Derivatives (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended											3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2013 MMcf	Dec. 31, 2012	Jun. 30, 2013 Commodity Contract [Member] Designated as Hedging Instrument [Member] Other current assets [Member]	Dec. 31, 2012 Commodity Contract [Member] Designated as Hedging Instrument [Member] Other current assets [Member]	Jun. 30, 2013 Commodity Contract [Member] Designated as Hedging Instrument [Member] Other current liabilities [Member]	Dec. 31, 2012 Commodity Contract [Member] Designated as Hedging Instrument [Member] Other current liabilities [Member]	Jun. 30, 2013 Commodity Contract [Member] Designated as Hedging Instrument [Member] Other non-current assets [Member]	Dec. 31, 2012 Commodity Contract [Member] Designated as Hedging Instrument [Member] Other non-current assets [Member]	Jun. 30, 2013 Commodity Contract [Member] Designated as Hedging Instrument [Member] Other non-current liabilities [Member]	Dec. 31, 2012 Commodity Contract [Member] Designated as Hedging Instrument [Member] Other non-current liabilities [Member]	Jun. 30, 2012 Term loan HP Storage [Member]	Jun. 30, 2012 Term loan HP Storage [Member]	Dec. 31, 2012 Term loan HP Storage [Member]
Derivatives, Fair Value [Line Items]
Gas stored underground		$ 1.4	$ 7.0
Capacity of gas available for sale (in mmcf)		9,700
Book value of additional gas available for sale		22.0
Amount of stored gas hedged with derivatives (in mmcf)		10,300
Notional amount												100.0	100.0
Long-term Debt, Gross												200.0	200.0	200.0
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments												0.3	0.8
Notional Amount of Cash Flow Hedge Instruments	300.0
Maximum Length of Time Hedged in Price Risk Cash Flow Hedge (in years)	10 years
Derivative, Description of Terms (in years)	10
Derivative, cash paid on hedge	6.8
Derivative Asset, Fair Value, Net				2.7	0.1			0.5	0
Derivative Liability, Fair Value, Net						$ 0.9	$ 0.1			$ 0	$ 0