ACCUMULATED OTHER COMPREHENSIVE LOSS (AOCL) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance (beginning)		$ 707,927
Other comprehensive income before reclassifications		(3,364)	$ (186)
Losses from reclassifications for the period net of tax		1,292	1,803
Net current period other comprehensive income	[1],[2]	(2,072)	1,617	$ (3,610)
Balance (ending)		$ 839,900	$ 707,927
Combined statutory tax rate		40.00%	40.00%	40.00%
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance (beginning)		$ (12,862)	$ (14,479)
Net current period other comprehensive income	[2],[3]	(2,072)	1,617	$ (3,610)
Balance (ending)		(14,934)	(12,862)	(14,479)
Postretirement Liability Adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance (beginning)		(12,220)	(13,837)
Other comprehensive income before reclassifications		(3,687)	(110)
Losses from reclassifications for the period net of tax		1,490	1,727
Net current period other comprehensive income	[3]	(2,197)	1,617	(3,165)
Balance (ending)		(14,417)	(12,220)	(13,837)
Unrealized Gain (Loss) on Derivatives-Other
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance (beginning)		(544)	(567)
Other comprehensive income before reclassifications		0	0
Losses from reclassifications for the period net of tax		27	23
Net current period other comprehensive income	[3]	27	23	27
Balance (ending)		(517)	(544)	(567)
Unrealized Gain (Loss) on Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance (beginning)		(98)	(75)
Other comprehensive income before reclassifications		323	(76)
Losses from reclassifications for the period net of tax		(225)	53
Net current period other comprehensive income	[3]	98	(23)	(472)
Balance (ending)		$ 0	$ (98)	$ (75)

[1] Determined using a combined statutory tax rate of 40% in each of 2016, 2015 and 2014.

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

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