REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	$ 54,123	$ 54,420
Regulatory Assets, Noncurrent	416,140	437,479
Regulatory Liability, Current	(48,784)	(48,394)
Regulatory Liability, Noncurrent	(506,961)	(520,866)
Net regulatory assets (liabilities), current	5,339	6,026
Net regulatory assets (liabilities), noncurrent	(90,821)	(83,387)
Net Regulatory Assets	(85,482)	(77,361)
Federal income tax rate changes [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current	(14,263)	(30,934)
Regulatory Liability, Noncurrent	(506,961)	(520,866)
Regulatory Liabilities	(521,224)	(551,800)
Over-recovered purchased-gas costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current	(27,543)	(13,668)
Regulatory Liability, Noncurrent	0	0
Regulatory Liabilities	(27,543)	(13,668)
Total regulated liabilities [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current	(48,784)	(48,394)
Regulatory Liability, Noncurrent	(506,961)	(520,866)
Regulatory Liabilities	(555,745)	(569,260)
Under-recovered purchased-gas costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	18,058	25,083
Regulatory Assets, Noncurrent	0	0
Regulatory Assets	18,058	25,083
Pension and postretirement benefit costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	22,966	23,384
Regulatory Assets, Noncurrent	398,683	421,726
Regulatory Assets	421,649	445,110
Weather normalization [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current	(6,978)	(3,792)
Regulatory Liability, Noncurrent	0	0
Regulatory Liabilities	(6,978)	(3,792)
Reacquired debt costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	812	812
Regulatory Assets, Noncurrent	5,879	6,487
Regulatory Assets	6,691	7,299
MGP Costs [Member] [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	98	0
Regulatory Assets, Noncurrent	9,734	7,724
Regulatory Assets	9,832	7,724
Ad valorem tax [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	2,572	1,070
Regulatory Assets, Noncurrent	0	0
Regulatory Assets	2,572	1,070
Other regulatory assets [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	9,617	4,071
Regulatory Assets, Noncurrent	1,844	1,542
Regulatory Assets	11,461	5,613
Total regulatory assets, net of amortization [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	54,123	54,420
Regulatory Assets, Noncurrent	416,140	437,479
Regulatory Assets	$ 470,263	$ 491,899