PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 5,132,682	$ 4,850,201
Accumulated depreciation and amortization	(1,620,771)	(1,556,481)
Net property, plant and equipment	3,511,911	3,293,720
Regulated [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	5,132,682	4,850,201
Accumulated depreciation and amortization	(1,620,771)	(1,556,481)
Net property, plant and equipment	$ 3,511,911	$ 3,293,720
Average depreciation rates, minimum	2.00%	2.00%	2.00%
Average depreciation rates, maximum	3.00%	3.00%	3.00%
Interest costs capitalized	$ 2,600	$ 2,500	$ 1,300
Construction work in process expenditures incurred but not yet paid	15,000	7,000	$ 10,500
Natural gas distribution pipelines and related equipment | Regulated [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	4,114,090	3,909,881
Natural gas transmission pipelines and related equipment | Regulated [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	462,654	450,810
General plant and other | Regulated [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	498,906	418,157
Construction work in process | Regulated [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 57,032	$ 71,353