INVESTMENTS (Amortized Cost and Estimated Fair Values of Investments Available-for-sale) (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 749,017	$ 730,845
Gross Unrealized Gains	11,738	10,097
Gross Unrealized Losses	(4,686)	(7,388)
Estimated Fair Value	756,069	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	748,805	729,622
Gross Unrealized Gains	11,738	10,097
Gross Unrealized Losses	(4,686)	(7,388)
Estimated Fair Value	755,857	732,331
Debt Securities | U.S. Government Agencies
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	109,348	124,314
Gross Unrealized Gains	0	32
Gross Unrealized Losses	(2,008)	(2,556)
Estimated Fair Value	107,340	121,790
Debt Securities | State and municipal
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	312,656	281,090
Gross Unrealized Gains	8,534	7,180
Gross Unrealized Losses	(63)	(586)
Estimated Fair Value	321,127	287,684
Debt Securities | Mortgage-Backed
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	316,770	314,029
Gross Unrealized Gains	2,724	2,851
Gross Unrealized Losses	(2,615)	(4,169)
Estimated Fair Value	316,879	312,711
Debt Securities | Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	9,100	9,100
Gross Unrealized Gains	422	34
Gross Unrealized Losses	0	0
Estimated Fair Value	9,522	9,134
Debt Securities | Trust Preferred
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	931	1,089
Gross Unrealized Gains	58	0
Gross Unrealized Losses	0	(77)
Estimated Fair Value	$ 989	$ 1,012