INVESTMENTS (Amortized Cost and Estimated Fair Values of Investments Available-for-sale) (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 1,325,172	$ 1,067,954
Gross Unrealized Gains	35,405	8,984
Gross Unrealized Losses	(4,778)	(3,605)
Estimated Fair Value	1,355,799	1,073,333
U.S. treasuries and government agencies
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	171,465	260,294
Gross Unrealized Gains	1,126	887
Gross Unrealized Losses	(4,519)	(2,686)
Estimated Fair Value	168,072	258,495
State and municipal
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	289,513	229,309
Gross Unrealized Gains	8,194	4,377
Gross Unrealized Losses	(127)	(37)
Estimated Fair Value	297,580	233,649
Mortgage-Backed and asset-backed
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	852,063	568,373
Gross Unrealized Gains	26,021	3,268
Gross Unrealized Losses	(129)	(882)
Estimated Fair Value	877,955	570,759
Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	12,131	9,100
Gross Unrealized Gains	64	452
Gross Unrealized Losses	(3)	0
Estimated Fair Value	12,192	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	1,325,172	1,067,386
Gross Unrealized Gains	35,405	8,984
Gross Unrealized Losses	(4,778)	(3,605)
Estimated Fair Value	1,355,799	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