500				134,125				112,500	134,125	72,882
Parent Company [Member]
Cash Flows from Operating Activities:
Net income									53,209	48,250	45,355
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed income-subsidiary									(27,270)	(3,960)	(3,494)
Decrease in receivable from subsidiary bank									30,000	0	0
Share based compensation expense									2,164	2,139	1,979
Tax benefit from stock options exercised									0	125	350
Other-net									(4,028)	3,213	10
Net cash provided by operating activities									54,075	49,767	44,200
Cash Flows from Investing Activities:
Purchase of investment available-for-sale									3,179	(7,000)	(2,600)
Net cash (used) in investing activities									3,179	(7,000)	(2,600)
Cash Flows from Financing Activities:
Retirement of subordinated debt									(30,000)	(5,000)	0
Proceeds from issuance of common stock									1,200	1,580	1,174
Stock tendered for payment of withholding taxes									(952)	(683)	(687)
Repurchase of common stock									0	(13,273)	(22,624)
Dividends paid									(25,134)	(23,676)	(22,397)
Net cash provided by financing activities									(54,886)	(41,052)	(44,534)
Net increase (decrease) in cash and cash equivalents									2,368	1,715	(2,934)
Cash and cash equivalents at beginning of period				$ 10,869				$ 9,154	10,869	9,154	12,088
Cash and cash equivalents at end of period	$ 13,237				$ 10,869				$ 13,237	$ 10,869	$ 9,154