ACCUMULATED OTHER COMPREHENSIVE LOSS (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance				$ 707,927
Other comprehensive loss before reclassifications		$ 167		304
Amounts reclassified from AOCL		(122)		(238)
Other Comprehensive Income (Loss) - Net of Tax	[1]	45	$ (92)	66	$ (66)
Ending balance		$ 819,139		$ 819,139
Combined statutory tax rate		40.00%	40.00%	40.00%	40.00%
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		$ (12,841)		$ (12,862)
Ending balance		(12,796)		(12,796)
Postretirement Liability Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(12,220)		(12,220)
Other comprehensive loss before reclassifications		0		0
Amounts reclassified from AOCL		0		0
Other Comprehensive Income (Loss) - Net of Tax		0		0
Ending balance		(12,220)		(12,220)
Unrealized Gain (Loss) on Derivatives-Other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(530)		(544)
Other comprehensive loss before reclassifications		0		0
Amounts reclassified from AOCL		7		21
Other Comprehensive Income (Loss) - Net of Tax		7		21
Ending balance		(523)		(523)
Unrealized Gain (Loss) on Available-for-Sale Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(91)		(98)
Other comprehensive loss before reclassifications		167		304
Amounts reclassified from AOCL		(129)		(259)
Other Comprehensive Income (Loss) - Net of Tax		38		45
Ending balance		$ (53)		$ (53)

[1] Determined using a combined average statutory tax rate of 40%.