DERIVATIVE FINANCIAL INSTRUMENTS AND HEDGING ACTIVITIES (Fair Values of Expected Cash Flows of Commodity Options) (Details) $ in Millions		6 Months Ended	12 Months Ended
		Jun. 30, 2015 USD ($) bbl MMBTU $ / bbl $ / MMBTU	Dec. 31, 2014 USD ($)
Fair Value
Liability		$ (417.3)	$ (440.0)
Commodity Option [Member]
Fair Value
Credit valuation adjustments		$ (0.2)	$ (0.7)
Written [Member] | Portion of Put Option Contracts Maturing in 2015 - Natural Gas [Member]
Derivative [Line Items]
Notional | MMBTU	[1]	2,024,000
Strike Price | $ / MMBTU	[2]	3.90
Market Price | $ / MMBTU	[2]	2.91
Fair Value
Asset	[3]
Liability	[3]	$ (2.1)	$ (3.8)
Written [Member] | Portion of Put Option Contracts Maturing in 2015 - NGL [Member]
Derivative [Line Items]
Notional | bbl	[1]	46,000
Strike Price | $ / bbl	[2]	77.28
Market Price | $ / bbl	[2]	51.86
Fair Value
Asset	[3]
Liability	[3]	$ (1.2)
Written [Member] | Portion of Call Option Contracts Maturing in 2015 - Natural Gas [Member]
Derivative [Line Items]
Notional | MMBTU	[1]	644,000
Strike Price | $ / MMBTU	[2]	5.05
Market Price | $ / MMBTU	[2]	2.90
Fair Value
Asset	[3]
Liability	[3]
Written [Member] | Portion of Call Option Contracts Maturing in 2015 - NGL [Member]
Derivative [Line Items]
Notional | bbl	[1]	745,200
Strike Price | $ / bbl	[2]	45.80
Market Price | $ / bbl	[2]	24.93
Fair Value
Asset	[3]
Liability	[3]	$ (0.1)	$ (0.6)
Written [Member] | Portion of Call Option Contracts Maturing in 2015 - Crude Oil [Member]
Derivative [Line Items]
Notional | bbl	[1]	368,000
Strike Price | $ / bbl	[2]	88.39
Market Price | $ / bbl	[2]	60.38
Fair Value
Asset	[3]
Liability	[3]		$ (0.4)
Written [Member] | Portion of Put Option Contracts Maturing in 2016 - Natural Gas [Member]
Derivative [Line Items]
Notional | MMBTU	[1]	1,647,000
Strike Price | $ / MMBTU	[2]	3.75
Market Price | $ / MMBTU	[2]	3.17
Fair Value
Asset	[3]
Liability	[3]	$ (1.2)	$ (1.0)
Written [Member] | Portion of Put Option Contracts Maturing in 2016 - NGL [Member]
Derivative [Line Items]
Notional | bbl	[1]	91,500
Strike Price | $ / bbl	[2]	39.06
Market Price | $ / bbl	[2]	29.66
Fair Value
Asset	[3]
Liability	[3]	$ (1.1)
Written [Member] | Portion of Call Option Contracts Maturing in 2016 - Natural Gas [Member]
Derivative [Line Items]
Notional | MMBTU	[1]	1,647,000
Strike Price | $ / MMBTU	[2]	4.98
Market Price | $ / MMBTU	[2]	3.17
Fair Value
Asset	[3]
Liability	[3]	$ (0.1)	$ (0.1)
Written [Member] | Portion of Call Option Contracts Maturing in 2016 - NGL [Member]
Derivative [Line Items]
Notional | bbl	[1]	2,836,500
Strike Price | $ / bbl	[2]	45.14
Market Price | $ / bbl	[2]	27.06
Fair Value
Asset	[3]
Liability	[3]	$ (2.8)	$ (3.2)
Written [Member] | Portion of Call Option Contracts Maturing in 2016 - Crude Oil [Member]
Derivative [Line Items]
Notional | bbl	[1]	805,200
Strike Price | $ / bbl	[2]	86.68
Market Price | $ / bbl	[2]	62.08
Fair Value
Asset	[3]
Liability	[3]	$ (0.8)	$ (2.7)
Written [Member] | Portion of Call Option Contracts Maturing in 2017 - NGL [Member]
Derivative [Line Items]
Notional | bbl	[1]	547,500
Strike Price | $ / bbl	[2]	25.34
Market Price | $ / bbl	[2]	23.14
Fair Value
Asset	[3]
Liability	[3]	$ (0.6)	$ (0.7)
Written [Member] | Portion of Call Option Contracts Maturing in 2017 - Crude Oil [Member]
Derivative [Line Items]
Notional | bbl	[1]	547,500
Strike Price | $ / bbl	[2]	71.45
Market Price | $ / bbl	[2]	63.62
Fair Value
Asset	[3]
Liability	[3]	$ (2.7)	$ (3.3)
Purchased [Member] | Portion of Put Option Contracts Maturing in 2015 - Natural Gas [Member]
Derivative [Line Items]
Notional | MMBTU	[1]	2,024,000
Strike Price | $ / MMBTU	[2]	3.90
Market Price | $ / MMBTU	[2]	2.90
Fair Value
Asset	[3]	$ 2.1	$ 3.8
Liability	[3]
Purchased [Member] | Portion of Put Option Contracts Maturing in 2015 - NGL [Member]
Derivative [Line Items]
Notional | bbl	[1]	1,159,200
Strike Price | $ / bbl	[2]	43.32
Market Price | $ / bbl	[2]	25.70
Fair Value
Asset	[3]	$ 20.6	$ 40.2
Liability	[3]
Purchased [Member] | Portion of Put Option Contracts Maturing in 2015 - Crude Oil [Member]
Derivative [Line Items]
Notional | bbl	[1]	368,000
Strike Price | $ / bbl	[2]	81.56
Market Price | $ / bbl	[2]	60.38
Fair Value
Asset	[3]	$ 7.9	$ 18.8
Liability	[3]
Purchased [Member] | Portion of Call Option Contracts Maturing in 2015 - Natural Gas [Member]
Derivative [Line Items]
Notional | MMBTU	[1]	644,000
Strike Price | $ / MMBTU	[2]	5.05
Market Price | $ / MMBTU	[2]	2.90
Fair Value
Asset	[3]
Liability	[3]
Purchased [Member] | Portion of Put Option Contracts Maturing in 2016 - Natural Gas [Member]
Derivative [Line Items]
Notional | MMBTU	[1]	1,647,000
Strike Price | $ / MMBTU	[2]	3.75
Market Price | $ / MMBTU	[2]	3.17
Fair Value
Asset	[3]	$ 1.2	$ 1.0
Liability	[3]
Purchased [Member] | Portion of Put Option Contracts Maturing in 2016 - NGL [Member]
Derivative [Line Items]
Notional | bbl	[1]	2,836,500
Strike Price | $ / bbl	[2]	39.24
Market Price | $ / bbl	[2]	27.06
Fair Value
Asset	[3]	$ 38.9	$ 39.3
Liability	[3]
Purchased [Member] | Portion of Put Option Contracts Maturing in 2016 - Crude Oil [Member]
Derivative [Line Items]
Notional | bbl	[1]	805,200
Strike Price | $ / bbl	[2]	75.91
Market Price | $ / bbl	[2]	62.08
Fair Value
Asset	[3]	$ 12.9	$ 14.7
Liability	[3]
Purchased [Member] | Portion of Call Option Contracts Maturing in 2016 - Natural Gas [Member]
Derivative [Line Items]
Notional | MMBTU	[1]	1,647,000
Strike Price | $ / MMBTU	[2]	4.98
Market Price | $ / MMBTU	[2]	3.17
Fair Value
Asset	[3]	$ 0.1	$ 0.1
Liability	[3]
Purchased [Member] | Portion of Call Option Contracts Maturing in 2016 - NGL [Member]
Derivative [Line Items]
Notional | bbl	[1]	91,500
Strike Price | $ / bbl	[2]	46.41
Market Price | $ / bbl	[2]	29.66
Fair Value
Asset	[3]	$ 0.1
Liability	[3]
Purchased [Member] | Portion of Put Option Contracts Maturing in 2017 - NGL [Member]
Derivative [Line Items]
Notional | bbl	[1]	547,500
Strike Price | $ / bbl	[2]	21.70
Market Price | $ / bbl	[2]	23.14
Fair Value
Asset	[3]	$ 0.8	$ 1.2
Liability	[3]
Purchased [Member] | Portion of Put Option Contracts Maturing in 2017 - Crude Oil [Member]
Derivative [Line Items]
Notional | bbl	[1]	547,500
Strike Price | $ / bbl	[2]	63.00
Market Price | $ / bbl	[2]	63.62
Fair Value
Asset	[3]	$ 4.3	$ 4.1
Liability	[3]

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

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

[3] The fair value is determined based on quoted market prices at June 30, 2015 and December 31, 2014, 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 $0.2 million and $0.7 million at June 30, 2015 and December 31, 2014, respectively, as well as cash collateral received.