Annual report pursuant to Section 13 and 15(d)

DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Narrative) (Details)

v3.10.0.1
DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Narrative) (Details)
$ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
gal
MMcf
Dec. 31, 2017
USD ($)
gal
MMcf
Dec. 31, 2016
USD ($)
Derivative [Line Items]      
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ 422.6 $ 119.3 $ 57.8
Derivative asset, fair value 91.5 37.9  
Derivative liability, fair value 3.7 2.0  
Not Designated as Hedging Instrument [Member]      
Derivative [Line Items]      
Derivative asset, fair value 91.4 37.9  
Derivative liability, fair value 0.0 1.7  
Provisional Pricing Arrangements [Member] | Product Revenues [Member]      
Derivative [Line Items]      
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ (3.2) $ (42.7) $ 14.2
Natural Gas [Member]      
Derivative [Line Items]      
Natural Gas Hedges | MMcf 1,800,000.0 3,500,000.0  
Energy Related Derivative [Member]      
Derivative [Line Items]      
Natural Gas Hedges | gal 11,000,000.0 0  
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member]      
Derivative [Line Items]      
Derivative asset, fair value $ 89.3 $ 37.9  
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member]      
Derivative [Line Items]      
Derivative asset, fair value 2.1 0.0  
Valuation, Market Approach [Member] | Fair Value, Inputs, Level 3 [Member] | Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]      
Derivative [Line Items]      
Derivative liability, fair value $ 0.0 $ 1.7