Derivative Instruments and Hedging Activities, Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Derivative assets	$ 177.2	$ 60.2
Derivative liabilities	2.4	5.2
Current Assets from Risk Management Activities [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	126.0	44.4
Long-Term Assets from Risk Management Activities [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	51.2	15.8
Current Liabilities from Risk Management Activities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	0.6	5.2
Long-Term Liabilities from Risk Management Activities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	1.8	0
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	169.0	60.2
Derivative liabilities	2.2	0
Designated as Hedging Instrument [Member] | Commodity Contracts [Member] | Current Assets from Risk Management Activities [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	117.8	44.4
Designated as Hedging Instrument [Member] | Commodity Contracts [Member] | Long-Term Assets from Risk Management Activities [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	51.2	15.8
Designated as Hedging Instrument [Member] | Commodity Contracts [Member] | Current Liabilities from Risk Management Activities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	0.4	0
Designated as Hedging Instrument [Member] | Commodity Contracts [Member] | Long-Term Liabilities from Risk Management Activities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	1.8	0
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	8.2	0
Derivative liabilities	0.2	5.2
Not Designated as Hedging Instrument [Member] | Current Assets from Risk Management Activities [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	8.2	0
Not Designated as Hedging Instrument [Member] | Current Liabilities from Risk Management Activities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	$ 0.2	$ 5.2