INVESTMENTS (Amortized Cost and Estimated Fair Values of Investments Available-for-sale) (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 894,022	$ 946,308
Gross Unrealized Gains	8,857	3,243
Gross Unrealized Losses	(1,854)	(12,216)
Estimated Fair Value	901,025	937,335
U.S. treasuries and government agencies
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	314,564	300,338
Gross Unrealized Gains	1,072	370
Gross Unrealized Losses	(867)	(4,030)
Estimated Fair Value	314,769	296,678
State and municipal
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	245,086	280,725
Gross Unrealized Gains	4,499	2,080
Gross Unrealized Losses	(9)	(781)
Estimated Fair Value	249,576	282,024
Mortgage-Backed and asset-backed
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	324,394	355,267
Gross Unrealized Gains	2,930	653
Gross Unrealized Losses	(978)	(7,405)
Estimated Fair Value	326,346	348,515
Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	9,100	9,100
Gross Unrealized Gains	356	140
Gross Unrealized Losses	0	0
Estimated Fair Value	9,456	9,240
Trust Preferred
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	310	310
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	310	310
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	893,454	945,740
Gross Unrealized Gains	8,857	3,243
Gross Unrealized Losses	(1,854)	(12,216)
Estimated Fair Value	900,457	936,767
Marketable Equity Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	568	568
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 568	$ 568