OTHER COMPREHENSIVE INCOME : OTHER COMPREHENSIVE INCOME (LOSS)-Reclassifications (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains (losses)										$ 59	$ 34	$ 17
Income tax expense										(20,622)	(19,883)	(10,392)
NET INCOME		$ 2,616	$ 8,794	$ 8,352	$ 9,369	$ 8,344	$ 8,162	$ 8,232	$ 13,675	29,131	38,413	30,196
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
NET INCOME										(698)	(1,195)	(4,909)
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)										59	34	17
Income tax expense										(23)	(13)	(6)
NET INCOME										36	21	11
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]									(1,204)	(1,992)	(8,066)
Income tax expense										470	776	3,146
NET INCOME										$ (734)	$ (1,216)	$ (4,920)

[1] Included in the computation of net periodic benefit cost which is included in salaries and benefits. (see Footnote 15 for additional details).