INVESTMENTS (Amortized Cost and Estimated Fair Values of Investments Available-for-sale) (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 1,168,134	$ 1,067,954
Gross Unrealized Gains	28,478	8,984
Gross Unrealized Losses	(9,005)	(3,605)
Estimated Fair Value	1,187,607	1,073,333
U.S. treasuries and government agencies
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	186,184	260,294
Gross Unrealized Gains	1,559	887
Gross Unrealized Losses	(7,399)	(2,686)
Estimated Fair Value	180,344	258,495
State and municipal
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	271,771	229,309
Gross Unrealized Gains	6,106	4,377
Gross Unrealized Losses	(1,428)	(37)
Estimated Fair Value	276,449	233,649
Mortgage-Backed and asset-backed
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	698,029	568,373
Gross Unrealized Gains	20,631	3,268
Gross Unrealized Losses	(178)	(882)
Estimated Fair Value	718,482	570,759
Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	12,150	9,100
Gross Unrealized Gains	182	452
Gross Unrealized Losses	0	0
Estimated Fair Value	12,332	9,552
Trust Preferred
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	0	310
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	0	310
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	1,168,134	1,067,386
Gross Unrealized Gains	28,478	8,984
Gross Unrealized Losses	(9,005)	(3,605)
Estimated Fair Value	1,187,607	1,072,765
Marketable Equity Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	0	568
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 0	$ 568