INVESTMENTS (Amortized Cost and Estimated Fair Values of Investments Available-for-sale) (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 886,372	$ 946,308
Gross Unrealized Gains	9,613	3,243
Gross Unrealized Losses	(1,713)	(12,216)
Estimated Fair Value	894,272	937,335
U.S. treasuries and government agencies
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	286,478	300,338
Gross Unrealized Gains	1,036	370
Gross Unrealized Losses	(1,133)	(4,030)
Estimated Fair Value	286,381	296,678
State and municipal
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	216,770	280,725
Gross Unrealized Gains	4,716	2,080
Gross Unrealized Losses	(5)	(781)
Estimated Fair Value	221,481	282,024
Mortgage-Backed and asset-backed
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	373,146	355,267
Gross Unrealized Gains	3,450	653
Gross Unrealized Losses	(575)	(7,405)
Estimated Fair Value	376,021	348,515
Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	9,100	9,100
Gross Unrealized Gains	411	140
Gross Unrealized Losses	0	0
Estimated Fair Value	9,511	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	885,804	945,740
Gross Unrealized Gains	9,613	3,243
Gross Unrealized Losses	(1,713)	(12,216)
Estimated Fair Value	893,704	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