RISK MANAGEMENT AND HEDGING ACTIVITIES USING DERIVATIVES - Part 1 (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Assets	$ 74,415,000	$ 53,168,000
(Liabilities)	(58,749,000)	(72,136,000)
Derivative, Net Liability Position, Aggregate Fair Value	0
Derivatives designated as hedging instruments
Derivatives, Fair Value [Line Items]
Assets	66,938,000	48,822,000
(Liabilities)	(51,438,000)	(67,897,000)
Derivatives not designated as hedging instruments
Derivatives, Fair Value [Line Items]
Assets	7,477,000	4,346,000
(Liabilities)	(7,311,000)	(4,239,000)
Natural Gas Pipelines
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	0	0
Other Current Assets [Member] \| Commodity contracts \| Financial contracts \| Derivatives designated as hedging instruments
Derivatives, Fair Value [Line Items]
Assets	16,978,000	1,155,000
Other Current Assets [Member] \| Commodity contracts \| Financial contracts \| Derivatives not designated as hedging instruments
Derivatives, Fair Value [Line Items]
Assets	7,477,000	4,346,000
Other Current Assets [Member] \| Commodity contracts \| Physical contracts \| Derivatives designated as hedging instruments
Derivatives, Fair Value [Line Items]
Assets	0	0
Other Current Assets [Member] \| Interest Rate Contracts \| Derivatives designated as hedging instruments
Derivatives, Fair Value [Line Items]
Assets	1,330,000	0
Other Current Liabilities [Member] \| Commodity contracts \| Financial contracts \| Derivatives designated as hedging instruments
Derivatives, Fair Value [Line Items]
(Liabilities)	(42,819,000)	(49,938,000)
Other Current Liabilities [Member] \| Commodity contracts \| Financial contracts \| Derivatives not designated as hedging instruments
Derivatives, Fair Value [Line Items]
(Liabilities)	(7,311,000)	(4,239,000)
Other Current Liabilities [Member] \| Commodity contracts \| Physical contracts \| Derivatives designated as hedging instruments
Derivatives, Fair Value [Line Items]
(Liabilities)	(4,781,000)	(3,022,000)
Other Current Liabilities [Member] \| Interest Rate Contracts \| Derivatives designated as hedging instruments
Derivatives, Fair Value [Line Items]
(Liabilities)	0	(12,795,000)
Other Assets [Member] \| Commodity contracts \| Financial contracts \| Derivatives designated as hedging instruments
Derivatives, Fair Value [Line Items]
Assets	0	210,000
Other Assets [Member] \| Interest Rate Contracts \| Derivatives designated as hedging instruments
Derivatives, Fair Value [Line Items]
Assets	48,630,000	47,457,000
Other Deferred Credits \| Commodity contracts \| Financial contracts \| Derivatives designated as hedging instruments
Derivatives, Fair Value [Line Items]
(Liabilities)	(3,838,000)	(2,142,000)
Other Deferred Credits \| Interest Rate Contracts \| Derivatives designated as hedging instruments
Derivatives, Fair Value [Line Items]
(Liabilities)	$ 0	$ 0

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

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