REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	$ 38,760	$ 54,420
Regulatory Assets, Noncurrent	429,518	437,479
Regulatory Liability, Current	(54,791)	(48,394)
Regulatory Liability, Noncurrent	(511,743)	(520,866)
Net regulatory assets (liabilities), current	(16,031)	6,026
Net regulatory assets (liabilities), noncurrent	(82,225)	(83,387)
Net Regulatory Assets	(98,256)	(77,361)
Federal income tax rate changes [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current	(32,312)	(30,934)
Regulatory Liability, Noncurrent	(511,743)	(520,866)
Regulatory Liabilities	(544,055)	(551,800)
Over-recovered purchased-gas costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current	(14,197)	(13,668)
Regulatory Liability, Noncurrent	0	0
Regulatory Liabilities	(14,197)	(13,668)
Total regulated liabilities [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current	(54,791)	(48,394)
Regulatory Liability, Noncurrent	(511,743)	(520,866)
Regulatory Liabilities	(566,534)	(569,260)
Under-recovered purchased-gas costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current		25,083
Regulatory Assets, Noncurrent		0
Regulatory Assets		25,083
Pension and postretirement benefit costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	22,967	23,384
Regulatory Assets, Noncurrent	414,253	421,726
Regulatory Assets	437,220	445,110
Weather normalization [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current	(8,282)	(3,792)
Regulatory Liability, Noncurrent	0	0
Regulatory Liabilities	(8,282)	(3,792)
Reacquired debt costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	812	812
Regulatory Assets, Noncurrent	6,285	6,487
Regulatory Assets	7,097	7,299
MGP Costs [Member] [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	98	0
Regulatory Assets, Noncurrent	7,611	7,724
Regulatory Assets	7,709	7,724
Other regulatory assets [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	14,883	5,141
Regulatory Assets, Noncurrent	1,369	1,542
Regulatory Assets	16,252	6,683
Total regulatory assets, net of amortization [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	38,760	54,420
Regulatory Assets, Noncurrent	429,518	437,479
Regulatory Assets	$ 468,278	$ 491,899