REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	$ 49,039	$ 88,180
Regulatory Assets, Noncurrent	375,059	405,189
Regulatory Liability, Current	(41,665)	(9,438)
Regulatory Liability, Noncurrent	(521,717)	(519,421)
Net regulatory assets (liabilities), current	7,374	78,742
Net regulatory assets (liabilities), noncurrent	(146,658)	(114,232)
Net Regulatory Assets	(139,284)	(35,490)
Federal income tax rate changes [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current	(19,447)	0
Regulatory Liability, Noncurrent	(521,717)	(519,421)
Regulatory Liabilities	(541,164)	(519,421)
Over-recovered purchased-gas costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current		(9,434)
Regulatory Liability, Noncurrent		0
Regulatory Liabilities		(9,434)
Ad valorem tax [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	1,029
Regulatory Assets, Noncurrent	0
Regulatory Assets	1,029
Regulatory Liability, Current		(4)
Regulatory Liability, Noncurrent		0
Regulatory Liabilities		(4)
Total regulated liabilities [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current	(41,665)	(9,438)
Regulatory Liability, Noncurrent	(521,717)	(519,421)
Regulatory Liabilities	(563,382)	(528,859)
Under-recovered purchased-gas costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	16,727	41,238
Regulatory Assets, Noncurrent	0	0
Regulatory Assets	16,727	41,238
Pension and postretirement benefit costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	25,109	25,156
Regulatory Assets, Noncurrent	357,723	387,582
Regulatory Assets	382,832	412,738
Weather normalization [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current		17,461
Regulatory Assets, Noncurrent		0
Regulatory Assets		17,461
Regulatory Liability, Current	(22,218)
Regulatory Liability, Noncurrent	0
Regulatory Liabilities	(22,218)
Reacquired debt costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	812	812
Regulatory Assets, Noncurrent	6,690	7,298
Regulatory Assets	7,502	8,110
MGP Costs [Member] [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Noncurrent	7,724	6,104
Regulatory Assets	7,724	6,104
Other regulatory assets [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	5,362	3,513
Regulatory Assets, Noncurrent	2,922	4,205
Regulatory Assets	8,284	7,718
Total regulatory assets, net of amortization [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	49,039	88,180
Regulatory Assets, Noncurrent	375,059	405,189
Regulatory Assets	$ 424,098	$ 493,369