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SIGNIFICANT ACCOUNTING POLICIES - Impairment of Equity Method Investments (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2015
Dec. 31, 2015
EQUITY INVESTMENTS    
Impairment of equity-method investment $ 199 $ 199 [1]
Great Lakes | Nonrecurring fair value measurement    
EQUITY INVESTMENTS    
Impairment of equity-method investment $ 199 $ 199
[1] Recast to consolidate PNGTS for all periods presented (Refer to Note 2).