INVESTMENTS (Summary of Amortized Costs and Estimated Fair Values of Available-For-Sale Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
AFS Investments (carried at fair value):
Available-for-sale securities, at fair value (book value of $1,010,325 and $913,978, respectively)	$ 1,047,663	$ 918,118
HTM Investments (carried at amortized cost):
Amortized Cost	1,299	1,302
Unrealized Gains	89	57
Unrealized Losses	0	0
Fair Value	1,388	1,359
Obligations of states and political subdivisions
AFS Investments (carried at fair value):
Amortized Cost	121,347	115,632
Unrealized Gains	5,973	2,779
Unrealized Losses	(193)	(328)
Available-for-sale securities, at fair value (book value of $1,010,325 and $913,978, respectively)	127,127	118,083
HTM Investments (carried at amortized cost):
Amortized Cost	1,299	1,302
Unrealized Gains	89	57
Unrealized Losses	0	0
Fair Value	1,388	1,359
Mortgage-backed securities issued or guaranteed by U.S. government-sponsored enterprises
AFS Investments (carried at fair value):
Amortized Cost	516,580	462,593
Unrealized Gains	19,687	3,398
Unrealized Losses	(42)	(2,605)
Available-for-sale securities, at fair value (book value of $1,010,325 and $913,978, respectively)	536,225	463,386
Collateralized mortgage obligations issued or guaranteed by U.S. government-sponsored enterprises
AFS Investments (carried at fair value):
Amortized Cost	361,860	325,200
Unrealized Gains	11,781	3,183
Unrealized Losses	(109)	(2,478)
Available-for-sale securities, at fair value (book value of $1,010,325 and $913,978, respectively)	373,532	325,905
Subordinated corporate bonds
AFS Investments (carried at fair value):
Amortized Cost	10,538	10,553
Unrealized Gains	285	191
Unrealized Losses	(44)	0
Available-for-sale securities, at fair value (book value of $1,010,325 and $913,978, respectively)	10,779	10,744
Total AFS investments
AFS Investments (carried at fair value):
Amortized Cost	1,010,325	913,978
Unrealized Gains	37,726	9,551
Unrealized Losses	(388)	(5,411)
Available-for-sale securities, at fair value (book value of $1,010,325 and $913,978, respectively)	$ 1,047,663	$ 918,118