REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	$ 82,302	$ 83,146
Regulatory Assets, Noncurrent	421,094	440,522
Net regulatory assets (liabilities), current	67,647	71,224
Net regulatory assets (liabilities), noncurrent	421,094	440,522
Net Regulatory Assets	488,741	511,746
Over-recovered purchased-gas costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current	(14,049)	(10,154)
Regulatory Liability, Noncurrent	0	0
Regulatory Liabilities	(14,049)	(10,154)
Ad valorem tax [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current	(606)	(1,768)
Regulatory Liability, Noncurrent	0	0
Regulatory Liabilities	(606)	(1,768)
Total regulated liabilities [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current	(14,655)	(11,922)
Regulatory Liability, Noncurrent	0	0
Regulatory Liabilities	(14,655)	(11,922)
Under-recovered purchased-gas costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	26,500	29,901
Regulatory Assets, Noncurrent	0	0
Regulatory Assets	26,500	29,901
Pension and postretirement benefit costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	31,498	31,498
Regulatory Assets, Noncurrent	408,593	427,448
Regulatory Assets	440,091	458,946
Weather normalization [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	20,311	17,661
Regulatory Assets, Noncurrent	0	0
Regulatory Assets	20,311	17,661
Reacquired debt costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	812	812
Regulatory Assets, Noncurrent	7,703	8,108
Regulatory Assets	8,515	8,920
Other regulatory assets [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	3,181	3,274
Regulatory Assets, Noncurrent	4,798	4,966
Regulatory Assets	7,979	8,240
Total regulatory assets, net of amortization [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	82,302	83,146
Regulatory Assets, Noncurrent	421,094	440,522
Regulatory Assets	$ 503,396	$ 523,668