DERIVATIVE FINANCIAL INSTRUMENTS AND HEDGING ACTIVITIES (Fair Values of Expected Cash Flows of Commodity Options) (Details) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2016 USD ($) MMBTU Boe $ / MillionBarrels-MMBbls bbl	Dec. 31, 2015 USD ($)
Fair Value
Asset		$ 128.1	$ 163.6
Liability		(289.7)	(221.5)
Commodity Options [Member]
Fair Value
Credit Valuation Adjustments		$ 0.3	0.4
Purchased [Member] | Portion of Put Option Contracts Maturing in 2016 - Natural Gas [Member]
Derivative [Line Items]
Notional (MMBtu) | Boe		1,237,500
Strike Price | $ / MillionBarrels-MMBbls	[1]	3.75
Market Price | $ / MillionBarrels-MMBbls	[1]	2.22
Fair Value
Asset	[2]	$ 1.9	2.1
Liability	[2]	$ 0.0	0.0
Purchased [Member] | Portion of Put Option Contracts Maturing in 2016 - NGL [Member]
Derivative [Line Items]
Notional (Bbl) | bbl	[3]	2,227,500
Strike Price | $ / MillionBarrels-MMBbls	[1]	39.29
Market Price | $ / MillionBarrels-MMBbls	[1]	22.66
Fair Value
Asset	[2]	$ 37.6	54.4
Liability	[2]	$ 0.0	0.0
Purchased [Member] | Portion Of Put Option Contracts Maturing In 2016 - Crude Oil [Member]
Derivative [Line Items]
Notional (Bbl) | bbl	[3]	605,000
Strike Price | $ / MillionBarrels-MMBbls	[1]	75.91
Market Price | $ / MillionBarrels-MMBbls	[1]	41.68
Fair Value
Asset	[2]	$ 20.7	27.7
Liability	[2]	$ 0.0	0.0
Purchased [Member] | Portion of Call Option Contracts Maturing in 2016 - Natural Gas [Member]
Derivative [Line Items]
Notional (MMBtu) | MMBTU	[3]	1,237,500
Strike Price | $ / MillionBarrels-MMBbls	[1]	4.98
Market Price | $ / MillionBarrels-MMBbls	[1]	2.22
Fair Value
Asset	[2]	$ 0.0	0.0
Liability	[2]	$ 0.0	0.0
Purchased [Member] | Portion of Call Option Contracts Maturing in 2016 - NGL [Member]
Derivative [Line Items]
Notional (Bbl) | bbl	[3]	68,750
Strike Price | $ / MillionBarrels-MMBbls	[1]	46.41
Market Price | $ / MillionBarrels-MMBbls	[1]	23.25
Fair Value
Asset	[2]	$ 0.0	0.0
Liability	[2]	$ 0.0	0.0
Purchased [Member] | Portion of Put Option Contracts Maturing in 2017 - NGL [Member]
Derivative [Line Items]
Notional (Bbl) | bbl	[3]	1,277,500
Strike Price | $ / MillionBarrels-MMBbls	[1]	25.26
Market Price | $ / MillionBarrels-MMBbls	[1]	23.44
Fair Value
Asset	[2]	$ 5.1	5.8
Liability	[2]	$ 0.0	0.0
Purchased [Member] | Portion of Put Option Contracts Maturing in 2017 - Crude Oil [Member]
Derivative [Line Items]
Notional (Bbl) | bbl	[3]	638,750
Strike Price | $ / MillionBarrels-MMBbls	[1]	59.86
Market Price | $ / MillionBarrels-MMBbls	[1]	44.92
Fair Value
Asset	[2]	$ 11.1	10.0
Liability	[2]	$ 0.0	0.0
Purchased [Member] | Portion of Call Option Contracts Maturing in 2017 - NGL [Member]
Derivative [Line Items]
Notional (Bbl) | bbl	[3]	1,277,500
Strike Price | $ / MillionBarrels-MMBbls	[1]	29.46
Market Price | $ / MillionBarrels-MMBbls	[1]	23.44
Fair Value
Asset	[2]	$ 0.0	0.0
Liability	[2]	$ (0.8)	(0.8)
Purchased [Member] | Portion of Call Option Contracts Maturing in 2017 - Crude Oil [Member]
Derivative [Line Items]
Strike Price | $ / MillionBarrels-MMBbls	[1]	68.19
Market Price | $ / MillionBarrels-MMBbls	[1]	44.92
Fair Value
Asset	[2]	$ 0.0	0.0
Liability	[2]	$ (0.9)	(0.6)
Purchased [Member] | Portion Of Put Option Contracts Maturing In 2018 - Crude Oil [Member]
Derivative [Line Items]
Notional (Bbl) | bbl		91,250
Strike Price | $ / MillionBarrels-MMBbls	[1]	42.00
Market Price | $ / MillionBarrels-MMBbls	[1]	47.03
Fair Value
Asset	[2]	$ 0.5	0.0
Liability	[2]	$ 0.0	0.0
Written [Member] | Portion of Put Option Contracts Maturing in 2016 - Natural Gas [Member]
Derivative [Line Items]
Notional (MMBtu) | MMBTU	[3]	1,237,500
Strike Price | $ / MillionBarrels-MMBbls	[1]	3.75
Market Price | $ / MillionBarrels-MMBbls	[1]	2.22
Fair Value
Asset	[2]	$ 0.0	0.0
Liability	[2]	$ (1.9)	(2.1)
Written [Member] | Portion of Put Option Contracts Maturing in 2016 - NGL [Member]
Derivative [Line Items]
Notional (Bbl) | bbl	[3]	68,750
Strike Price | $ / MillionBarrels-MMBbls	[1]	39.06
Market Price | $ / MillionBarrels-MMBbls	[1]	23.25
Fair Value
Asset	[2]	$ 0.0	0.0
Liability	[2]	$ (1.1)	(1.5)
Written [Member] | Portion of Call Option Contracts Maturing in 2016 - Natural Gas [Member]
Derivative [Line Items]
Notional (MMBtu) | MMBTU	[3]	1,237,500
Strike Price | $ / MillionBarrels-MMBbls	[1]	4.98
Market Price | $ / MillionBarrels-MMBbls	[1]	2.22
Fair Value
Asset	[2]	$ 0.0	0.0
Liability	[2]	$ 0.0	0.0
Written [Member] | Portion of Call Option Contracts Maturing in 2016 - NGL [Member]
Derivative [Line Items]
Notional (Bbl) | bbl	[3]	2,227,500
Strike Price | $ / MillionBarrels-MMBbls	[1]	45.09
Market Price | $ / MillionBarrels-MMBbls	[1]	22.66
Fair Value
Asset	[2]	$ 0.0	0.0
Liability	[2]	$ (0.4)	(0.3)
Written [Member] | Portion of Call Option Contracts Maturing in 2016 - Crude Oil [Member]
Derivative [Line Items]
Notional (Bbl) | bbl	[3]	605,000
Strike Price | $ / MillionBarrels-MMBbls	[1]	86.68
Market Price | $ / MillionBarrels-MMBbls	[1]	41.68
Fair Value
Asset	[2]	$ 0.0	0.0
Liability	[2]	$ 0.0	0.0
Written [Member] | Portion of Call Option Contracts Maturing in 2017 - Crude Oil [Member]
Derivative [Line Items]
Notional (Bbl) | bbl	[3]	638,750
Written [Member] | Portion Of Put Option Contracts Maturing In 2018 - Crude Oil [Member]
Derivative [Line Items]
Notional (Bbl) | bbl		91,250
Written [Member] | Portion Of Call Option Contracts Maturing In 2018 - Crude Oil [Member]
Derivative [Line Items]
Strike Price | $ / MillionBarrels-MMBbls	[1]	51.75
Market Price | $ / MillionBarrels-MMBbls	[1]	47.03
Fair Value
Asset	[2]	$ 0.0	0.0
Liability	[2]	$ (0.5)	$ 0.0

[1] Strike and market prices are in $/MMBtu for natural gas and in $/Bbl for NGL and crude oil.

[2] The fair value is determined based on quoted market prices at March 31, 2016 and December 31, 2015, respectively, discounted using the swap rate for the respective periods to consider the time value of money. Fair values exclude credit valuation adjustment losses of approximately $0.3 million and $0.4 million at March 31, 2016 and December 31, 2015, respectively, as well as cash collateral received.

[3] Volumes of natural gas are measured in MMBtu, whereas volumes of NGL and crude oil are measured in Bbl.