ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS), Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017		Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance		$ 602,081	$ 598,567
Other comprehensive income (loss) before reclassifications		1,374	499
Amounts reclassified from accumulated other comprehensive loss		(106)	(774)
Other comprehensive income (loss), net of tax		1,268	(275)
Reclassification of tax effects on other comprehensive income	[1]		32
Balance		$ 606,274	607,957	$ 598,567
Corporate income tax rate		21.00%		35.00%
ASU 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance			598,567
Reclassification for adoption of ASU 2016-01				$ 0
Balance				598,567
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance		$ (6,500)	(3,641)
Balance		(5,232)	(4,037)	(3,641)
Accumulated Other Comprehensive Loss [Member] | ASU 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance			(3,794)
Reclassification for adoption of ASU 2016-01				(153)	[2]	$ (153)
Balance				(3,794)
Securities Available-for-Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance		(1,957)	285
Other comprehensive income (loss) before reclassifications		3,129	(786)
Amounts reclassified from accumulated other comprehensive loss		51	(922)
Other comprehensive income (loss), net of tax		3,180	(1,708)
Reclassification of tax effects on other comprehensive income	[1]		0
Balance		1,223	(1,576)	285
Securities Available-for-Sale [Member] | ASU 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance			132
Reclassification for adoption of ASU 2016-01	[2]			(153)
Balance				132
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance		(6,290)	(6,633)
Other comprehensive income (loss) before reclassifications		252	(88)
Amounts reclassified from accumulated other comprehensive loss		82	192
Other comprehensive income (loss), net of tax		334	104
Reclassification of tax effects on other comprehensive income	[1]		32
Balance		(5,956)	(6,497)	(6,633)
Defined Benefit Plans [Member] | ASU 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance			(6,633)
Reclassification for adoption of ASU 2016-01	[2]			0
Balance				(6,633)
Derivative Asset [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance		1,747	2,707
Other comprehensive income (loss) before reclassifications		(2,007)	1,373
Amounts reclassified from accumulated other comprehensive loss		(239)	(44)
Other comprehensive income (loss), net of tax		(2,246)	1,329
Reclassification of tax effects on other comprehensive income	[1]		0
Balance		$ (499)	4,036	2,707
Derivative Asset [Member] | ASU 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance			$ 2,707
Reclassification for adoption of ASU 2016-01	[2]			0
Balance				$ 2,707

[1] Represents the impact of adopting ASU 2018-02 allowing the reclassification of certain stranded income tax effects in accumulated other comprehensive income resulting from the Tax Cuts and Jobs Act of 2017 (the "Tax Act") from accumulated other comprehensive income to retained earnings in each period in which the effect of the change in the U.S. federal corporate income tax rate in the Tax Act (or portion thereof) is recorded. The amount of the reclassification is an adjustment for the difference between the historical corporate income tax rate (35%) and the newly enacted 21% corporate income tax rate.

[2] Represents the impact of adopting ASU 2016-01 allowing the reclassification of unrealized gains and losses on available-for-sale equity securities from accumulated other comprehensive income to retained earnings.