PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 5,404,168	$ 5,132,682
Accumulated depreciation and amortization	(1,672,548)	(1,620,771)
Net property, plant and equipment	3,731,620	3,511,911
Regulated [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	5,404,168	5,132,682
Accumulated depreciation and amortization	(1,672,548)	(1,620,771)
Net property, plant and equipment	$ 3,731,620	$ 3,511,911
Average depreciation rates, minimum	2.00%	2.00%	2.00%
Average depreciation rates, maximum	3.00%	3.00%	3.00%
Interest costs capitalized	$ 3,600	$ 2,600	$ 2,500
Construction work in process expenditures incurred but not yet paid	11,900	15,000	$ 7,000
Natural gas distribution pipelines and related equipment | Regulated [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	4,321,429	4,114,090
Natural gas transmission pipelines and related equipment | Regulated [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	481,953	462,654
General plant and other | Regulated [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	530,459	498,906
Construction work in process | Regulated [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 70,327	$ 57,032