Derivatives (Derivatives Outstanding) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Net fair value of derivatives	$ 17.4	$ 21.7
Not Designated as Hedging Instrument
Derivative [Line Items]
Net fair value of derivatives	17.4
Liquids | Short Contracts | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	54,700,000
Net fair value of derivatives	21.7
Liquids | Long Contracts | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	44,600,000
Net fair value of derivatives	(4.1)
Gas | Short Contracts | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	600,000
Net fair value of derivatives	0.1
Gas | Long Contracts | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	400,000
Net fair value of derivatives	(0.3)
Crude and Condensate | Short Contracts | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Net fair value of derivatives	(0.1)
Crude and Condensate | Long Contracts | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0
Net fair value of derivatives	$ 0.1