INVESTMENTS (Amortized Cost and Estimated Fair Values of Investments Available-for-sale) (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 773,139	$ 730,845
Gross Unrealized Gains	12,090	10,097
Gross Unrealized Losses	(5,151)	(7,388)
Estimated Fair Value	780,078	733,554
Marketable Equity Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	212	1,223
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	212	1,223
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	772,927	729,622
Gross Unrealized Gains	12,090	10,097
Gross Unrealized Losses	(5,151)	(7,388)
Estimated Fair Value	779,866	732,331
Debt Securities | U.S. Government Agencies
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	114,346	124,314
Gross Unrealized Gains	0	32
Gross Unrealized Losses	(1,985)	(2,556)
Estimated Fair Value	112,361	121,790
Debt Securities | State and municipal
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	315,972	281,090
Gross Unrealized Gains	9,039	7,180
Gross Unrealized Losses	(78)	(586)
Estimated Fair Value	324,933	287,684
Debt Securities | Mortgage-Backed
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	332,578	314,029
Gross Unrealized Gains	2,762	2,851
Gross Unrealized Losses	(3,065)	(4,169)
Estimated Fair Value	332,275	312,711
Debt Securities | Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	9,100	9,100
Gross Unrealized Gains	289	34
Gross Unrealized Losses	0	0
Estimated Fair Value	9,389	9,134
Debt Securities | Trust Preferred
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	931	1,089
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(23)	(77)
Estimated Fair Value	$ 908	$ 1,012