REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2017	Dec. 31, 2019
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%		35.00%
Regulatory Assets, Current	$ 39,808			$ 47,440
Regulatory Assets, Noncurrent	380,686			391,036
Regulatory Liability, Current	(33,649)			(45,201)
Regulatory Liability, Noncurrent	(494,210)			(503,518)
Net regulatory assets (liabilities), current	6,159			2,239
Net regulatory assets (liabilities), noncurrent	(113,524)			(112,482)
Net Regulatory Assets	(107,365)			(110,243)
Reduction in income tax expense for the amortization of the regulatory liability associated with excess ADIT that was returned to customers	6,900	$ 6,700
Federal income tax rate changes [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current	(6,282)			(10,297)
Regulatory Liability, Noncurrent	(494,210)			(503,518)
Regulatory Liabilities	(500,492)			(513,815)
Over-recovered purchased-gas costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current	(27,367)			(27,623)
Regulatory Liability, Noncurrent	0			0
Regulatory Liabilities	(27,367)			(27,623)
Total regulated liabilities [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Liability, Current	(33,649)			(45,201)
Regulatory Liability, Noncurrent	(494,210)			(503,518)
Regulatory Liabilities	(527,859)			(548,719)
Under-recovered purchased-gas costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current				17,172
Regulatory Assets, Noncurrent				0
Regulatory Assets				17,172
Pension and postretirement benefit costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	21,199			21,213
Regulatory Assets, Noncurrent	363,467			373,266
Regulatory Assets	384,666			394,479
Weather normalization [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	3,713
Regulatory Assets, Noncurrent	0
Regulatory Assets	3,713
Regulatory Liability, Current				(7,281)
Regulatory Liability, Noncurrent				0
Regulatory Liabilities				(7,281)
Reacquired debt costs [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	812			812
Regulatory Assets, Noncurrent	5,474			5,677
Regulatory Assets	6,286			6,489
MGP Costs [Member] [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	98			98
Regulatory Assets, Noncurrent	9,685			9,709
Regulatory Assets	9,783			9,807
Ad valorem tax [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	3,593			2,921
Regulatory Assets, Noncurrent	0			0
Regulatory Assets	3,593			2,921
Other regulatory assets [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	10,393			5,224
Regulatory Assets, Noncurrent	2,060			2,384
Regulatory Assets	12,453			7,608
Total regulatory assets, net of amortization [Member]
SCHEDULE OF REGULATED ASSETS AND LIABILITIES [Line Items]
Regulatory Assets, Current	39,808			47,440
Regulatory Assets, Noncurrent	380,686			391,036
Regulatory Assets	$ 420,494			$ 438,476