INVESTMENTS (Amortized Cost and Estimated Fair Values of Investments Available-for-sale) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 926,689	$ 946,308
Gross Unrealized Gains	5,361	3,243
Gross Unrealized Losses	(5,520)	(12,216)
Estimated Fair Value	926,530	937,335
U.S. treasuries and government agencies
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	314,928	300,338
Gross Unrealized Gains	546	370
Gross Unrealized Losses	(1,886)	(4,030)
Estimated Fair Value	313,588	296,678
State and municipal
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	261,126	280,725
Gross Unrealized Gains	3,491	2,080
Gross Unrealized Losses	(37)	(781)
Estimated Fair Value	264,580	282,024
Mortgage-Backed and asset-backed
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	340,657	355,267
Gross Unrealized Gains	1,037	653
Gross Unrealized Losses	(3,597)	(7,405)
Estimated Fair Value	338,097	348,515
Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	9,100	9,100
Gross Unrealized Gains	287	140
Gross Unrealized Losses	0	0
Estimated Fair Value	9,387	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	926,121	945,740
Gross Unrealized Gains	5,361	3,243
Gross Unrealized Losses	(5,520)	(12,216)
Estimated Fair Value	925,962	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