INVESTMENTS (Amortized Cost and Estimated Fair Values of Investments Available-for-sale) (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 809,887	$ 730,845
Gross Unrealized Gains	11,331	10,097
Gross Unrealized Losses	(7,122)	(7,388)
Estimated Fair Value	814,096	733,554
Marketable Equity Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	1,223	1,223
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	1,223	1,223
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	808,664	729,622
Gross Unrealized Gains	11,331	10,097
Gross Unrealized Losses	(7,122)	(7,388)
Estimated Fair Value	812,873	732,331
Debt Securities | U.S. Government Agencies
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	124,316	124,314
Gross Unrealized Gains	0	32
Gross Unrealized Losses	(2,547)	(2,556)
Estimated Fair Value	121,769	121,790
Debt Securities | State and municipal
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	326,743	281,090
Gross Unrealized Gains	8,207	7,180
Gross Unrealized Losses	(462)	(586)
Estimated Fair Value	334,488	287,684
Debt Securities | Mortgage-Backed
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	347,574	314,029
Gross Unrealized Gains	2,801	2,851
Gross Unrealized Losses	(4,113)	(4,169)
Estimated Fair Value	346,262	312,711
Debt Securities | Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	9,100	9,100
Gross Unrealized Gains	304	34
Gross Unrealized Losses	0	0
Estimated Fair Value	9,404	9,134
Debt Securities | Trust Preferred
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	931	1,089
Gross Unrealized Gains	19	0
Gross Unrealized Losses	0	(77)
Estimated Fair Value	$ 950	$ 1,012