INVESTMENTS (Summary of Amortized Costs and Estimated Fair Values of Available-For-Sale Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
AFS Investments (carried at fair value):
Available-for-sale securities, at fair value (book value of $915,099 and $933,399, respectively)	$ 920,083	$ 910,692
HTM Investments (carried at amortized cost):
Amortized Cost	1,304	1,307
Unrealized Gains	31	8
Unrealized Losses	0	(24)
Fair Value	1,335	1,291
Obligations of states and political subdivisions
AFS Investments (carried at fair value):
Amortized Cost	87,223	94,430
Unrealized Gains	1,932	216
Unrealized Losses	(26)	(894)
Available-for-sale securities, at fair value (book value of $915,099 and $933,399, respectively)	89,129	93,752
HTM Investments (carried at amortized cost):
Amortized Cost	1,304	1,307
Unrealized Gains	31	8
Unrealized Losses	0	(24)
Fair Value	1,335	1,291
Mortgage-backed securities issued or guaranteed by U.S. government-sponsored enterprises
AFS Investments (carried at fair value):
Amortized Cost	455,330	466,613
Unrealized Gains	3,250	583
Unrealized Losses	(2,203)	(13,524)
Available-for-sale securities, at fair value (book value of $915,099 and $933,399, respectively)	456,377	453,672
Collateralized mortgage obligations issued or guaranteed by U.S. government-sponsored enterprises
AFS Investments (carried at fair value):
Amortized Cost	351,149	351,958
Unrealized Gains	3,940	1,007
Unrealized Losses	(3,018)	(10,071)
Available-for-sale securities, at fair value (book value of $915,099 and $933,399, respectively)	352,071	342,894
Subordinated corporate bonds
AFS Investments (carried at fair value):
Amortized Cost	21,397	20,398
Unrealized Gains	1,109	23
Unrealized Losses	0	(47)
Available-for-sale securities, at fair value (book value of $915,099 and $933,399, respectively)	22,506	20,374
Total AFS investments
AFS Investments (carried at fair value):
Amortized Cost	915,099	933,399
Unrealized Gains	10,231	1,829
Unrealized Losses	(5,247)	(24,536)
Available-for-sale securities, at fair value (book value of $915,099 and $933,399, respectively)	$ 920,083	$ 910,692

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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