INVESTMENTS - Amortized Cost and Estimated Fair Values of Investments Available-for-sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,323,184	$ 1,067,954
Gross Unrealized Gains	36,710	8,984
Gross Unrealized Losses	(2,689)	(3,605)
Estimated Fair Value	1,357,205	1,073,333
U.S. treasuries and government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	140,454	260,294
Gross Unrealized Gains	914	887
Gross Unrealized Losses	(1,963)	(2,686)
Estimated Fair Value	139,405	258,495
State and municipal
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	344,168	229,309
Gross Unrealized Gains	9,435	4,377
Gross Unrealized Losses	(616)	(37)
Estimated Fair Value	352,987	233,649
Mortgage-backed and asset-backed
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	826,450	568,373
Gross Unrealized Gains	25,967	3,268
Gross Unrealized Losses	(103)	(882)
Estimated Fair Value	852,314	570,759
Corporate debt
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,112	9,100
Gross Unrealized Gains	394	452
Gross Unrealized Losses	(7)	0
Estimated Fair Value	12,499	9,552
Trust preferred
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	0	310
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	0	310
Debt Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,323,184	1,067,386
Gross Unrealized Gains	36,710	8,984
Gross Unrealized Losses	(2,689)	(3,605)
Estimated Fair Value	1,357,205	1,072,765
Marketable equity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	0	568
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 0	$ 568