PENSION AND OTHER POSTRETIREMENT BENEFITS - DETAILS OF ACTIVITY WITHIN THE REGULATORY ASSET AND ACCUMULATED OTHER COMPREHENSIVE LOSS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Regulatory Assets
Beginning Balance	$ 64,432	$ 71,177
Amounts Arising during the Period: Net Actuarial (Loss) Gain	9,706	(463)
Amounts Arising during the Period: Prior Service Credit	0
Amounts Amortized to Net Periodic Costs: Net Actuarial Loss	(5,485)	(6,079)
Amounts Amortized to Net Periodic Costs: Prior Service Cost	(203)	(203)
Ending Balance	68,450	64,432
Accumulated Other Comprehensive Loss (pre-tax)
Beginning Balance	35,066	39,468
Amounts Arising during the Period: Net Actuarial (Loss) Gain	8,370	59
Amounts Arising during the Period: Prior Service Credit	0
Amounts Amortized to Net Periodic Costs: Net Actuarial Loss	(3,838)	(4,452)
Amounts Amortized to Net Periodic Costs: Prior Service Cost	(8)	(9)
Ending Balance	39,590	35,066
Other Postretirement Benefits
Regulatory Assets
Beginning Balance	15,347	27,863
Amounts Arising during the Period: Net Actuarial (Loss) Gain	2,584	(3,155)
Amounts Arising during the Period: Prior Service Credit	257	(499)
Amounts Amortized to Net Periodic Costs: Net Actuarial Loss	(945)	(1,107)
Amounts Amortized to Net Periodic Costs: Prior Service Cost	0	(7,755)
Ending Balance	17,243	15,347
Accumulated Other Comprehensive Loss (pre-tax)
Beginning Balance	2,062	6,755
Amounts Arising during the Period: Net Actuarial (Loss) Gain	829	(2,285)
Amounts Arising during the Period: Prior Service Credit	84	(106)
Amounts Amortized to Net Periodic Costs: Net Actuarial Loss	(154)	(221)
Amounts Amortized to Net Periodic Costs: Prior Service Cost	0	(2,081)
Ending Balance	$ 2,821	$ 2,062