OTHER COMPREHENSIVE INCOME : OTHER COMPREHENSIVE INCOME (LOSS)-Reclassifications (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains (losses)										$ 233	$ 44	$ 2
Income tax expense										(11,692)	(12,127)	(11,145)
NET INCOME		$ 15,739	$ 14,000	$ 11,924	$ 12,181	$ 14,364	$ 12,257	$ 12,569	$ 9,682	53,844	48,872	46,583
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
NET INCOME										(1,300)	(1,135)	(1,083)
Unrealized Gains and Losses on Available-for-Sale Securities | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains (losses)										233	44	2
Income tax expense										(58)	(11)	0
NET INCOME										175	33	2
Retirement Plans | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Change in funded status of post retirement benefits	[1]									(1,967)	(1,558)	(1,447)
Income tax expense										492	390	362
NET INCOME										$ (1,475)	$ (1,168)	$ (1,085)

[1] Included in the computation of net periodic benefit cost which is included in salaries and benefits. (see Footnote 16 for additional details).